boost/python
2
0
Fork 0
mirror of https://github.com/boostorg/python.git synced 2026-01-20 04:42:28 +00:00
Code Issues Projects Releases Wiki Activity
Files
c56cec0b9775fb51d0747c613205c6f0bd710505
python/doc/develop/html/tutorial/tutorial/functions.html
stefanseefeld d5d03ea3e7 deploy: 45e28cc0346f7c1c7588a860c218426cbc1cb05b
2020-12-25 18:34:38 +00:00

588 lines
45 KiB
HTML
Raw Blame History

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Functions</title>
<link rel="stylesheet" href="../../boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="../index.html" title="Boost.Python Tutorial">
<link rel="up" href="../index.html" title="Boost.Python Tutorial">
<link rel="prev" href="exposing.html" title="Exposing Classes">
<link rel="next" href="object.html" title="Object Interface">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="" width="" height="" src="../../images/bpl.png"></td></tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="exposing.html"><img src="../../images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="object.html"><img src="../../images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="tutorial.functions"></a><a class="link" href="functions.html" title="Functions">Functions</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="functions.html#tutorial.functions.call_policies">Call Policies</a></span></dt>
<dt><span class="section"><a href="functions.html#tutorial.functions.overloading">Overloading</a></span></dt>
<dt><span class="section"><a href="functions.html#tutorial.functions.default_arguments">Default Arguments</a></span></dt>
<dt><span class="section"><a href="functions.html#tutorial.functions.auto_overloading">Auto-Overloading</a></span></dt>
</dl></div>
<p>
In this chapter, we'll look at Boost.Python powered functions in closer detail.
We will see some facilities to make exposing C++ functions to Python safe from
potential pifalls such as dangling pointers and references. We will also see
facilities that will make it even easier for us to expose C++ functions that
take advantage of C++ features such as overloading and default arguments.
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="emphasis"><em>Read on...</em></span>
</p></blockquote></div>
<p>
But before you do, you might want to fire up Python 2.2 or later and type
<code class="literal">&gt;&gt;&gt; import this</code>.
</p>
<pre class="programlisting">&gt;&gt;&gt; import this
The Zen of Python, by Tim Peters
Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than <span class="bold"><strong>right</strong></span> now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!
</pre>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="tutorial.functions.call_policies"></a><a class="link" href="functions.html#tutorial.functions.call_policies" title="Call Policies">Call Policies</a>
</h3></div></div></div>
<p>
In C++, we often deal with arguments and return types such as pointers and
references. Such primitive types are rather, ummmm, low level and they really
don't tell us much. At the very least, we don't know the owner of the pointer
or the referenced object. No wonder languages such as Java and Python never
deal with such low level entities. In C++, it's usually considered a good
practice to use smart pointers which exactly describe ownership semantics.
Still, even good C++ interfaces use raw references and pointers sometimes,
so Boost.Python must deal with them. To do this, it may need your help. Consider
the following C++ function:
</p>
<pre class="programlisting"><span class="identifier">X</span><span class="special">&amp;</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Y</span><span class="special">&amp;</span> <span class="identifier">y</span><span class="special">,</span> <span class="identifier">Z</span><span class="special">*</span> <span class="identifier">z</span><span class="special">);</span>
</pre>
<p>
How should the library wrap this function? A naive approach builds a Python
X object around result reference. This strategy might or might not work out.
Here's an example where it didn't
</p>
<pre class="programlisting"><span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">y</span><span class="special">,</span> <span class="identifier">z</span><span class="special">)</span> <span class="special">#</span> <span class="identifier">x</span> <span class="identifier">refers</span> <span class="identifier">to</span> <span class="identifier">some</span> <span class="identifier">C</span><span class="special">++</span> <span class="identifier">X</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">del</span> <span class="identifier">y</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">some_method</span><span class="special">()</span> <span class="special">#</span> <span class="identifier">CRASH</span><span class="special">!</span>
</pre>
<p>
What's the problem?
</p>
<p>
Well, what if f() was implemented as shown below:
</p>
<pre class="programlisting"><span class="identifier">X</span><span class="special">&amp;</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Y</span><span class="special">&amp;</span> <span class="identifier">y</span><span class="special">,</span> <span class="identifier">Z</span><span class="special">*</span> <span class="identifier">z</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">y</span><span class="special">.</span><span class="identifier">z</span> <span class="special">=</span> <span class="identifier">z</span><span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">y</span><span class="special">.</span><span class="identifier">x</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
The problem is that the lifetime of result X&amp; is tied to the lifetime
of y, because the f() returns a reference to a member of the y object. This
idiom is is not uncommon and perfectly acceptable in the context of C++.
However, Python users should not be able to crash the system just by using
our C++ interface. In this case deleting y will invalidate the reference
to X. We have a dangling reference.
</p>
<p>
Here's what's happening:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<code class="literal">f</code> is called passing in a reference to <code class="literal">y</code>
and a pointer to <code class="literal">z</code>
</li>
<li class="listitem">
A reference to <code class="literal">y.x</code> is returned
</li>
<li class="listitem">
<code class="literal">y</code> is deleted. <code class="literal">x</code> is a dangling reference
</li>
<li class="listitem">
<code class="literal">x.some_method()</code> is called
</li>
<li class="listitem">
<span class="bold"><strong>BOOM!</strong></span>
</li>
</ol></div>
<p>
We could copy result into a new object:
</p>
<pre class="programlisting"><span class="special">&gt;&gt;&gt;</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">y</span><span class="special">,</span> <span class="identifier">z</span><span class="special">).</span><span class="identifier">set</span><span class="special">(</span><span class="number">42</span><span class="special">)</span> <span class="comment"># Result disappears</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">y</span><span class="special">.</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="comment"># No crash, but still bad</span>
<span class="number">3.14</span>
</pre>
<p>
This is not really our intent of our C++ interface. We've broken our promise
that the Python interface should reflect the C++ interface as closely as
possible.
</p>
<p>
Our problems do not end there. Suppose Y is implemented as follows:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Y</span>
<span class="special">{</span>
<span class="identifier">X</span> <span class="identifier">x</span><span class="special">;</span> <span class="identifier">Z</span><span class="special">*</span> <span class="identifier">z</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">z_value</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">z</span><span class="special">-&gt;</span><span class="identifier">value</span><span class="special">();</span> <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
Notice that the data member <code class="literal">z</code> is held by class Y using
a raw pointer. Now we have a potential dangling pointer problem inside Y:
</p>
<pre class="programlisting"><span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">y</span><span class="special">,</span> <span class="identifier">z</span><span class="special">)</span> <span class="special">#</span> <span class="identifier">y</span> <span class="identifier">refers</span> <span class="identifier">to</span> <span class="identifier">z</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">del</span> <span class="identifier">z</span> <span class="special">#</span> <span class="identifier">Kill</span> <span class="identifier">the</span> <span class="identifier">z</span> <span class="identifier">object</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">y</span><span class="special">.</span><span class="identifier">z_value</span><span class="special">()</span> <span class="special">#</span> <span class="identifier">CRASH</span><span class="special">!</span>
</pre>
<p>
For reference, here's the implementation of <code class="literal">f</code> again:
</p>
<pre class="programlisting"><span class="identifier">X</span><span class="special">&amp;</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Y</span><span class="special">&amp;</span> <span class="identifier">y</span><span class="special">,</span> <span class="identifier">Z</span><span class="special">*</span> <span class="identifier">z</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">y</span><span class="special">.</span><span class="identifier">z</span> <span class="special">=</span> <span class="identifier">z</span><span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">y</span><span class="special">.</span><span class="identifier">x</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
Here's what's happening:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<code class="literal">f</code> is called passing in a reference to <code class="literal">y</code>
and a pointer to <code class="literal">z</code>
</li>
<li class="listitem">
A pointer to <code class="literal">z</code> is held by <code class="literal">y</code>
</li>
<li class="listitem">
A reference to <code class="literal">y.x</code> is returned
</li>
<li class="listitem">
<code class="literal">z</code> is deleted. <code class="literal">y.z</code> is a dangling
pointer
</li>
<li class="listitem">
<code class="literal">y.z_value()</code> is called
</li>
<li class="listitem">
<code class="literal">z-&gt;value()</code> is called
</li>
<li class="listitem">
<span class="bold"><strong>BOOM!</strong></span>
</li>
</ol></div>
<h3>
<a name="tutorial.functions.call_policies.h0"></a>
<span class="phrase"><a name="tutorial.functions.call_policies.call_policies"></a></span><a class="link" href="functions.html#tutorial.functions.call_policies.call_policies">Call
Policies</a>
</h3>
<p>
Call Policies may be used in situations such as the example detailed above.
In our example, <code class="literal">return_internal_reference</code> and <code class="literal">with_custodian_and_ward</code>
are our friends:
</p>
<pre class="programlisting"><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">f</span><span class="special">,</span>
<span class="identifier">return_internal_reference</span><span class="special">&lt;</span><span class="number">1</span><span class="special">,</span>
<span class="identifier">with_custodian_and_ward</span><span class="special">&lt;</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">&gt;</span> <span class="special">&gt;());</span>
</pre>
<p>
What are the <code class="literal">1</code> and <code class="literal">2</code> parameters, you
ask?
</p>
<pre class="programlisting"><span class="identifier">return_internal_reference</span><span class="special">&lt;</span><span class="number">1</span>
</pre>
<p>
Informs Boost.Python that the first argument, in our case <code class="literal">Y&amp;
y</code>, is the owner of the returned reference: <code class="literal">X&amp;</code>.
The "<code class="literal">1</code>" simply specifies the first argument.
In short: "return an internal reference <code class="literal">X&amp;</code> owned
by the 1st argument <code class="literal">Y&amp; y</code>".
</p>
<pre class="programlisting"><span class="identifier">with_custodian_and_ward</span><span class="special">&lt;</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">&gt;</span>
</pre>
<p>
Informs Boost.Python that the lifetime of the argument indicated by ward
(i.e. the 2nd argument: <code class="literal">Z* z</code>) is dependent on the lifetime
of the argument indicated by custodian (i.e. the 1st argument: <code class="literal">Y&amp;
y</code>).
</p>
<p>
It is also important to note that we have defined two policies above. Two
or more policies can be composed by chaining. Here's the general syntax:
</p>
<pre class="programlisting"><span class="identifier">policy1</span><span class="special">&lt;</span><span class="identifier">args</span><span class="special">...,</span>
<span class="identifier">policy2</span><span class="special">&lt;</span><span class="identifier">args</span><span class="special">...,</span>
<span class="identifier">policy3</span><span class="special">&lt;</span><span class="identifier">args</span><span class="special">...&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
</pre>
<p>
Here is the list of predefined call policies. A complete reference detailing
these can be found <a href="../../reference/function_invocation_and_creation/models_of_callpolicies.html" target="_top">here</a>.
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<span class="bold"><strong>with_custodian_and_ward</strong></span>: Ties lifetimes
of the arguments
</li>
<li class="listitem">
<span class="bold"><strong>with_custodian_and_ward_postcall</strong></span>: Ties
lifetimes of the arguments and results
</li>
<li class="listitem">
<span class="bold"><strong>return_internal_reference</strong></span>: Ties lifetime
of one argument to that of result
</li>
<li class="listitem">
<span class="bold"><strong>return_value_policy&lt;T&gt; with T one of:</strong></span>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem">
<span class="bold"><strong>reference_existing_object</strong></span>: naive
(dangerous) approach
</li>
<li class="listitem">
<span class="bold"><strong>copy_const_reference</strong></span>: Boost.Python
v1 approach
</li>
<li class="listitem">
<span class="bold"><strong>copy_non_const_reference</strong></span>:
</li>
<li class="listitem">
<span class="bold"><strong>manage_new_object</strong></span>: Adopt a pointer
and hold the instance
</li>
</ul></div>
</li>
</ul></div>
<div class="blurb">
<div class="titlepage"><div><div><p class="title"><b></b></p></div></div></div>
<p>
<span class="inlinemediaobject"><img src="../../images/smiley.png"></span>
<span class="bold"><strong>Remember the Zen, Luke:</strong></span>
</p>
<p>
"Explicit is better than implicit"
</p>
<p>
"In the face of ambiguity, refuse the temptation to guess"
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="tutorial.functions.overloading"></a><a class="link" href="functions.html#tutorial.functions.overloading" title="Overloading">Overloading</a>
</h3></div></div></div>
<p>
The following illustrates a scheme for manually wrapping an overloaded member
functions. Of course, the same technique can be applied to wrapping overloaded
non-member functions.
</p>
<p>
We have here our C++ class:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span>
<span class="special">{</span>
<span class="keyword">bool</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">bool</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">b</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">bool</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">b</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">c</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">b</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">c</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">};</span>
</pre>
<p>
Class X has 4 overloaded functions. We will start by introducing some member
function pointer variables:
</p>
<pre class="programlisting"><span class="keyword">bool</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx1</span><span class="special">)(</span><span class="keyword">int</span><span class="special">)</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
<span class="keyword">bool</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx2</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">)</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
<span class="keyword">bool</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx3</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">char</span><span class="special">)=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
<span class="keyword">int</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx4</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
</pre>
<p>
With these in hand, we can proceed to define and wrap this for Python:
</p>
<pre class="programlisting"><span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx1</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx2</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx3</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx4</span><span class="special">)</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="tutorial.functions.default_arguments"></a><a class="link" href="functions.html#tutorial.functions.default_arguments" title="Default Arguments">Default Arguments</a>
</h3></div></div></div>
<p>
Boost.Python wraps (member) function pointers. Unfortunately, C++ function
pointers carry no default argument info. Take a function <code class="literal">f</code>
with default arguments:
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span> <span class="special">=</span> <span class="number">3.14</span><span class="special">,</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span> <span class="special">=</span> <span class="string">"hello"</span><span class="special">);</span>
</pre>
<p>
But the type of a pointer to the function <code class="literal">f</code> has no information
about its default arguments:
</p>
<pre class="programlisting"><span class="keyword">int</span><span class="special">(*</span><span class="identifier">g</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">*)</span> <span class="special">=</span> <span class="identifier">f</span><span class="special">;</span> <span class="comment">// defaults lost!</span>
</pre>
<p>
When we pass this function pointer to the <code class="literal">def</code> function,
there is no way to retrieve the default arguments:
</p>
<pre class="programlisting"><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">f</span><span class="special">);</span> <span class="comment">// defaults lost!</span>
</pre>
<p>
Because of this, when wrapping C++ code, we had to resort to manual wrapping
as outlined in the <a class="link" href="functions.html#tutorial.functions.overloading" title="Overloading">previous
section</a>, or writing thin wrappers:
</p>
<pre class="programlisting"><span class="comment">// write "thin wrappers"</span>
<span class="keyword">int</span> <span class="identifier">f1</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span> <span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">f2</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span><span class="identifier">y</span><span class="special">);</span> <span class="special">}</span>
<span class="comment">/*...*/</span>
<span class="comment">// in module init</span>
<span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">f</span><span class="special">);</span> <span class="comment">// all arguments</span>
<span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">f2</span><span class="special">);</span> <span class="comment">// two arguments</span>
<span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">f1</span><span class="special">);</span> <span class="comment">// one argument</span>
</pre>
<p>
When you want to wrap functions (or member functions) that either:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
have default arguments, or
</li>
<li class="listitem">
are overloaded with a common sequence of initial arguments
</li>
</ul></div>
<h3>
<a name="tutorial.functions.default_arguments.h0"></a>
<span class="phrase"><a name="tutorial.functions.default_arguments.boost_python_function_overloads"></a></span><a class="link" href="functions.html#tutorial.functions.default_arguments.boost_python_function_overloads">BOOST_PYTHON_FUNCTION_OVERLOADS</a>
</h3>
<p>
Boost.Python now has a way to make it easier. For instance, given a function:
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">unsigned</span> <span class="identifier">c</span> <span class="special">=</span> <span class="number">2</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">3</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
</pre>
<p>
The macro invocation:
</p>
<pre class="programlisting"><span class="identifier">BOOST_PYTHON_FUNCTION_OVERLOADS</span><span class="special">(</span><span class="identifier">foo_overloads</span><span class="special">,</span> <span class="identifier">foo</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">4</span><span class="special">)</span>
</pre>
<p>
will automatically create the thin wrappers for us. This macro will create
a class <code class="literal">foo_overloads</code> that can be passed on to <code class="literal">def(...)</code>.
The third and fourth macro argument are the minimum arguments and maximum
arguments, respectively. In our <code class="literal">foo</code> function the minimum
number of arguments is 1 and the maximum number of arguments is 4. The <code class="literal">def(...)</code>
function will automatically add all the foo variants for us:
</p>
<pre class="programlisting"><span class="identifier">def</span><span class="special">(</span><span class="string">"foo"</span><span class="special">,</span> <span class="identifier">foo</span><span class="special">,</span> <span class="identifier">foo_overloads</span><span class="special">());</span>
</pre>
<h3>
<a name="tutorial.functions.default_arguments.h1"></a>
<span class="phrase"><a name="tutorial.functions.default_arguments.boost_python_member_function_ove"></a></span><a class="link" href="functions.html#tutorial.functions.default_arguments.boost_python_member_function_ove">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</a>
</h3>
<p>
Objects here, objects there, objects here there everywhere. More frequently
than anything else, we need to expose member functions of our classes to
Python. Then again, we have the same inconveniences as before when default
arguments or overloads with a common sequence of initial arguments come into
play. Another macro is provided to make this a breeze.
</p>
<p>
Like <code class="literal">BOOST_PYTHON_FUNCTION_OVERLOADS</code>, <code class="literal">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</code>
may be used to automatically create the thin wrappers for wrapping member
functions. Let's have an example:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">george</span>
<span class="special">{</span>
<span class="keyword">void</span>
<span class="identifier">wack_em</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">c</span> <span class="special">=</span> <span class="char">'x'</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
The macro invocation:
</p>
<pre class="programlisting"><span class="identifier">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</span><span class="special">(</span><span class="identifier">george_overloads</span><span class="special">,</span> <span class="identifier">wack_em</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">3</span><span class="special">)</span>
</pre>
<p>
will generate a set of thin wrappers for george's <code class="literal">wack_em</code>
member function accepting a minimum of 1 and a maximum of 3 arguments (i.e.
the third and fourth macro argument). The thin wrappers are all enclosed
in a class named <code class="literal">george_overloads</code> that can then be used
as an argument to <code class="literal">def(...)</code>:
</p>
<pre class="programlisting"><span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"wack_em"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">george</span><span class="special">::</span><span class="identifier">wack_em</span><span class="special">,</span> <span class="identifier">george_overloads</span><span class="special">());</span>
</pre>
<p>
See the <a href="../../reference/function_invocation_and_creation/boost_python_overloads_hpp.html#function_invocation_and_creation.boost_python_overloads_hpp.macros" target="_top">overloads
reference</a> for details.
</p>
<h3>
<a name="tutorial.functions.default_arguments.h2"></a>
<span class="phrase"><a name="tutorial.functions.default_arguments.init_and_optional"></a></span><a class="link" href="functions.html#tutorial.functions.default_arguments.init_and_optional">init and
optional</a>
</h3>
<p>
A similar facility is provided for class constructors, again, with default
arguments or a sequence of overloads. Remember <code class="literal">init&lt;...&gt;</code>?
For example, given a class X with a constructor:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span>
<span class="special">{</span>
<span class="identifier">X</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">b</span> <span class="special">=</span> <span class="char">'D'</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">c</span> <span class="special">=</span> <span class="string">"constructor"</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">);</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
</pre>
<p>
You can easily add this constructor to Boost.Python in one shot:
</p>
<pre class="programlisting"><span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">init</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;())</span>
</pre>
<p>
Notice the use of <code class="literal">init&lt;...&gt;</code> and <code class="literal">optional&lt;...&gt;</code>
to signify the default (optional arguments).
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="tutorial.functions.auto_overloading"></a><a class="link" href="functions.html#tutorial.functions.auto_overloading" title="Auto-Overloading">Auto-Overloading</a>
</h3></div></div></div>
<p>
It was mentioned in passing in the previous section that <code class="literal">BOOST_PYTHON_FUNCTION_OVERLOADS</code>
and <code class="literal">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</code> can also be
used for overloaded functions and member functions with a common sequence
of initial arguments. Here is an example:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">bool</span> <span class="identifier">a</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">bool</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">b</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">bool</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">b</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">c</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
</pre>
<p>
Like in the previous section, we can generate thin wrappers for these overloaded
functions in one-shot:
</p>
<pre class="programlisting"><span class="identifier">BOOST_PYTHON_FUNCTION_OVERLOADS</span><span class="special">(</span><span class="identifier">foo_overloads</span><span class="special">,</span> <span class="identifier">foo</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="number">3</span><span class="special">)</span>
</pre>
<p>
Then...
</p>
<pre class="programlisting"><span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"foo"</span><span class="special">,</span> <span class="special">(</span><span class="keyword">void</span><span class="special">(*)(</span><span class="keyword">bool</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">char</span><span class="special">))</span><span class="number">0</span><span class="special">,</span> <span class="identifier">foo_overloads</span><span class="special">());</span>
</pre>
<p>
Notice though that we have a situation now where we have a minimum of zero
(0) arguments and a maximum of 3 arguments.
</p>
<h3>
<a name="tutorial.functions.auto_overloading.h0"></a>
<span class="phrase"><a name="tutorial.functions.auto_overloading.manual_wrapping"></a></span><a class="link" href="functions.html#tutorial.functions.auto_overloading.manual_wrapping">Manual
Wrapping</a>
</h3>
<p>
It is important to emphasize however that <span class="bold"><strong>the overloaded
functions must have a common sequence of initial arguments</strong></span>. Otherwise,
our scheme above will not work. If this is not the case, we have to wrap
our functions <a class="link" href="functions.html#tutorial.functions.overloading" title="Overloading">manually</a>.
</p>
<p>
Actually, we can mix and match manual wrapping of overloaded functions and
automatic wrapping through <code class="literal">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</code>
and its sister, <code class="literal">BOOST_PYTHON_FUNCTION_OVERLOADS</code>. Following
up on our example presented in the section <a class="link" href="functions.html#tutorial.functions.overloading" title="Overloading">on
overloading</a>, since the first 4 overload functins have a common sequence
of initial arguments, we can use <code class="literal">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</code>
to automatically wrap the first three of the <code class="literal">def</code>s and
manually wrap just the last. Here's how we'll do this:
</p>
<pre class="programlisting"><span class="identifier">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</span><span class="special">(</span><span class="identifier">xf_overloads</span><span class="special">,</span> <span class="identifier">f</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">4</span><span class="special">)</span>
</pre>
<p>
Create a member function pointers as above for both X::f overloads:
</p>
<pre class="programlisting"><span class="keyword">bool</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx1</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">char</span><span class="special">)</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
<span class="keyword">int</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx2</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
</pre>
<p>
Then...
</p>
<pre class="programlisting"><span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx1</span><span class="special">,</span> <span class="identifier">xf_overloads</span><span class="special">());</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx2</span><span class="special">)</span>
</pre>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005 Joel
de Guzman, David Abrahams<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>
</p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="exposing.html"><img src="../../images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="object.html"><img src="../../images/next.png" alt="Next"></a>
</div>
</body>
</html>
Reference in New Issue View Git Blame Copy Permalink