调用模板类成员时非法使用此类型作为表达式

Question

I have a problem with the usage of a static class member size of a class SizeObj , which is used as template parameter for a template class SizeTemplate . See below the code snippet, which I reduced to the minimum.

In fact that code below ran well up to MS VS2008, but now the compilation fails when compiling with VS2010. The following error messages are displayed:

error C2275: 'K' : illegal use of this type as an expression
error C2228: left of '.size' must have class/struct/union

It need to be said, that the compilation fails only, if the getSize method is called at least once.

Please don't ask about the sense of the code below, as said I reduced it to the essential minimum to explain. However, I need to admit that the usage of the member 'size' is not very elegant due to several reasons, and maybe there are lots of better solutions, but at the moment, I don't have any choice to keep it like that.

Do you know what may be wrong here? Is it possible to solve that by build settings or something similar? I didn't find yet anything suitable yet.

In the following posts it was easy, because an instance of class K is available, but for my problem, I don't know how to get that instance properly:

• Illegal use of type in template
• Simple templated function to convert std::vectors - "illegal use of this type as an expression"

//myTemplate.h


class SizeObj
{
public:
    static const int size = 1;
};

template<class K>
class SizeTemplate
{
public:
    int getSize();
};

template<class K>
int SizeTemplate<K>::getSize()
{
    return K.size;
}

//main.cpp

int main(...)
{
    SizeTemplate<SizeObj> sizeObj;
    printf("size:%d", sizeObj.getSize());
}

Thank you a lot in advance!

tangoal

Answer 1

Unlike Java, in C++ you cannot use the dot operator on classes, you need use the scope resolution operator (ie :: ) to get things from within the class scope (for example the size static variable), so replace return K.size with return K::size

Also marking the method to be constexpr is likely going to help here.