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c++constructordestructortemporary-objects

C++ destruction of temporary object in an expression

Given the following code:

#include <iostream>

struct implicit_t
{
    implicit_t(int x) :
        x_m(x)
    {
        std::cout << "ctor" << std::endl;
    }

    ~implicit_t()
    {
        std::cout << "dtor" << std::endl;
    }

    int x_m;
};

std::ostream& operator<<(std::ostream& s, const implicit_t& x)
{
    return s << x.x_m;
}

const implicit_t& f(const implicit_t& x)
{
    return x;
}

int main()
{
    std::cout << f(42) << std::endl;

    return 0;
}

I get the following output:

ctor
42
dtor

While I know this is correct, I'm not certain why. Is there anyone with stdc++ knowledge who can explain it to me?

Solution

  • Temporary objects are destroyed as the last step in evaluating the full-expression (1.9) that (lexically) contains the point where they were created. [12.2/3]