const参考变量的生命周期未延长

Question

Binding a temporary to a const reference extends its lifetime; cf. GotW #88 .

Why does not this work on this snippet? Live here .

#include <iostream>
#include <string>

struct A {
    A() : s("abc") {}
    const std::string& s;
};

struct B {
    const std::string& s = "def";
};

int main() {
    A a;
    std::cout << a.s << std::endl;
    B b;
    std::cout << b.s << std::endl;
}

Bonus question: How to trigger a warning with gcc?

Answer 1

In the article that you linked to, you will find:

(Note this only applies to stack-based references. It doesn't work for references that are members of objects.)

That's why the references in a and b are not valid. They don't extend the life of the temporaries.

Answer 2

From C++14 [class.temporary]/5:

The temporary to which the reference is bound or the temporary that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference except :

• A temporary bound to a reference member in a constructor's ctor-initializer persists until the constructor exits.

• [...]

Answer 3

cppreference.com says:

a temporary bound to a reference member in a constructor initializer list persists only until the constructor exits, not as long as the object exists.

http://en.cppreference.com/w/cpp/language/reference_initialization

Answer 4

The lifetime is only extended to the end of the compiler generator constructor for B . When the constructor returns, the temporary string created to hold "def" will be destroyed leaving you with a dangling reference.