Reference to an unnamed temporary object (life time)

Question

After reading this answer from ildjarn, I wrote the following example, and it looks like an unnamed temporary object has the same life time as its reference!

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Answer 1

Lightness Races in Orbit is right. And I think this example would be more concise.

#include <iostream>  //cout
#include <string>

int main ()
{
    using namespace std;
    int a = 123;
    int b = 123;
//  int       & a_b = a + b; // error!
    int const & a_b = a + b;
    cout<<"hello world!"<<endl;
    cout<<a_b<<endl;
}

Answer 2

How come this is possible?

Because the standard says so, because it's deemed useful. rvalue references and const lvalue references extend the lifetime of temporaries:

[C++11: 12.2/5]: [..] 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 [..]

and exhaustive wording in [C++11: 8.5.3/5] requires that we shall not bind temporaries to non-const lvalue references.

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Is it specified in the C++ standard? Which version?

Yes. All of them.

Answer 3

A temporary bound to a const reference increases the lifetime of the temporary till the lifetime of the constant reference.

Good Read:

GotW #88: A Candidate For the “Most Important const”

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Yes it is specified in the C++ standard from the time references were introduced.
So if you are wondering if this is C++11 feature, no it is not. It already existed in C++03.