Template problem porting from 7.0 to 7.1

[Jay]

Hi,

Can anyone tell me why the following code doesn't work on 7.1? It's been ok
with previous versions. The error given by the linker is:

rtwnd.obj : error LNK2019: unresolved external symbol "class TestT<bool>
__cdecl operator+(class TestT<bool> const &,class TestT<bool> const &)"
(??H@YA?AV?$TestT@_N@@ABV0@0@Z) referenced in function "void __cdecl
f(void)" (?f@@YAXXZ)


template <class T> class TestT
{
public:

friend TestT <T> operator+ (const TestT <T>&, const TestT <T>&);
};

template <class T> TestT <T> operator+ (const TestT <T>&, const TestT <T>&)
{
TestT <T> result;

return result;
}


void f ()
{
TestT <bool> t1, t2, t3;

t3 = t1 + t2;
}


Thanks,

Jay.

 

[Jay]

It seems that replacing

friend TestT <T> operator+ (const TestT <T>&, const TestT <T>&);

with

friend TestT <T> operator+ <T> (const TestT <T>&, const TestT <T>&);

cures the problem. But why? And why no compiler error? I've seen many
examples of the original code. Is this a change in the standard? I'm
confused

Jay.

 

[Carl Daniel [VC++ MVP]]

It seems that replacing

friend TestT <T> operator+ (const TestT <T>&, const TestT <T>&);

This declares that a namespace-scoped function named operator+ that accepts

with

friend TestT <T> operator+ <T> (const TestT <T>&, const TestT <T>&);

This declares that an instantiation of a namespace scoped function template
named operator + which is instantiated with a single template parameter <T>
and accepts and returns TestT<T> is a friend. In your example, the
definition of operator+ that you supplied is a function template, so this is
the proper form of friend declaration.

cures the problem. But why? And why no compiler error? I've seen many
examples of the original code. Is this a change in the standard? I'm
confused

VC7 was wrong, VC7.1 is right. Under VC7, a non-template function and a
template instantiation were "mangled" to the same name at the linker level,
as long as the actual function parameters and return types agreed. VC7.1
correctly distiguishes these cases by mapping them to distinct names.

-cd