c++ - Calling inline function in .cu file from C++ class

Question

I'm trying to call a kernel wrapper foo from a C++ class. I've tried to do as suggested here below:

// In CPP.h:
class cls {
    extern "C" inline void foo();
}

// In Kernels.cu:
#include "CPP.h"
extern "C" inline void cls::foo() {
    // call kernels here
}

but this has not worked - I get a compiler errors:

• CPP.h: invalid storage class for a class member
• CPP.h: "cls::foo" was referenced but not defined
• Kernels.cu: label "cls" was declared but never referenced

What's going wrong?