noir wrote:
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?
You can interpret the Standard as having two things that are called
"temporary". The Standard is not always clear what thing it refers to in all
places, apparently assuming the reader can deduce that from context.
The first thing is a temporary *object*. The reference that "append" returns
surely refers to that temporary object.
The second thing is a temporary *expression*. append does not return a
temporary expression. Temporary expressions are the ones that directly
represent the creation of a temporary object. I.e 'string("hello")'.
I doubt that this two-fold identity of "temporary" is actually intended by
the committee, but such is definitely present. See http://www.open-
std.org/jtc1/sc22/wg21/docs/cwg_active.html#462 for a discussion where the
property of an *expression* that was temporary would have been lost, and
explicit wording needs added to restore that property. On the other side,
other wording like at [class.copy]/p15 uses the *object* property of
"temporary" when it says "when a temporary class object that has not been
bound to a reference (12.2) would be copied ...". In the end, I think this
is disgusting.
The issue you mention was posted by me to usenet months ago, but it wasn't
forwarded as an issue report to the committee it seems. (And since the
reflector mailing list is kept behind closed doors, I have no clue whether
the committee ever discussed it).
thank you for your reply. The two different definitions of temporary