Coroutines (C++20)

 

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A coroutine is a function that can suspend execution to be resumed later. Coroutines are stackless: they suspend execution by returning to the caller.

A function is a coroutine if its definition does any of the following:

• uses the keyword co_return for its return statement
• uses the keyword co_yield to suspend execution with a value

generator<int> gen(int next, int inc = 1) {
  for(;;) {
    co_yield next;
    next += inc;
  }
}

• uses the co_await operator or the co_await form of of range-for loop

task<int> f();
task<void> g1() {
  int i = co_await f();
  std::cout << "f() => " << i << '\n';
}
task<void> g2() {
  for co_await (auto&& n : gen()) 
    std::cout << "generator yielded " << n << '\n';
}

Restrictions

Coroutines cannot use variadic arguments, plain return statements, or placeholder return types (auto or Concept).

Constexpr functions, constructors, destructors, and the main function cannot be coroutines.