Question

Please consider the following mini example

// CFoo.hpp
class CFoo{
private:
    static const double VPI = 0.5;    
public:
    double getVpi();
};

// CFoo.cpp
#include "CFoo.hpp"    
double CFoo::getVpi(){
    double x = -VPI;
    return x;
}

// main.cpp
#include "CFoo.hpp"    
int main(){
    CFoo aFoo();
    return 0;
}

Lining the program with gcc version 4.5.1 produces the error CFoo.cpp: undefined reference to CFoo::VPI. The error dose not occur if

• VPI is not negated
• the negation is written as double x = -1 * VPI;
• Declaration and definition of class CFoo happen in the same file

Do you know the reason for this error?

Answer 1

There are multiple problems with your code. Primarily, this is not valid C++03:

class CFoo{
private:
    static const double VPI = 0.5;    
// ...
};

The declaration of a static data member can specify a constant initializer if and only if that initializer is const integral or const enumeration type. 0.5 is neither of these, and hence your code is not valid C++. 9.4.2 Static data members covers this:

2/ The declaration of a static data member in its class definition is
not a definition […]The definition for a static data member shall
appear in a namespace scope enclosing the member’s class definition.
[…]

4/ If a static data member is of const integral or const enumeration
type, its declaration in the class definition can specify a
constant-initializer which shall be an integral constant expression
(5.19).

In order to initialize VPI, you must do so in the CPP file:

header:

class CFoo{
private:
    static const double VPI;    
};

cpp :

const double CFoo::VPI = 0.5;

Another problem, unrelated, is here:

int main(){
    CFoo aFoo(); // NOT OK
    return 0;

The expression CFoo aFoo(); doesn’t do what you think it does. You think it declares an object aFoo of type CFoo and initializes it using CFoo‘s default constructor. But what it actually does is declare a function named aFoo taking no parameters, returning a CFoo by value. This is known as the most vexing parse. In order to do what you want, simple omit the parenthesis:

CFoo aFoo;