CODEWITHSUNDEEP
c++parameter-passingmove-semantics
Antonio Santoro
Antonio Santoro

Reputation: 907

std::move argument passed by value

I need some help regarding this piece of code:

void foo(std::promise<std::string> p) {
    try {
        std::string result = "ok";
        p.set_value(std::move(result));
    } catch (...) {
        std::current_exception();
    }

}

int main() {
    std::promise<std::string> promise;
    std::future<std::string> f = promise.get_future();
    std::thread t(foo, std::move(promise));
    t.detach();

    std::string res = f.get();

    return 0;
}

What's the meaning of the std::move() usage? Is p passed by value (so a copy)?

Upvotes: 1

Views: 177

Answers (1)

rafix07
rafix07

Reputation: 20928

p is local variable inside foo. Before foo is invoked, p must be created. There are two possibilities to create it: by copy constructor or by move constructor. std::promise is movable class, it cannot be copied. So only way to create it is call promise(promise&&) - move constructor. By 

std::move(promise)

you are casting p to promise&&, then the compiler can select promise(promise&&) to move promise from main into p inside foo.

So no copy of promise is made here.

Upvotes: 4

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