Python/C# style yield statements in C++

This library allows the user to create generators- functions that can be paused and resumed, yielding values to the caller. These functions maintain their stacks between yields.

Generator objects are templated on the type of object they yield. To create one, create an object of the type, and pass a callable object that takes a Yield object as its argument:

// A generator that yields 1,2,3,4
Generator<int> generator(
[](Yield<int> yield)
{
	yield(1); yield(2); yield(3); yield(4);
});

When a Generator is first constructed, it immediately launches. The function is executed in its own stack. When it calls yield(value), execution of the of the function is suspended, and a pointer to the value is passed to the calling context. The calling context, in turn, can call the generator.get() to get this pointer. The caller can then call generator.advance() to resume the generator function. This causes the function's stack to be restored, and for execution to continue from the site the most recent yield until the next one.

If the function returns, the generator becomes stopped. All the allocated resources are reclaimed immediately, and execution returns to the last time the generator was resumed (either construction or a call to advance). Once the generator is stopped, it cannot be restarted; get() will always return nullptr, advance becomes a no-op, and stopped() returns true.

#include <iostream>
#include "Generator.hpp"

namespace gen = generator;
using namespace std;

int main()
{
	cout << "[main] creating generator\n";

	gen::Generator<int> generator(
	[](gen::Yield<int> yield)
	{
		cout << "[generator] generator started\n";
		for(int i = 0; i < 5; ++i)
		{
			cout << "[generator] local i: " << i << "\n";
			cout << "[generator] yielding i\n";
			yield(i);
		}
		cout << "[generator] exiting generator\n";
	});

	cout << "[main] beginning for() loop\n";

	for(int i : generator)
	{
	    cout << "[main] got " << i << " from generator\n";
	}

	cout << "[main] for() loop complete\n";
	cout << "[main] exiting\n";
}

[main] creating generator
[generator] generator started
[generator] local i: 0
[generator] yielding i
[main] beginning for() loop
[main] got 0 from generator
[generator] local i: 1
[generator] yielding i
[main] got 1 from generator
[generator] local i: 2
[generator] yielding i
[main] got 2 from generator
[generator] local i: 3
[generator] yielding i
[main] got 3 from generator
[generator] local i: 4
[generator] yielding i
[main] got 4 from generator
[generator] exiting generator
[main] for() loop complete
[main] exiting

This library is header-only, and all contained in a single file. However, it depends on boost (specifically boost::context and boost::iterator_facade). boost::context in particular needs to be build and linked against in order to use the generator; see the boost docs at https://www.boost.org/doc/libs/1_55_0/more/getting_started/index.html to learn how to build this library on your system.

A Generator can be constructed with any callable object or function pointer with the correct signature- /* ignored return type */ function(Yield<YieldType>). The template parameter of the Yield argument must be the same as that of the Generator object; doing otherwise is a compiler error. This callable can be passed as an lvalue or an rvalue. If it is passed as an lvalue, a reference is kept to the original object and used- this means that any changes to the object in the generator's context are reflected in the original object. If it is passed as an rvalue, a new instance is move-constructed in the generator's local stack and destroyed when the generator ends.

A Generator can also optionally receive a size parameter as a second argument; this is the requested size in bytes of the stack that will be allocated for the general. The actual size may be adjusted to match your OS page layout or other considerations, but it will always be at least that many bytes; you can call stack_size() to get the actual allocated size.

The generator is immediately launched when first constructed. Execution is paused once the first yield is called; if no yield is called the generator becomes stopped immediately.

Generators can also be move-constructed from other generators. In this case, the new generator takes ownership of the internal context of the old generator, and the old generator becomes stopped, if it wasn't already. Unlike normal construction, move-constructing a generator does not resume or launch the internal context.

Generators cannot be copied. They currently cannot be move-assigned, though this will change in a future release.

#include <iostream>
#include "Generator.hpp"

using namespace generator;
using namespace std;

class FuncObj
{
public:
    int x;

    void operator() (Yield<int> yield)
    {
        cout << "[FuncObj] x is " << x << "\n";
        cout << "[FuncObj] setting x to 10\n";
        x = 10;
        cout << "[FuncObj] yielding\n";
        yield();
        cout << "[FuncObj] x is " << x << "\n";
    }

    FuncObj(int i): x(i) {}

    FuncObj(const FuncObj& cpy): x(cpy.x)
    { cout << "FuncObj copied\n"; }

    FuncObj(FuncObj&& mve): x(mve.x)
    { cout << "FuncObj moved\n"; }

    ~FuncObj()
    { cout << "FuncObj destructed\n"; }
};

int main()
{
    //lvalue
    cout << "[main] creating FuncObj, with value 20\n";
    FuncObj func(20);

    cout << "[main] creating gen1 with lvalue\n";
    Generator<int> gen1(func);

    cout << "[main] func.x: " << func.x << "\n";

    cout << "[main] setting func.x to 15\n";
    func.x = 15;

    cout << "[main] advancing gen1\n";
    gen1.advance();

    //rvalue
    cout << "[main] creating gen2 with rvalue, initial value 30\n";
    Generator<int> gen2(FuncObj(30));

    cout << "[main] advancing gen2\n";
    gen2.advance();
}

[main] creating gen1 with rvalue, initial value 30
FuncObj moved
[FuncObj] x is 30
[FuncObj] setting x to 10
[FuncObj] yielding
FuncObj destructed
[main] advancing gen1
[FuncObj] x is 10
FuncObj destructed
[main] creating FuncObj, with value 20
[main] creating gen2 with lvalue
[FuncObj] x is 20
[FuncObj] setting x to 10
[FuncObj] yielding
[main] func.x: 10
[main] setting func.x to 15
[main] advancing gen2
[FuncObj] x is 15
FuncObj destructed

• 3 FuncObjs are destroyed. The first is the temporary passed into the Generator, which isn't destroyed until control returns to the constructor. The second is the local copy created inside of gen1, which is destroyed when the generator becomes stopped. The last is the local one in main, which is destroyed when main returns.

• Notice that no copies or moves were made of the FuncObj passed to gen2.

The generator function receives a single Yield object as its argument. This object is used to yield values to the calling context; for this section we will assume the object is called yield.

The generator yields a value by calling yield(object). Objects are yielded by reference- the calling context calls get() to get a pointer to this object. Any changes to the object are reflected when the generator resumes. Both lvalues and rvalues may be yielded. The generator function can also call yield() with no argument; in this case, calls to get() will return nullptr.

If the generator function returns, the generator stops. Execution returns to the last time the generator was resumed (either the constructor or a call to advance), and the stack memory is recovered. A stopped generator cannot be restarted or otherwise be made to have a new context, though this may change if move-assignment is introduced.

A generator function may also stop by calling yield.done() or yield.exit(). yield.exit() will cause the context to be exited and the stack destroyed immediately, without calling any local destructors, similar to a call to std::exit(). yield.done() performs a clean exit by throwing an exception of an internal type and catching it below the generator function itself; this allows proper stack unwinding to happen and destructors to be called.

If an exception is allowed to leave the generator function without being caught (besides the internal exception type used by yield.done() and stop(), it is undefined behavior. In my limited testing it is treated the same as an exception leaving main- terminate called, etc.

Once the generator object is created and control is returned to the calling context, the generator can be used. The caller can use the get() function to get a pointer to the most recently yielded object, and the advance() function to advance the generator to the next yield. The convenience function next() is also provided, which advances the generator then returns the pointer to the new value.

The pointers returned by get() point to the yielded values, so the calling can context can communicate with the generator by manipulating. Additionally, if a function object was passed by lvalue reference into the generator, that objects members are available to both the caller and the generator function.

The generator can be iterated over, and has the standard functions begin() and end() to get iterators. The iterators are standard InputIterators, but also have some features of OutputIterators. Dereferencing them gets a reference to the yielded value, and incrementing them advances the generator, invalidating old iterators. An iterator will compare equal to the end() iterator when the generator becomes stopped. Postfix-incrementing the iterator creates a copy of the yielded object and stores it in a proxy before advancing the generator. This is why these iterator do not model standard OutputIterators; they do not have postfix-increment-assignment (*it++ = x; will not assign x to the yielded value in the generator). If the generator yields forever, iteration will never terminate. Yielding nothing to an iterator is undefined behavior, as it is the same as dereferencing a nullptr.

The calling context can call stack_size() to get the size in bytes of the stack allocated to the generator. It can call stopped() to test if the generator is stopped.

The caller has many options for prematurely stopping a generator. The normal way is to call stop(). This will cause the generator to attempt to cleanly exit- as with yield.done(), an exception of internal type is thrown at the last yield site, allowing destructors to be called and the stack to be unwound. The exception is caught when it leaves the generator function, and the generator's resources are freed. If, for whatever reason, this exception is NOT allowed to propogate out of the generator function, the context it still cleared the next time control returns to the calling context.

The caller can choose instead to call kill(). This wipes out the context without performing any cleanup and should be used sparingly.

If the generator is destructed before it is stopped, it calls stop().