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## Class execution_context

| Important
execution_context is supported only by C++14.

Class execution_context encapsulates fcontext_t and related functions ( jump_fcontext() and make_fcontext()) as well as stack management. execution_context permits access to the current, active context via `execution_context::current()`.

    /*
     * grammar:
     *   P ---> E '\0'
     *   E ---> T {('+'|'-') T}
     *   T ---> S {('*'|'/') S}
     *   S ---> digit | '(' E ')'
     */
    class Parser{
        // implementation omitted; see examples directory
    };

    int main() {
        std::istringstream is("1+1");
        bool done=false;
        char c;

        // create handle to main execution context
        boost::context::execution_context main_ctx(
            boost::context::execution_context::current() );

        // executes parser in new execution context
        boost::context::execution_context parser_ctx(
            boost::context::fixedsize_stack(),
            [&main_ctx,&is,&c,&done](){
                // create parser with callback function
                Parser p(is,
                         [&main_ctx,&c](char ch){
                             c=ch;
                             // resume main execution context
                             main_ctx.resume();
                         });
                // start recursive parsing
                p.run();
                done=true;
                // return to main execution context
                main_ctx.resume();
            });

        // user-code pulls parsed data from parser
        // inverted control flow
        parser_ctx.resume();
        do {
            printf("Parsed: %c\n",c);
            parser_ctx.resume();
        } while( ! done);
    }

    output:
        Parsed: 1
        Parsed: +
        Parsed: 1

In this example a recursive descent parser uses a callback to emit a newly passed symbol. Using execution_context the control flow can be inverted, e.g. the user-code pulls parsed symbols from the parser - instead to get pushed from the parser (via callback).

The interface of execution_context does not transfer data. This is not required because usually sharing data's address (pointer/reference) sufficient.

If the code executed by execution_context emits an exception, `std::terminate()` will be called.

| Important
Do not jump from inside a catch block and than re-throw exceptions in another execution context.

Sometimes it is necessary to unwind the stack of an unfinished context to destroy local stack variables so they can release allocated resources (RAII pattern). The user is responsible for this task.

#### allocating control strutures on top of stack

Allocating control structures on top of the stack requires to allocated the stack_context and create the control structure with placement new before execution_context is created.

| Note
The user is responsible for destructing the control structure at the top of the stack.

    // stack-alloctor used for (de-)allocating stack
    fixedsize_stack salloc( 4048);
    // allocate stack space
    stack_context sctx( salloc.allocate() );
    // reserve space for control structure on top of the stack
    void * sp = static_cast< char * >( sctx.sp) - sizeof( my_control_structure);
    std::size_t size = sctx.size - sizeof( my_control_structure);
    // placement new creates control structure on reserved space
    my_control_structure * cs = new ( sp) my_control_structure( sp, size, sctx, salloc);
    ...
    // destructing the control structure
    cs->~my_control_structure();
    ...
    struct my_control_structure  {
        // execution context
        execution_context ectx;

        template< typename StackAllocator >
        my_control_structure( void * sp, std::size_t size, stack_context sctx, StackAllocator salloc) :
            // create execution context
            ectx( preallocated( sp, size, sctx), salloc, entry_func) {
        }
        ...
    };

#### exception handling

If the function executed inside a execution_context emitts ans exception, `std::terminate()` is called - `std::exception_ptr` can used to store exceptions thrown inside the other context.

#### parameter passing

Input and output parameters are transfered via a lambda capture list and references/pointers.

    class X {
    private:
        int * inp_;
        std::string outp_;
        std::exception_ptr excptr_;
        boost::context::execution_context caller_;
        boost::context::execution_context callee_;

    public:
        X() :
            inp_( nullptr),
            outp_(),
            excptr_(),
            caller_( boost::context::execution_context::current() ),
            callee_( boost::context::fixedsize_stack(),
                     [=] () {
                        try {
                            int i = * inp_;
                            outp_ = boost::lexical_cast< std::string >( i);
                            caller_.resume();
                        } catch (...) {
                            excptr_ = std::current_exception();
                        }
                     })
        {}

        std::string operator()( int i) {
            inp_ = & i;
            callee_.resume();
            if ( excptr_) {
                std::rethrow_exception( excptr_);
            }
            return outp_;
        }
    };

    int main() {
        X x;
        std::cout << x( 7) << std::endl;
        std::cout << "done" << std::endl;
    }

#### Class `execution_context`

    class execution_context {
    public:
        static execution_context current() noexcept;

        template< typename StackAlloc, typename Fn >
        execution_context( StackAlloc salloc, Fn && fn);

        template< typename StackAlloc, typename Fn, typename ... Args >
        execution_context( StackAlloc salloc, Fn && fn, Args && ... args);

        template< typename StackAlloc, typename Fn >
        execution_context( preallocated palloc, StackAlloc salloc, Fn && fn);

        template< typename StackAlloc, typename Fn, typename ... Args >
        execution_context( preallocated palloc, StackAlloc salloc, Fn && fn, Args && ... args);

        void resume() noexcept;

        explicit operator bool() const noexcept;

        bool operator!() const noexcept;
    };

#### `static execution_context current()`

Returns:

  Returns an instance of excution_context pointing to the active execution context.

Throws:

  Nothing.

#### `template< typename StackAlloc, typname Fn > execution_context( StackAlloc salloc, Fn && fn)`

Effects:

  Creates a new execution context and prepares the context to execute `fn`.

#### `template< typename StackAlloc, typname Fn, typename ... Args > execution_context( StackAlloc salloc, Fn && fn, Args && ... args)`

Effects:

  Creates a new execution context and prepares the context to execute `fn`.

#### `template< typename StackAlloc, typname Fn > execution_context( preallocated palloc, StackAlloc salloc, Fn && fn)`

Effects:

  Creates a new execution context and prepares the context to execute `fn`. Used to store control structures on top of the stack.

#### `template< typename StackAlloc, typname Fn, typename ... Args > execution_context( preallocated palloc, StackAlloc salloc, Fn && fn, Args && ... args)`

Effects:

  Creates a new execution context and prepares the context to execute `fn`. Used to store control structures on top of the stack.

#### `void resume()`

Effects:

  Stores internally the current context data (stack pointer, instruction pointer, and CPU registers) to the current active context and restores the context data from `*this`, which implies jumping to `*this`'s execution context.

Note:

  The behaviour is undefined if `resume()` is called while `execution_context::current()` returns `*this` (e.g. resuming an alredy running cotnext).

Throws:

  Nothing.

#### `explicit operator bool() const`

Returns:

  If `*this` refers to an invalid context or the context-function has returned (completed), the function returns `false`. Otherwise `true`.

Throws:

  Nothing.

#### `bool operator!() const`

Returns:

  If `*this` refers to an invalid context or the context-function has returned (completed), the function returns `true`. Otherwise `false`.

Throws:

  Nothing.

#### Struct `preallocated`

    struct preallocated {
        void        *   sp;
        std::size_t     size;
        stack_context   sctx;

        preallocated( void * sp, std:size_t size, stack_allocator sctx) noexcept;
    };

#### `preallocated( void * sp, std:size_t size, stack_allocator sctx)`

Effects:

  Crreates an object of preallocated.