C++ addition overload ambiguity
- by Nate
I am coming up against a vexing conundrum in my code base. I can't quite tell why my code generates this error, but (for example) std::string does not.
class String {
public:
String(const char*str);
friend String operator+ ( const String& lval, const char *rval );
friend String operator+ ( const char *lval, const String& rval );
String operator+ ( const String& rval );
};
The implementation of these is easy enough to imagine on your own.
My driver program contains the following:
String result, lval("left side "), rval("of string");
char lv[] = "right side ", rv[] = "of string";
result = lv + rval;
printf(result);
result = (lval + rv);
printf(result);
Which generates the following error in gcc 4.1.2:
driver.cpp:25: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
String.h:22: note: candidate 1: String operator+(const String&, const char*)
String.h:24: note: candidate 2: String String::operator+(const String&)
So far so good, right? Sadly, my String(const char *str) constructor is so handy to have as an implicit constructor, that using the explicit keyword to solve this would just cause a different pile of problems.
Moreover... std::string doesn't have to resort to this, and I can't figure out why. For example, in basic_string.h, they are declared as follows:
template<typename _CharT, typename _Traits, typename _Alloc>
basic_string<_CharT, _Traits, _Alloc>
operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
const basic_string<_CharT, _Traits, _Alloc>& __rhs)
template<typename _CharT, typename _Traits, typename _Alloc>
basic_string<_CharT,_Traits,_Alloc>
operator+(const _CharT* __lhs,
const basic_string<_CharT,_Traits,_Alloc>& __rhs);
and so on. The basic_string constructor is not declared explicit. How does this not cause the same error I'm getting, and how can I achieve the same behavior??
