How to define a static const variable of a template class

Question

I'm trying to make a vector class with predefined static constants for up, right and forward because these should be the same in all cases. How should this be defined and is it even possible?

I'm trying to do something like this:

template <class T> class vec3
{
public:
    vec3(T x = 0, T y = 0, T z = 0) :
        x(x),
        y(y),
        z(z)
    {
    }

    static const vec3<T> right;
    static const vec3<T> up;
    static const vec3<T> forward;

    T x, y, z;
}

cpp:

#include "vec3.h"

template <typename T>
const vec3<T>::right(1, 0, 0);

template <typename T>
const vec3<T>::up(0, 1, 0);

template <typename T>
const vec3<T>::forward(0, 0, 1);

This results in a syntax error.

BTW, I doubt that you want the definition in .cpp as it continue to depend on `T`. — Jarod42, Dec 01 '15 at 17:41
@Jarod42 how would it be defined in the header? If I try `= vec3(1,0,0);`'vec3::right': a static data member with an in-class initializer must have non-volatile const integral type` — Duckdoom5, Dec 01 '15 at 17:43
You likely want `template const vec3 vec3::right(1, 0, 0);` in the header! — , Dec 01 '15 at 17:45
@DieterLücking That gives a compiler error with right(, , ) — Duckdoom5, Dec 01 '15 at 17:47
@Duckdoom5 compiles fine with g++ after fixing: `error: expected ‘;’ after class definition` — , Dec 01 '15 at 17:52

score 13 · Accepted Answer · answered Dec 01 '15 at 17:46

It should be (all in header (you may use .inl or .hxx if you want to split declaration from definition)):

template <class T> class vec3
{
public:
    vec3(T x = 0, T y = 0, T z = 0) :
        x(x),
        y(y),
        z(z)
    {
    }

    static const vec3 right;
    static const vec3 up;
    static const vec3 forward;

    T x, y, z;
};

template <typename T> const vec3<T> vec3<T>::right(1, 0, 0);
template <typename T> const vec3<T> vec3<T>::up(0, 1, 0);
template <typename T> const vec3<T> vec3<T>::forward(0, 0, 1);

Demo

score 2 · Answer 2 · answered Dec 01 '15 at 17:46

template <class T> class vec3
{
public:
    vec3(T x = 0, T y = 0, T z = 0) :
        x(x),
        y(y),
        z(z)
    {
    }

    static const vec3 right;
    static const vec3 up;
    static const vec3 forward;

    T x, y, z;
};


template <typename T>
const vec3<T> vec3<T>::right(1, 0, 0);

template <typename T>
const vec3<T> vec3<T>::up(0, 1, 0);

template <typename T>
const vec3<T> vec3<T>::forward(0, 0, 1);

How to define a static const variable of a template class

2 Answers2

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