Question

Lets say I have two functions A and B.

where function A returns an object X and function B gets object X as an argument. For example.

X A() {
    X x;
    return x;
}

void B(X x) {
    write(x.data, x.size);
}

int main() { B(A()); }

Is this object X constructed only once as a temporary of B using RVO or do I need to use move semantics.

Answer 1

If copy elision is used, the X will only be constructed once.

Even if copy elision is disabled (e.g. passing the -fno-elide-constructors to gcc), the move constructors will be used automatically.

You can visualize it yourself:

#include <cstdio>

struct X
{
    X() { printf("default constructed\n"); }
    ~X() { printf("destructed\n"); }
    X(const X&) { printf("copy constructed\n"); }
    X(X&&) { printf("move constructed\n"); }
};

X A() {
    X x;
    return x;
}

void B(X x) {

}

int main() {
    B(A());
}

With RVO it prints

default constructed
destructed

With no RVO it prints

default constructed
move constructed
destructed
move constructed
destructed
destructed