Re: C++0x: returning rvalue references, recycling temporaries

SG <>
Tue, 16 Sep 2008 16:28:53 CST
On 16 Sep., 01:16, wrote:

Would there be any ill effect to changing the standard so that the
lifetime extension of 12.2 also applies to this case? It looks like a
fairly natural expectation.

I also thought about this. The problem is however that the compiler
doesn't know just by looking at a function prototype what reference is
returned. Consider:

    // insert more operator+ overloads here (#1...#3)
    string&& operator+(string&& a, string&& b); // #4

In this case the implementation of #4 could return a reference to 'a',
a reference to 'b' or just any other reference. Under the hood
references are probably returned in form of a pointer. So, to extend
the life time of the right temporary the compiler would need to check
what temporary is actually referenced (if any). This is not known
during compile time by simply looking at the function prototype. It
might even be a runtime dependent reference like in this case:

    string&& operator+(string&& a, string&& b)
       if (a.capacity()-a.length()< b.length()
       && (b.capacity()-b.length()>=a.length()) {
          return b;
       } else {
          a += b;
          return a;

This makes extending the life time of the right temporary almost


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