Re: user-defined conversion combined with argument-dependent lookup

"Alf P. Steinbach" <>
Fri, 30 Nov 2007 23:56:20 +0100
* Belebele:

Suppose that I have a class that has a user-defined conversion to
std::string, and I would like to compare objects of that class to
"strings" (e.g. literals, std::strings):

class InUpperCase {
    operator std::string() const;

InUpperCase()=="a string"; // This does not compile.
    // fine, bool operator==(InUpperCase const& , char const* );
    // is not declared anywhere.

InUpperCase()==std::string("a string"); // This one does not compile
    // even though, there is a definition for an operator== in std
    // that takes two std::strings.

I expected the second expression to compile fine due to ADL combined
with the user-defined conversion. Any idea why?


     #include <string>

     struct Foo
         operator std::string() const { return ""; }

     template< typename Ch >
     void bar( std::basic_string<Ch> const& ) {}

     int main()
         bar( Foo() );

Template matching doesn't succeed.

When you're the one writing the bar() function you can always support
future classes like Foo e.g. by making the argument a
ConvertibleTo<std::string>, with each Foo class publicly derived from

Cheers, & hth.,

- Alf

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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