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

为了使用一个固定的模板论点,其价值必须放在目前的翻译单位。当你将<代码>Travel:Air的定义移至不同的来源档案中时,其价值不再在main上提供给汇编者。

既然是惯犯,你可以宣布:

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

现在,这些数值可用作包括本类定义的任何翻译单位的模板论据。

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