Explicit template declaration/definition in VS 2012

Question

The following code declares a template, declares an explicit instantiation definition, then declares an explicit instantiation declaration:

template <typename T>
T Double(T number)
{
    return number * 2;
}

extern template int Double<int>(int);  // declaration
template int Double<int>(int t);       // definition

int main(int argc, char* argv[])
{
    int n = Double(10);

    return 0;
}

gives an error:

error C2929: 'int Double<int>(int)' : explicit instantiation; cannot explicitly force and suppress instantiation of template-class member

in Visual Studio 2012.

I'm under the impression from http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1987.htm that this should be valid, since the definition follows the declaration.

Am I missing something?

Andy Prowl Andy Prowl · Accepted Answer · 2013-05-28T17:26:38

Your program is well-formed. Paragraph 14.7.2/11 of the C++11 Standard specifies:

If an entity is the subject of both an explicit instantiation declaration and an explicit instantiation definition in the same translation unit, the definition shall follow the declaration. [...]

Your program respects this constraint and does not break any other rule. Therefore, this qualifies as a bug in VC11.

Explicit template declaration/definition in VS 2012

3 Answers