Re: issue with temporaries and const references
* noir, on 17.10.2010 10:44:
Hello,
why does 2) below fail?
#include <cassert>
#include <string>
int main()
{
// 1) WORKS
std::string s = std::string("hello").append("world");
assert(s == "helloworld");
// 2) FAILS
const std::string& t = std::string("hello").append("world");
assert(t == "helloworld");
return 0;
}
My understanding about case 2) is:
- a temp string is created (OK)
- a non-const member (append) is called on the temp string (OK)
- a non-const reference to the temp string is returned by append (OK)
- the non-const reference is bound to a const-reference (OK)
- the const-reference extends the lifetime of the temp string until the end of
the block.
However, I see "t" contains trash when it is tested in the assert, so I guess
the destructor for the string has been called somewhere.
Any clues?
(Btw, I am testing this on VS 2008 SP2)
Because 'append' returns a reference. The named reference is bound directly to
that reference, which is to a temporary whose lifetime ends after evaluation of
the initialization expression. If you tack on a '+ ""' you should avoid this,
but for your concrete example there's no point: the first declaration works.
Cheers & hth.,
- Alf
--
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