templating a class member function!

[Ardiani]

Im trying to write a template member function but something goes wrong. I get this compiler error:

error C2275: "MY_STRUCT" : illegal use of this type as an expression

Can anybody help me?

////////////////////////////////////////////////////////////////////////
// MyClass.h

typedef struct
{
int n;
long l;
} MY_STRUCT;

class MyClass
{
public:
template <typename T> int GetSizeOf(T myStruct);
MyClass();
virtual ~MyClass();
};
///////////////////////////////////////////////////////////////////////////

///////////////////////////////////////////////////////////////////////////
// MyClass.cpp

MyClass::MyClass()
{
}

MyClass::~MyClass()
{
}

template <typename T> int MyClass::GetSizeOf(T myStruct)
{
return sizeof(T);
}
////////////////////////////////////////////////////////////////////////////

////////////////////////////////////////////////////////////////////////////
// Main.cpp

void main ()
{
MyClass TestObj;
MY_STRUCT myStruct;

int nSize = TestObj.GetSizeOf<MY_STRUCT>(myStruct);

return;
}

[Graham]

1) You can't put the definition of GetSizeOf in the .cpp file - it has to go in the header.

2) Why have you made it a class member? The implementation you've given (apart from resolving to sizeof) would be better off as a free function.

[Ardiani]

Actually the sample I've posted used to be just an example of a problem. The reason I made this function a class member is that I have to use some member variables in it.

[Graham]

Fair enough, but point 1 is still valid.

[Ardiani]

Well, I've put the definition of 'GetSizeOf' function in the header file, unfortunately I'm still getting the same compiler error if I try using it in main function. Thank you anyway...

[zdf]

It looks like a compiler bug. Are you using VC++?

[zdf]

Yes, it is a bug! Works fine with Borland 5.5 and g++ 2.95.3-5.

Regards,

[Ardiani]

Yes. I'm using VC++6.0.

[Graham]

Ah, well. VC++ is seriously brain-damaged when it comes to template member functions.

[amag]

Actually the compiler deduces the type of GetSize() by itself. If you call GetSize() without the explicit template-argument you'll be fine!

[Ardiani]

Maybe I didn't get it right. Do you mean something like this:

MyClass TestObj;
MY_STRUCT myStruct;
myStruct.l = 1;
myStruct.n = 2;

int nSize = TestObj.GetSizeOf(myStruct);


I get linker error:
unresolved external symbol "public: int __thiscall MyClass::GetSizeOf(struct MY_STRUCT)"

[jfaust]

As Graham said, "You can't put the definition of GetSizeOf in the .cpp file - it has to go in the header."

This is where your unresolved is coming from.

Jeff

[Ardiani]

Thanks to all, this solved my problem.