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141 lines
4.5 KiB
Plaintext
141 lines
4.5 KiB
Plaintext
[/
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Copyright Oliver Kowalke 2014.
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt
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]
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[section:econtext Class execution_context]
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[important __econtext__ is supported only by C++11.]
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Class __econtext__ encapsulates __fcontext__ and related functions (
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__jump_fcontext__ and __make_fcontext__) as well as stack management.
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__econtext__ permits access to the current, active context via
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`execution_context::current()`.
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/*
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* grammar:
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* P ---> E '\0'
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* E ---> T {('+'|'-') T}
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* T ---> S {('*'|'/') S}
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* S ---> digit | '(' E ')'
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*/
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class Parser{
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// implementation omitted; see examples directory
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};
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int main() {
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std::istringstream is("1+1");
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bool done=false;
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char c;
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// create handle to main execution context
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boost::context::execution_context main_ctx(
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boost::context::execution_context::current() );
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// executes parser in new execution context
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boost::context::execution_context parser_ctx(
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boost::context::fixedsize(),
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[&is,&main_ctx,&done,&c](){
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// create parser with callback function
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// jumping back to main context
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Parser p(is,
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[&main_ctx,&c](char ch){
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c=ch;
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// resume main execution context
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main_ctx.jump_to();
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});
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// start recursive parsing
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p.run();
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done=true;
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});
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// user-code pulls parsed data from parser
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// inverted control flow
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while(!done){
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// resume parser execution context
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parser_ctx.jump_to();
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printf("Parsed: %c\n",c);
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}
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}
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output:
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Parsed: 1
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Parsed: +
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Parsed: 1
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In this example a recursive descent parser uses a callback to emit a newly passed
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symbol. Using __econtext__ the control flow can be inverted, e.g. the user-code
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pulls parsed symbols from the parser - instead to get pushed from the parser
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(via callback).
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The interface of __econtext__ does not transfer data. This is not required
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because usually sharing data's address (pointer/reference) sufficient.
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If the code executed by __econtext__ emits an exception, `std::terminate()` will
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be called.
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[important Do not jump from inside a catch block and than re-throw exceptions in
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another execution context.]
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Sometimes it is necessary to unwind the stack of an unfinished context to
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destroy local stack variables so they can release allocated resources (RAII
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pattern). The user is responsible for this task.
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[heading Class `execution_context`]
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class execution_context {
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public:
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static execution_context current() noexcept;
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template< typename StackAlloc, typename Fn >
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execution_context( StackAlloc salloc, Fn && fn);
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void jump_to() noexcept;
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explicit operator bool() const noexcept;
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bool operator!() const noexcept;
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};
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[heading `static execution_context current()`]
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[variablelist
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[[Returns:] [Returns an instance of excution_context pointing to the active
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execution context.]]
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[[Throws:] [Nothing.]]
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]
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[heading `template< typename StackAlloc, typname Fn > execution_context( StackAlloc salloc, Fn && fn)`]
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[variablelist
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[[Effects:] [Creates a new execution context and prepares the context to execute
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`fn`.]]
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]
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[heading `void jump_to()`]
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[variablelist
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[[Effects:] [Stores internally the current context data (stack pointer,
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instruction pointer, and CPU registers) to the current active context and
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restores the context data from `*this`, which implies jumping to `*this`'s
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execution context.]]
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[[Throws:] [Nothing.]]
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]
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[heading `explicit operator bool() const`]
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[variablelist
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[[Returns:] [If `*this` refers to an invalid context or the context-function
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has returned (completed), the function returns `false`. Otherwise `true`.]]
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[[Throws:] [Nothing.]]
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]
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[heading `bool operator!() const`]
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[variablelist
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[[Returns:] [If `*this` refers to an invalid context or the context-function
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has returned (completed), the function returns `true`. Otherwise `false`.]]
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[[Throws:] [Nothing.]]
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]
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[endsect]
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