Static const double in c++

Question

Is this the proper way to use a static const variable? In my top level class (Shape)

#ifndef SHAPE_H
#define SHAPE_H

class Shape
{
public:

    static const double pi;
private:
    double originX;
    double originY;
};

const double Shape::pi = 3.14159265;

#endif

And then later in a class that extends Shape, I use Shape::pi. I get a linker error. I moved the const double Shape::pi = 3.14... to the Shape.cpp file and my program then compiles. Why does that happen? thanks.

Answer 1

If you have a way to add C++11 (or later) flag to your compiler, you would've been able to do:

ifndef SHAPE_H
#define SHAPE_H

class Shape
{
public:

    static constexpr double pi = 3.14159265;
private:
    double originX;
    double originY;
};

#endif

Since C++11 you are able to use const expressions to types other than integral ones. This enables you to declare and define in place your constant variable.

Further details: https://en.cppreference.com/w/cpp/language/constexpr