Re: basic_ostream, wchar_t * and implicit conversions

From:
"Bo Persson" <bop@gmb.dk>
Newsgroups:
comp.lang.c++.moderated
Date:
Thu, 30 Sep 2010 08:23:00 CST
Message-ID:
<8ghgvsFg2rU1@mid.individual.net>
Marco Francescangeli wrote:

Hello

I have a class MyString defining a type conversion operator.
In this first case, when the type conversion converts MyString to an
int, the output is "1", as expected.
___________________________________________________

#include <iostream>

struct MyString{
operator const int() const{
return 1;
}
};

int main (void) {
MyString str;

std::wcout << str; //outputs "1"
std::wcout << 1; //outputs "1"

return 0;
}
___________________________________________________

In this case instead when converting implicitly to a const wchar_t
*, it outputs its memory address, instead of "Hello".
___________________________________________________

#include <iostream>

struct MyString{
operator const wchar_t *() const{
return L"Hello";
}
};

int main (void) {
MyString str;

std::wcout << str; // outputs a memory address
std::wcout << L"Hello"; // outputs "Hello"

return 0;
}
___________________________________________________

This happens because in std::basic_ostream there is an
operator<<(int),
and an operator<<(void const *) but there is no operator<<(wchar_t
const *).

When you pass directly a wchar_t const *, that is correctly handled
by the template function operator<<(basic_ostream &, const T *).
But it is a template accepting only pointers types, so when I pass
a MyString object it is not taken in consideration.

Isn't this an inconsistent behaviour?


Yes, and no.

The treatment of built in types is not totally consistent with the
treatment of user defined types, History, I guess.

However, when trying to instantiate a template, the compiler does not
have to consider if your type would be convertible to some possible T'
satisfying the template. That would be a lot of work!

So, instead it considers the non-template overloaded members of the
stream. And there it finds that your class is convertible to a
pointer, that can be passed to the void* overload. As that is the only
match, it is chosen.

If you want something else for your type, you could add you own
overload for operator<<.

Bo Persson

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