Boost C++ Libraries

Class execution_context encapsulates context switching and manages the associated context' stack (allocation/deallocation).

execution_context allocates the context stack (using its StackAllocator argument) and creates a control structure on top of it. This structure is responsible for managing context' stack. The address of the control structure is stored in the first frame of context' stack (e.g. it can not directly accessed from within execution_context). In contrast to execution_context (v1) the ownership of the control structure is not shared (no member variable to control structure in execution_context). execution_context keeps internally a state that is moved by a call of execution_context::operator() (*this will be invalidated), e.g. after a calling execution_context::operator(), *this can not be used for an additional context switch.

execution_context is only move-constructible and move-assignable.

The moved state is assigned to a new instance of execution_context. This object becomes the first argument of the context-function, if the context was resumed the first time, or the first element in a tuple returned by execution_context::operator() that has been called in the resumed context. In contrast to execution_context (v1), the context switch is faster because no global pointer etc. is involved.

On return the context-function of the current context has to specify an execution_context to which the execution control is transferred after termination of the current context.

If an instance with valid state goes out of scope and the context-function has not yet returned, the stack is traversed in order to access the control structure (address stored at the first stack frame) and context' stack is deallocated via the StackAllocator. The stack walking makes the destruction of execution_context slow and should be prevented if possible.

execution_context expects a context-function with signature execution_context(execution_context ctx, Args ... args). The parameter ctx represents the context from which this context was resumed (e.g. that has called execution_context::operator() on *this) and args are the data passed to execution_context::operator(). The return value represents the execution_context that has to be resumed, after termiantion of this context.

Benefits of execution_context (v2) over execution_context (v1) are: faster context switch, type-safety of passed/returned arguments.

usage of execution_context

int n=35;
ctx::execution_context<int> source(
    [n](ctx::execution_context<int> && sink,int) mutable {
        int a=0;
        int b=1;
        while(n-->0){
            auto result=sink(a);
            sink=std::move(std::get<0>(result));
            auto next=a+b;
            a=b;
            b=next;
        }
        return std::move(sink);
    });
for(int i=0;i<10;++i){
    auto result=source(i);
    source=std::move(std::get<0>(result));
    std::cout<<std::get<1>(result)<<" ";
}

output:
    0 1 1 2 3 5 8 13 21 34

This simple example demonstrates the basic usage of execution_context as a generator. The context sink represents the main-context (function main() running). sink is generated by the framework (first element of lambda's parameter list). Because the state is invalidated (== changed) by each call of execution_context::operator(), the new state of the execution_context, returned by execution_context::operator(), needs to be assigned to sink after each call.

The lambda that calculates the Fibonacci numbers is executed inside the context represented by source. Calculated Fibonacci numbers are transferred between the two context' via expression sink(a) (and returned by source()). Note that this example represents a generator thus the value transferred into the lambda via source() is not used. Using boost::optional<> as transferred type, might also appropriate to express this fact.

The locale variables a, b and next remain their values during each context switch (yield(a)). This is possible due source has its own stack and the stack is exchanged by each context switch.

parameter passing

With execution_context<void> no data will be transferred, only the context switch is executed.

boost::context::execution_context<void> ctx1([](boost::context::execution_context<void> && ctx2){
            std::printf("inside ctx1\n");
            return ctx2();
        });
ctx1();

output:
    inside ctx1

ctx1() resumes ctx1, e.g. the lambda passed at the constructor of ctx1 is entered. Argument ctx2 represents the context that has been suspended with the invocation of ctx1(). When the lambda returns ctx2, context ctx1 will be terminated while the context represented by ctx2 is resumed, hence the control of execution returns from ctx1().

The arguments passed to execution_context::operator(), in one context, is passed as the last arguments of the context-function if the context is started for the first time. In all following invocations of execution_context::operator() the arguments passed to execution_context::operator(), in one context, is returned by execution_context::operator() in the other context.

boost::context::execution_context<int> ctx1([](boost::context::execution_context<int> && ctx2,int j){
            std::printf("inside ctx1,j==%d\n",j);
            std::tie(ctx2,j)=ctx2(j+1);
            return std::move(ctx2);
        });
int i=1;
std::tie(ctx1,i)=ctx1(i);
std::printf("i==%d\n",i);

output:
    inside ctx1,j==1
    i==2

ctx1(i) enters the lambda in context ctx1 with argument j=1. The expression ctx2(j+1) resumes the context represented by ctx2 and transfers back an integer of j+1. On return of ctx1(i), the variable i contains the value of j+1.

If more than one argument has to be transferred, the signature of the context-function is simply extended.

boost::context::execution_context<int,int> ctx1([](boost::context::execution_context<int,int> && ctx2,int i,int j){
            std::printf("inside ctx1,i==%d,j==%d\n",i,j);
            std::tie(ctx2,i,j)=ctx2(i+j,i-j);
            return std::move(ctx2);
        });
int i=2,j=1;
std::tie(ctx1,i,j)=ctx1(i,j);
std::printf("i==%d,j==%d\n",i,j);

output:
    inside ctx1,i==2,j==1
    i==3,j==1

For use-cases, that require to transfer data of different type in each direction, boost::variant<> could be used.

class X{
private:
    std::exception_ptr excptr_;
    boost::context::execution_context<boost::variant<int,std::string>> ctx_;

public:
    X():
        excptr_(),
        ctx_([=](boost::context::execution_context<boost::variant<int,std::string>> && ctx,boost::variant<int,std::string> data){
                try {
                    for (;;) {
                        int i=boost::get<int>(data);
                        data=boost::lexical_cast<std::string>(i);
                        auto result=ctx(data);
                        ctx=std::move(std::get<0>(result));
                        data=std::get<1>(result);
                } catch (std::bad_cast const&) {
                    excptr_=std::current_exception();
                }
                return std::move(ctx);
             })
    {}

    std::string operator()(int i){
        boost::variant<int,std::string> data=i;
        auto result=ctx_(data);
        ctx_=std::move(std::get<0>(result));
        data=std::get<1>(result);
        if(excptr_){
            std::rethrow_exception(excptr_);
        }
        return boost::get<std::string>(data);
    }
};

X x;
std::cout << x(7) << std::endl;

output:
7

In the case of unidirectional transfer of data, boost::optional<> or a pointer are appropriate.

exception handling

If the function executed inside a execution_context emits an exception, the application is terminated by calling std::terminate(). std::exception_ptr can be used to transfer exceptions between different execution contexts.

Executing function on top of a context

Sometimes it is useful to execute a new function on top of a resumed context. For this purpose execution_context::operator() with first argument exec_ontop_arg has to be used. The function passed as argument must return a tuple of execution_context and arguments.

boost::context::execution_context<int> f1(boost::context::execution_context<int> && ctx,int data) {
    std::cout << "f1: entered first time: " << data << std::endl;
    std::tie(ctx,data)=ctx(data+1);
    std::cout << "f1: entered second time: " << data << std::endl;
    std::tie(ctx,data)=ctx(data+1);
    std::cout << "f1: entered third time: " << data << std::endl;
    return std::move(ctx);
}

int f2(int data) {
    std::cout << "f2: entered: " << data << std::endl;
    return -1;
}

int data=0;
boost::context::execution_context< int > ctx(f1);
std::tie(ctx,data)=ctx(data+1);
std::cout << "f1: returned first time: " << data << std::endl;
std::tie(ctx,data)=ctx(data+1);
std::cout << "f1: returned second time: " << data << std::endl;
std::tie(ctx,data)=ctx(ctx::exec_ontop_arg,f2,data+1);

output:
    f1: entered first time: 1
    f1: returned first time: 2
    f1: entered second time: 3
    f1: returned second time: 4
    f2: entered: 5
    f1: entered third time: -1

The expression ctx(ctx::exec_ontop_arg,f2,data+1) executes f2() on top of context ctx, e.g. an additional stack frame is allocated on top of the context stack (in front of f1()). f2() returns argument -1 that will returned by the second invocation of ctx(data+1) in f1().

Destructor

~execution_context();

Move constructor

execution_context( execution_context && other) noexcept;

Move assignment operator

execution_context & operator=( execution_context && other) noexcept;

Member function operator bool()

explicit operator bool() const noexcept;

Member function operator!()

bool operator!() const noexcept;

Member function operator()()

std::tuple< execution_context< Args ... >, Args ... > operator()( Args ... args); // member of generic execution_context template

execution_context< void > operator()(); // member of execution_context< void >

Member function operator()()

template< typename Fn >
std::tuple< execution_context< Args ... >, Args ... > operator()( exec_ontop_arg_t, Fn && fn, Args ... args); // member of generic execution_context

template< typename Fn >
execution_context< void > operator()( exec_ontop_arg_t, Fn && fn); // member of execution_context< void >

Member function operator==()

bool operator==( execution_context const& other) const noexcept;

Member function operator!=()

bool operator!=( execution_context const& other) const noexcept;

Member function operator<()

bool operator<( execution_context const& other) const noexcept;

Member function operator>()

bool operator>( execution_context const& other) const noexcept;

Member function operator<=()

bool operator<=( execution_context const& other) const noexcept;

Member function operator>=()

bool operator>=( execution_context const& other) const noexcept;

Non-member function operator<<()

template< typename charT, class traitsT >
std::basic_ostream< charT, traitsT > &
operator<<( std::basic_ostream< charT, traitsT > & os, execution_context const& other);