tmp/tmps9mc_p1z/{from.md → to.md}
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| 1 |
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### Yielding a value <a id="expr.yield">[[expr.yield]]</a>
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``` bnf
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yield-expression:
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'co_yield' assignment-expression
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'co_yield' braced-init-list
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```
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A *yield-expression* shall appear only within a suspension context of a
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function [[expr.await]]. Let *e* be the operand of the
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*yield-expression* and *p* be an lvalue naming the promise object of the
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enclosing coroutine [[dcl.fct.def.coroutine]], then the
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*yield-expression* is equivalent to the expression
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`co_await `*p*`.yield_value(`*e*`)`.
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[*Example 1*:
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``` cpp
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template <typename T>
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struct my_generator {
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struct promise_type {
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T current_value;
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...
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auto yield_value(T v) {
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current_value = std::move(v);
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return std::suspend_always{};
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}
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};
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struct iterator { ... };
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iterator begin();
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iterator end();
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};
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my_generator<pair<int,int>> g1() {
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for (int i = i; i < 10; ++i) co_yield {i,i};
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}
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my_generator<pair<int,int>> g2() {
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for (int i = i; i < 10; ++i) co_yield make_pair(i,i);
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}
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auto f(int x = co_yield 5); // error: yield-expression outside of function suspension context
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int a[] = { co_yield 1 }; // error: yield-expression outside of function suspension context
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int main() {
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auto r1 = g1();
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auto r2 = g2();
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assert(std::equal(r1.begin(), r1.end(), r2.begin(), r2.end()));
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}
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```
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— *end example*]
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