What is the correct syntax in C++ for calling a method stored inside of a class using a different class as the 'this' for the method?

Question

I have a template class like this:

class template <class T1, class T2, class CTHIS> class cachedValuesClass
{
typedef T2 (CTHIS::*ptrToInternalMethodType)(T1) const;
ptrToInternalMethodType classFuncVar;
T2 getValue(CTHIS* _this, T1 x, bool *error = NULL) const;
}

The getValue code is supposed to call for the method stored at the point this->classFuncVar using the _this parameter as the "this" for this call. I tried writing this:

 template <class T1, class T2, class CTHIS>
 T2 cachedValuesClass<T1, T2, CTHIS>::getValue(CTHIS* _this, T1 x, bool *error /*=NULL*/) const
 {
 return *_this.*this.*classFuncVar(x);
 }

But it doesn't work, I get this error:

 130|error: must use ‘.*’ or ‘->*’ to call pointer-to-member function in ‘((const cachedValuesClass<float, float, vocationForTheorical>*)this)->cachedValuesClass<float, float, vocationForTheorical>::classFunc (...)’, e.g. ‘(... ->* ((const cachedValuesClass<float, float, vocationForTheorical>*)this)->cachedValuesClass<float, float, vocationForTheorical>::classFunc) (...)’|
 130|error: ‘this’ cannot be used as a member pointer, since it is of type ‘const cachedValuesClass<float, float, vocationForTheorical>* const’|

I tried several variations, including parenthesis but I coudn't make it work. How should be correct syntax for this line?

Thanks in advance!

Answer 1

You need a few more parenthesis in your getValue method code, in order to bind the method pointer to its target object before calling the member function:

return (_this->*classFuncVar)(x);
// or more complete
return (_this->*(this->classFuncVar))(x);

See also: How to call through a member function pointer?