I have the following code:
struct simple
{
simple (int a1, int a2) : member1(a1), member2(a2) {}
int member1;
int member2;
};
std::ofstream &operator << (std::ofstream &f, const simple &obj)
{
f<<obj.member1<<", "<<obj.member2;
return f;
}
int main(int argc, const char *argv[])
{
std::ofstream f("streamout.txt");
simple s(7,5);
f << s; //#1 This works
f << "label: " << s; //#2 This fails
return 0;
}
I'm trying to understand why #1 works, while there are problems when trying to use the overloaded operator concatenating it as in #2 which fails with the following error (gcc 4.5.3 on MacOSX):
error: cannot bind 'std::basic_ostream' lvalue to 'std::basic_ostream&&' /GCC-FACTORY/4.5/INSTALL/lib/gcc/x86_64-apple-darwin10.5.0/4.5.3/../../../../include/c++/4.5.3/ostream:579:5: error: initializing argument 1 of 'std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char, _Traits = std::char_traits, _Tp = simple]'
Everything is instead fine if I define my operator as
std::ostream &operator << (std::ostream &f, const simple &obj)
{ ... }
Sounds like something related to overload resolution, where having a something inserted in the ofstream for which there's already a provided overload (the const char * "label" in this case) breaks up following overload resolution, but I can't really understand what exactly is going on here. I'd like to get a clear picture of what the compiler's trying to do..
On the line :
f << "label: " << s;
Because the first call to operator<<
returns a std::ostream &
, the second fails to compile : the left operand to the operator is not of type std::ofstream
anymore and your overload is not found.
You should really use the second signature, as I see no reason for restricting your type to be outputted to std::ofstream
.