Re: Is this legal? assigning return value to a const ref?
On Oct 31, 12:26 pm, "Jim Langston" <tazmas...@rocketmail.com> wrote:
"James Kanze" <james.ka...@gmail.com> wrote in message
news:1193737109.281910.11190@k79g2000hse.googlegroups.com...
On Oct 29, 8:45 pm, flopbucket <flopbuc...@hotmail.com> wrote:
OP here, in bar() it should be a const reference:
std::string foo()
{
std::string xyz = "FOO";
return xyz;
}
void bar()
{
const std::string& s = foo();
// ... use s now
}
It's perfectly legal, but what's the point in using the const
reference (preferably written "std::string const&") rather than
a value?
==================
I tested this, and the output of the following program is indeed
One Two Unknown
#include <vector>
#include <iostream>
#include <string>
std::string Foo( int Val )
{
switch (Val)
{
case 1:
return "One";
break;
case 2:
return "Two";
break;
default:
return "Unknown";
break;
}
}
int main()
{
const std::string& One = Foo(1);
const std::string& Two = Foo(2);
const std::string& Three = Foo(3);
std::cout << One << " " << Two << " " << Three << "\n";
}
My question is, what is the lifetime of the returned string? The lifetime
of the references? I understand that normally the lifetime of a temporary
variable returned by a function is the statement it is called on, yet the
std::strings returned by Foo is beyond this.
Where do things std::strings reside, in what variable? I had always thou=
ght
that a reference was just a glorified pointer, but this code seems to
indicate more than this. The compiler seems to treat the std::strings
returned by Foo as being owned by the variables One, Two and Three, even
though they are references.
Dang, I thought I actually was starting to understand C++ yet there is
always something that throws me a suprise.
"References to variables and references to constants are distinguished
because the introduction of
a temporary in the case of the variable is highly errorprone;
an assignment to the variable would
become an assignment to the - soon to disappear - temporary. No such
problem exists for references
to constants, and references to constants are often important as
function arguments."
TC++PL =A75.5 P98