Fix stream detection in op<<, op>>

.github/workflows/ci.yml

@@ -17,24 +17,13 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - toolset: gcc-4.7
-            cxxstd: "03,11"
-            os: ubuntu-16.04
-            install: g++-4.7
-          - toolset: gcc-4.8
-            cxxstd: "03,11"
-            os: ubuntu-16.04
-            install: g++-4.8
-          - toolset: gcc-4.9
-            cxxstd: "03,11"
-            os: ubuntu-16.04
-            install: g++-4.9
           - toolset: gcc-5
             cxxstd: "03,11,14,1z"
-            os: ubuntu-16.04
+            os: ubuntu-18.04
+            install: g++-5
           - toolset: gcc-6
             cxxstd: "03,11,14,1z"
-            os: ubuntu-16.04
+            os: ubuntu-18.04
             install: g++-6
           - toolset: gcc-7
             cxxstd: "03,11,14,17"
@@ -45,7 +34,7 @@ jobs:
             install: g++-8
           - toolset: gcc-9
             cxxstd: "03,11,14,17,2a"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
           - toolset: gcc-10
             cxxstd: "03,11,14,17,2a"
             os: ubuntu-18.04
@@ -53,40 +42,20 @@ jobs:
             cxxstd: "03,11,14,17,2a"
             os: ubuntu-18.04
             install: g++-11
-          - toolset: clang
-            compiler: clang++-3.5
-            cxxstd: "03,11"
-            os: ubuntu-16.04
-            install: clang-3.5
-          - toolset: clang
-            compiler: clang++-3.6
-            cxxstd: "03,11,14"
-            os: ubuntu-16.04
-            install: clang-3.6
-          - toolset: clang
-            compiler: clang++-3.7
-            cxxstd: "03,11,14"
-            os: ubuntu-16.04
-            install: clang-3.7
-          - toolset: clang
-            compiler: clang++-3.8
-            cxxstd: "03,11,14"
-            os: ubuntu-16.04
-            install: clang-3.8
           - toolset: clang
             compiler: clang++-3.9
             cxxstd: "03,11,14"
-            os: ubuntu-16.04
+            os: ubuntu-18.04
             install: clang-3.9
           - toolset: clang
             compiler: clang++-4.0
             cxxstd: "03,11,14"
-            os: ubuntu-16.04
+            os: ubuntu-18.04
             install: clang-4.0
           - toolset: clang
             compiler: clang++-5.0
             cxxstd: "03,11,14,1z"
-            os: ubuntu-16.04
+            os: ubuntu-18.04
             install: clang-5.0
           - toolset: clang
             compiler: clang++-6.0

README.md

@@ -5,15 +5,15 @@ described in [this post](https://pdimov.github.io/blog/2020/07/22/a-c14-lambda-l
 
 It has no dependencies and consists of a [single header](include/boost/lambda2/lambda2.hpp).
 
-See [the documentation](https://www.boost.org/doc/libs/develop/libs/lambda2/) for more information.
+See [the documentation](https://www.boost.org/libs/lambda2/) for more information.
 
 ## Supported Compilers
 
 * g++ 5 or later with `-std=c++14` or above
-* clang++ 3.5 or later with `-std=c++14` or above
+* clang++ 3.9 or later with `-std=c++14` or above
 * Visual Studio 2015, 2017, 2019
 
-Tested on [Travis](https://travis-ci.org/github/pdimov/lambda2/) and
+Tested on [Github Actions](https://github.com/boostorg/lambda2/actions) and
 [Appveyor](https://ci.appveyor.com/project/pdimov/lambda2).
 
 ## License

doc/.gitignore

@@ -1 +1,2 @@
 /pdf/
+/html/

doc/lambda2.adoc

@@ -14,6 +14,7 @@ Peter Dimov
 :leveloffset: +1
 
 include::lambda2/overview.adoc[]
+include::lambda2/changelog.adoc[]
 include::lambda2/reference.adoc[]
 include::lambda2/copyright.adoc[]
 

doc/lambda2/changelog.adoc

@@ -0,0 +1,14 @@
+////
+Copyright 2021 Peter Dimov
+Distributed under the Boost Software License, Version 1.0.
+https://www.boost.org/LICENSE_1_0.txt
+////
+
+[#changelog]
+# Revision History
+:idprefix:
+
+## Changes in 1.78.0
+
+* Added special cases in `operator<<` and `operator>>` when
+  the first argument is a stream, to allow `std::cout << _1`.

doc/lambda2/copyright.adoc

@@ -8,5 +8,5 @@ https://www.boost.org/LICENSE_1_0.txt
 # Copyright and License
 :idprefix:
 
-This documentation is copyright 2020 Peter Dimov and is distributed under
+This documentation is copyright 2020, 2021 Peter Dimov and is distributed under
 the http://www.boost.org/LICENSE_1_0.txt[Boost Software License, Version 1.0].

doc/lambda2/overview.adoc

@@ -1,5 +1,5 @@
 ////
-Copyright 2020 Peter Dimov
+Copyright 2020, 2021 Peter Dimov
 Distributed under the Boost Software License, Version 1.0.
 https://www.boost.org/LICENSE_1_0.txt
 ////
@@ -12,16 +12,18 @@ https://www.boost.org/LICENSE_1_0.txt
 
 This is a simple, but functional, {cpp}14 lambda library. It takes
 advantage of the fact that the standard `<functional>` header already
-provides placeholders `_1`, `_2`, `_3`, and so on, for use with
-`std::bind`, and function objects such as `std::plus`, `std::greater`,
-`std::logical_not`, and `std::bit_xor`, corresponding to arithmetic,
-relational, logical and bitwise operators.
+provides `std::bind` customization points (`is_placeholder`,
+`is_bind_expression`), and function objects such as `std::plus`,
+`std::greater`, `std::logical_not`, and `std::bit_xor`, corresponding
+to arithmetic, relational, logical and bitwise operators.
 
 This allows the library to provide a minimal implementation that
 still lets expressions such as `_1 + 5`, `_1 % 2 == 0`, `_1 > _2`,
 or `_1 == ' ' || _1 == '\t'` to be composed and used as function
 objects.
 
+For example, `_1 + 5` is implemented as `std::bind(std::plus<>, _1, 5)`.
+
 These "lambda" expressions can also be freely combined with `std::bind`.
 For example, `std::bind( f, _1 ) == std::bind( g, _1 )` and
 `std::bind( f, _1 + _2 )` both work and have the expected behavior.
@@ -64,8 +66,8 @@ None. A single, self-contained header.
 ## Supported Compilers
 
 * GCC 5 or later with `-std=c++14` or above
-* Clang 3.5 or later with `-std=c++14` or above
+* Clang 3.9 or later with `-std=c++14` or above
 * Visual Studio 2015, 2017, 2019
 
-Tested on https://travis-ci.org/github/pdimov/lambda2[Travis] and
+Tested on https://github.com/boostorg/lambda2/actions[Github Actions] and
 https://ci.appveyor.com/project/pdimov/lambda2[Appveyor].

doc/lambda2/reference.adoc

@@ -1,5 +1,5 @@
 ////
-Copyright 2020 Peter Dimov
+Copyright 2020, 2021 Peter Dimov
 Distributed under the Boost Software License, Version 1.0.
 https://www.boost.org/LICENSE_1_0.txt
 ////
@@ -16,6 +16,22 @@ https://www.boost.org/LICENSE_1_0.txt
 namespace boost {
 namespace lambda2 {
 
+// placeholders
+
+template<int I> struct lambda2_arg;
+
+inline constexpr lambda2_arg<1> _1{};
+inline constexpr lambda2_arg<1> _2{};
+inline constexpr lambda2_arg<1> _3{};
+inline constexpr lambda2_arg<1> _4{};
+inline constexpr lambda2_arg<1> _5{};
+inline constexpr lambda2_arg<1> _6{};
+inline constexpr lambda2_arg<1> _7{};
+inline constexpr lambda2_arg<1> _8{};
+inline constexpr lambda2_arg<1> _9{};
+
+// arithmetic operators
+
 template<class A, class B> auto operator+( A && a, B && b );
 template<class A, class B> auto operator-( A && a, B && b );
 template<class A, class B> auto operator*( A && a, B && b );
@@ -23,6 +39,8 @@ template<class A, class B> auto operator/( A && a, B && b );
 template<class A, class B> auto operator%( A && a, B && b );
 template<class A> auto operator-( A && a );
 
+// relational operators
+
 template<class A, class B> auto operator==( A && a, B && b );
 template<class A, class B> auto operator!=( A && a, B && b );
 template<class A, class B> auto operator>( A && a, B && b );
@@ -30,19 +48,84 @@ template<class A, class B> auto operator<( A && a, B && b );
 template<class A, class B> auto operator>=( A && a, B && b );
 template<class A, class B> auto operator<=( A && a, B && b );
 
+// logical operators
+
 template<class A, class B> auto operator&&( A && a, B && b );
 template<class A, class B> auto operator||( A && a, B && b );
 template<class A> auto operator!( A && a );
 
+// bitwise operators
+
 template<class A, class B> auto operator&( A && a, B && b );
 template<class A, class B> auto operator|( A && a, B && b );
 template<class A, class B> auto operator^( A && a, B && b );
 template<class A> auto operator~( A && a );
 
+template<class A, class B> auto operator<<( A && a, B && b );
+template<class A, class B> auto operator>>( A && a, B && b );
+
+// additional unary operators
+
+template<class A> auto operator+( A && a );
+template<class A> auto operator*( A && a );
+template<class A> auto operator++( A && a );
+template<class A> auto operator--( A && a );
+template<class A> auto operator++( A && a, int );
+template<class A> auto operator--( A && a, int );
+
+// compound assignment operators
+
+template<class A, class B> auto operator+=( A && a, B && b );
+template<class A, class B> auto operator-=( A && a, B && b );
+template<class A, class B> auto operator*=( A && a, B && b );
+template<class A, class B> auto operator/=( A && a, B && b );
+template<class A, class B> auto operator%=( A && a, B && b );
+template<class A, class B> auto operator&=( A && a, B && b );
+template<class A, class B> auto operator|=( A && a, B && b );
+template<class A, class B> auto operator^=( A && a, B && b );
+template<class A, class B> auto operator<<=( A && a, B && b );
+template<class A, class B> auto operator>>=( A && a, B && b );
+
 } // namespace lambda2
 } // namespace boost
 ```
 
+### Placeholders
+
+```
+template<int I> struct lambda2_arg
+{
+    template<class... A> decltype(auto) operator()( A&&... a ) const noexcept;
+    template<class T> auto operator[]( T&& t ) const;
+};
+```
+
+`lambda2_arg<I>` is the type of the library-provided placeholders `_I`. The
+standard customization point `std::is_placeholder` is specialized for it,
+enabling the use of Lambda2's placeholders with `std::bind`.
+
+The placeholders define `operator()`, which permits their direct use as
+function objects. E.g. `_1(x, y)` returns `x`.
+
+`operator[]` is also defined to allow expressions like `_1[x]` or `_1[_2]`.
+
+```
+template<class... A> decltype(auto) operator()( A&&... a ) const noexcept;
+```
+[none]
+* {blank}
++
+Returns: :: `std::get<std::size_t{I-1}>( std::tuple<A&&...>( std::forward<A>(a)... ) );`
+
+```
+template<class T> auto operator[]( T&& t ) const;
+```
+[none]
+* {blank}
++
+Returns: :: `std::bind( fn, *this, std::forward<T>(t) );`, where `fn` is
+  a function object such that `fn(x, y)` returns `x[y]`.
+
 ### Common Requirements
 
 All operators defined in the subsequent sections only participate in
@@ -214,3 +297,94 @@ template<class A> auto operator~( A && a );
 +
 Returns: ::
   `std::bind( std::bit_not<>(), std::forward<A>(a) );`
+
+```
+template<class A, class B> auto operator<<( A && a, B && b );
+```
+[none]
+* {blank}
++
+Returns: :: `std::bind( fn, std::forward<A>(a), std::forward<B>(b) );`,
+  where `fn` is a function object such that `fn(x, y)` returns `x << y`.
+
+```
+template<class A, class B> auto operator>>( A && a, B && b );
+```
+[none]
+* {blank}
++
+Returns: :: `std::bind( fn, std::forward<A>(a), std::forward<B>(b) );`,
+  where `fn` is a function object such that `fn(x, y)` returns `x >> y`.
+
+### Additional Unary Operators
+
+```
+template<class A> auto operator+( A && a );
+```
+[none]
+* {blank}
++
+Returns: ::
+  `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object
+  such that `fn(x)` returns `+x`.
+
+```
+template<class A> auto operator*( A && a );
+```
+[none]
+* {blank}
++
+Returns: ::
+  `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object
+  such that `fn(x)` returns `*x`.
+
+```
+template<class A> auto operator++( A && a );
+```
+[none]
+* {blank}
++
+Returns: ::
+  `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object
+  such that `fn(x)` returns `++x`.
+
+```
+template<class A> auto operator--( A && a );
+```
+[none]
+* {blank}
++
+Returns: ::
+  `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object
+  such that `fn(x)` returns `--x`.
+
+```
+template<class A> auto operator++( A && a, int );
+```
+[none]
+* {blank}
++
+Returns: ::
+  `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object
+  such that `fn(x)` returns `x++`.
+
+```
+template<class A> auto operator--( A && a, int );
+```
+[none]
+* {blank}
++
+Returns: ::
+  `std::bind( fn, std::forward<A>(a) );`, where `fn` is a function object
+  such that `fn(x)` returns `x--`.
+
+### Compound Assignment Operators
+
+```
+template<class A, class B> auto operator@=( A && a, B && b );
+```
+[none]
+* {blank}
++
+Returns: :: `std::bind( fn, std::forward<A>(a), std::forward<B>(b) );`,
+  where `fn` is a function object such that `fn(x, y)` returns `x @= y`.

include/boost/lambda2/lambda2.hpp

@@ -1,7 +1,7 @@
 #ifndef BOOST_LAMBDA2_LAMBDA2_HPP_INCLUDED
 #define BOOST_LAMBDA2_LAMBDA2_HPP_INCLUDED
 
-// Copyright 2020 Peter Dimov
+// Copyright 2020, 2021 Peter Dimov
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
@@ -10,10 +10,11 @@
 #include <utility>
 #include <tuple>
 #include <cstddef>
+#include <iosfwd>
 
 // Same format as BOOST_VERSION:
 //   major * 100000 + minor * 100 + patch
-#define BOOST_LAMBDA2_VERSION 107700
+#define BOOST_LAMBDA2_VERSION 107800
 
 namespace boost
 {
@@ -151,6 +152,8 @@ template<class A> using enable_unary_lambda =
 template<class A, class B> using enable_binary_lambda =
     std::enable_if_t<is_lambda_expression<A>::value || is_lambda_expression<B>::value>;
 
+template<class T> using is_stream = std::is_base_of<std::ios_base, remove_cvref_t<T>>;
+
 } // namespace lambda2_detail
 
 #define BOOST_LAMBDA2_UNARY_LAMBDA(op, fn) \
@@ -201,9 +204,6 @@ BOOST_LAMBDA2_UNARY_LAMBDA(~, std::bit_not<>)
 
 // additional
 
-BOOST_LAMBDA2_BINARY_LAMBDA(<<, lambda2_detail::left_shift)
-BOOST_LAMBDA2_BINARY_LAMBDA(>>, lambda2_detail::right_shift)
-
 BOOST_LAMBDA2_UNARY_LAMBDA(+, lambda2_detail::unary_plus)
 BOOST_LAMBDA2_UNARY_LAMBDA(*, lambda2_detail::dereference)
 
@@ -226,6 +226,38 @@ BOOST_LAMBDA2_BINARY_LAMBDA(^=, lambda2_detail::bit_xor_equal)
 BOOST_LAMBDA2_BINARY_LAMBDA(<<=, lambda2_detail::left_shift_equal)
 BOOST_LAMBDA2_BINARY_LAMBDA(>>=, lambda2_detail::right_shift_equal)
 
+// operator<<
+
+template<class A, class = std::enable_if_t<!lambda2_detail::is_stream<A>::value>,
+    class B, class = lambda2_detail::enable_binary_lambda<A, B>>
+auto operator<<( A&& a, B&& b )
+{
+    return std::bind( lambda2_detail::left_shift(), std::forward<A>(a), std::forward<B>(b) ); \
+}
+
+template<class A, class = std::enable_if_t<lambda2_detail::is_stream<A>::value>,
+    class B, class = lambda2_detail::enable_unary_lambda<B>>
+auto operator<<( A& a, B&& b )
+{
+    return std::bind( lambda2_detail::left_shift(), std::ref(a), std::forward<B>(b) );
+}
+
+// operator>>
+
+template<class A, class = std::enable_if_t<!lambda2_detail::is_stream<A>::value>,
+    class B, class = lambda2_detail::enable_binary_lambda<A, B>>
+auto operator>>( A&& a, B&& b )
+{
+    return std::bind( lambda2_detail::right_shift(), std::forward<A>(a), std::forward<B>(b) ); \
+}
+
+template<class A, class = std::enable_if_t<lambda2_detail::is_stream<A>::value>,
+    class B, class = lambda2_detail::enable_unary_lambda<B>>
+auto operator>>( A& a, B&& b )
+{
+    return std::bind( lambda2_detail::right_shift(), std::ref(a), std::forward<B>(b) );
+}
+
 } // namespace lambda2
 } // namespace boost
 

test/Jamfile

@@ -24,3 +24,5 @@ run placeholders.cpp ;
 run increment.cpp ;
 run subscript.cpp ;
 run compound.cpp ;
+run stream_insert.cpp ;
+run stream_extract.cpp ;

test/stream_extract.cpp

@@ -0,0 +1,42 @@
+// Copyright 2021 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/lambda2.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <sstream>
+#include <string>
+
+template<class T> T from_string_1( std::string const& s )
+{
+    using namespace boost::lambda2;
+
+    std::istringstream is( s );
+
+    T t{};
+
+    ( std::ref( is ) >> _1 )( t );
+
+    return t;
+}
+
+template<class T> T from_string_2( std::string const& s )
+{
+    using namespace boost::lambda2;
+
+    std::istringstream is( s );
+
+    T t{};
+
+    ( is >> _1 )( t );
+
+    return t;
+}
+
+int main()
+{
+    BOOST_TEST_EQ( from_string_1<int>( "123" ), 123 );
+    BOOST_TEST_EQ( from_string_2<int>( "456" ), 456 );
+
+    return boost::report_errors();
+}

test/stream_insert.cpp

@@ -0,0 +1,38 @@
+// Copyright 2021 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/lambda2.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <sstream>
+#include <string>
+
+template<class T> std::string to_string_1( T const& t )
+{
+    using namespace boost::lambda2;
+
+    std::ostringstream os;
+
+    ( std::ref( os ) << _1 )( t );
+
+    return os.str();
+}
+
+template<class T> std::string to_string_2( T const& t )
+{
+    using namespace boost::lambda2;
+
+    std::ostringstream os;
+
+    ( os << _1 )( t );
+
+    return os.str();
+}
+
+int main()
+{
+    BOOST_TEST_EQ( to_string_1( 123 ), "123" );
+    BOOST_TEST_EQ( to_string_2( 456 ), "456" );
+
+    return boost::report_errors();
+}