Question

The code below works as long as I keep it all in the “main.cpp” file.

//#include "Travel.h"
//#include "Obj.h"

// "Travel.h"
typedef int travel_t;

class Travel
{

public:

    static const travel_t AIR;
    static const travel_t WATER;
    static const travel_t LAND;

};

// "Travel.cpp"
// #ifndef TRAVEL_H
// #define TRAVEL_H
//
// #include "Travel.h"
const travel_t Travel::AIR = -2;
const travel_t Travel::WATER = -1;
const travel_t Travel::LAND = 0;
// #endif //TRAVEL_H

// "Obj.h"
// #ifndef OBJ_H
// #define OBJ_H
//
//#include "Travel.h"
template<typename T, travel_t travel>
class Obj
{
public:
    void foo(){};
};
// #endif //OBJ_H

// "main.cpp"
int main()
{
    Obj<int, Travel::AIR> objAirTravel;

    objAirTravel.foo();

    return 0;
}

However, as soon as I moved code to different headers and implementation files as indicated, it doesn’t compile any more. 🙁 How can I fix that problem? What is the problem/rule behind it? This is the compiler error I get (using gcc):

main.cpp|45|error: 'Travel::AIR' is not a valid template argument for type 'int' because it is a non-constant expression|
main.cpp|45|error: invalid type in declaration before ';' token|
main.cpp|47|error: request for member 'foo' in 'objAirTravel', which is of non-class type 'int'|

Answer 1

In order to use a constant as a template argument, its value must be available in the current translation unit. When you move the definition of Travel::Air to a different source file, its value is no longer available to the compiler in main.

Since it’s an integer constant, you can declare the value in the declaration inside the class:

class Travel
{
public:
    static const travel_t AIR = -2;
    static const travel_t WATER = -1;
    static const travel_t LAND = 0;
};

Now the values are available to use as template arguments in any translation unit that includes this class definition.