Question

Consider the following code:

#include <iostream>

struct A {
    const char *name;

    A() : name("A") {
        std::cout << "A()\n";
    }

    virtual ~A() {
        std::cout << "~A()\n";
    }
};

class B {
    const A& a;
public:

    B() : a(A()) {
    };

    void print_data() const {
        std::cout << a.name << '\n';
    }

    ~B() {
        std::cout << "~B()\n";
    }
};

int main() {
    B b;
    b.print_data();
    return 0;
}

The output from GCC 4.4 is:

A() ~A() A ~B()

This looks strange to me. I would have expected either a copy of the temporary instance of A being bound to the B::a or that temporary itself being destructed during ~B().

Basically, I thought B::a is always a valid reference during the life of b. In fact b.print_data() works apparently and the compiler does not give any warning.

What does the standard c++98/03 say about this matter?

Answer 1

Constant references don’t prolong the life of temporaries in classes, period. It’s just like that. They only do it in Foo const& f = Foo(); where foo returns by-value, but that’s it.

§12.2 [class.temporary]

p4 There are two contexts in which temporaries are destroyed at a different point than the end of the full expression. […]

p5 The second context is when a reference is bound to a temporary. The temporary to which the reference is bound or the temporary that is the complete object to a subobject of which the temporary is bound persists for the lifetime of the reference except as specified below. A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the constructor exits.