Why does negating a static member variable produce a linker error? - c++

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;