Normally a temporary object (such as one returned by a function call) has a lifetime that extends to the end of the “enclosing expression”. However, a temporary bound to a reference generally has it’s lifetime ‘promoted’ to the lifetime of the reference (which may or may not be the lifetime of the calling function), but there are a couple exceptions. This is covered by the standard in 12.2/5 “Temporary objects”:
The temporary to which the reference is bound or the temporary that is the complete object to a subobject of which the temporary is bound persists for the lifetime of the reference except as specified below. A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the constructor exits. A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of the full expression containing the call.
See the following for more information:
- C++ constant reference lifetime (container adaptor)
- GotW #88: A Candidate For the “Most Important const”
An example that might help visualize what’s going on:
#include <iostream>
#include <string>
class foo {
public:
foo( std::string const& n) : name(n) {
std::cout << "foo ctor - " << name + " created\n";
};
foo( foo const& other) : name( other.name + " copy") {
std::cout << "foo copy ctor - " << name + " created\n";
};
~foo() {
std::cout << name + " destroyed\n";
};
std::string getname() const { return name; };
foo getcopy() const { return foo( *this); };
private:
std::string name;
};
std::ostream& operator<<( std::ostream& strm, foo const& f) {
strm << f.getname();
return strm;
}
int main()
{
foo x( "x");
std::cout << x.getcopy() << std::endl;
std::cout << "note that the temp has already been destroyed\n\n\n";
foo const& ref( x.getcopy());
std::cout << ref << std::endl;
std::cout << "the temp won't be deleted until after this...\n\n";
std::cout << "note that the temp has *not* been destroyed yet...\n\n";
}
Which displays:
foo ctor - x created
foo copy ctor - x copy created
x copy
x copy destroyed
note that the temp has already been destroyed
foo copy ctor - x copy created
x copy
the temp won't be deleted until after this...
note that the temp has *not* been destroyed yet...
x copy destroyed
x destroyed