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Compiler Error C3206

Error Message

'function' : invalid type argument for 'param', missing type argument list on class type 'typename'

A template function is defined as taking a template type argument. However, a template template argument was passed.

The following sample generates C3206:

// C3206.cpp
template <class T>
void f() {}

template <class T>
struct S {};

void f1() {
   f<S>();   // C3206
   // try the following line instead
   // f<S<int> >();
}

Possible resolution:

// C3206b.cpp
// compile with: /c
template <class T>
void f() {}

template <class T>
struct S {};

void f1() {
   f<S<int> >();
}

C3206 can also occur when using generics:

// C3206c.cpp
// compile with: /clr
generic <class GT1>
void gf() {}

generic <class T>
value struct GS {};

int main() {
   gf<GS>();   // C3206
}

Possible resolution:

// C3206d.cpp
// compile with: /clr
generic <class GT1>
void gf() {}

generic <class T>
value struct GS {};

int main() {
   gf<GS<int> >();
}

This error can also be generated as a result of compiler conformance work for Visual C++ .NET 2003, where class templates are not allowed as template type argument.

A class template is not allowed as a template type argument. This worked in Visual C++ .NET 2003, but it is invalid C++.

For more information, see Summary of Compile-Time Breaking Changes.

The following sample compiles in Visual C++ .NET 2002 but will fail in Visual C++ .NET 2003:

// C3206e.cpp
template <class T>
struct S {};

template <class T>
void func() {   // takes a type
   T<int> t;
}

int main() {
   func<S>();   // C3206 S is not a type.
}

Possible resolution:

// C3206f.cpp
template <class T>
struct S {};

template <class T>
void func() {   // takes a type
   T t;
}

int main() {
   func<S<int> >();
}

If a template template parameter is necessary, then the resolution for the error that is valid in both the Visual C++ .NET 2003 and Visual C++ .NET 2002 versions require you to wrap the function in a template class that takes a template template parameter:

// C3206g.cpp
template <class T>
struct S {};

template<template<class> class TT>
struct X {
   static void func() {
      TT<int> t1;
      TT<char> t2;
   }
};

int main() {
   X<S>::func();
}

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