Reference to an unnamed temporary object (life tim

问题:

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!

• How come this is possible?
• Is it specified in the C++ standard?
• Which version?

Source code:

#include <iostream>  //cout
#include <sstream>   //ostringstream 

int main ()
{
        std::ostringstream oss;
        oss << 1234;

        std::string const& str = oss.str();
        char        const* ptr = str.c_str();

        // Change the stream content
        oss << "_more_stuff_";
        oss.str(""); //reset
        oss << "Beginning";
        std::cout << oss.str() <<'\n';

        // Fill the call stack
        // ... create many local variables, call functions...

        // Change again the stream content
        oss << "Again";
        oss.str(""); //reset
        oss << "Next should be '1234': ";
        std::cout << oss.str() <<'\n';

        // Check if the ptr is still unchanged
        std::cout << ptr << std::endl;
}

Execution:

> g++ --version
g++ (GCC) 4.1.2 20080704 (Red Hat 4.1.2-54)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> g++ main.cpp -O3
> ./a.out
Beginning
Next should be '1234':
1234

回答1:

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.

---

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

Yes. All of them.

回答2:

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”

---

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.

回答3:

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;
}