Initialize a template class private static variable in C++

Question

I'm trying to compile a sample program in C++ using templates. The template class has a priavte static member variable which seems to be undefined when trying to compile. Going through other answers on SO, I realized that this variable needs to be defined as well. However my attempts at defining this variable have been unsuccessful so far, likely due to my lack of experience working with templates. Here is my sample program:

#include <iostream>
#include <array>

enum FRUIT
{
    APPLE,
    ORANGE
};

using FunctionPtr = void(*)(void);

template <FRUIT T>
void FruitFunction(void);

template <FRUIT...TotalFruits>
class TestClass
{
public:

    struct fruitGroup
    {
        FRUIT fruit;
        FunctionPtr func;
    };

    static int find_fruit(FRUIT fruit, int arg)
    {
        for (auto i = pv_mem_.begin(); i != pv_mem_.end(); ++i) {
            if (i->fruit == fruit) {
                break;
            }
        }

        return 0;
    }

private:
    constexpr static std::array<fruitGroup, sizeof...(TotalFruits)> pv_mem_ 
    {
        fruitGroup{TotalFruits, &FruitFunction<TotalFruits>}...
    };
};

int main()
{
    TestClass<FRUIT::APPLE, FRUIT::ORANGE> test;
    test.find_fruit(FRUIT::APPLE, 0);

    return 0;
}

This yields:

$ g++ -std=c++11 fruit.cpp -o foo
/tmp/ccqaSBYm.o: In function `TestClass<(FRUIT)0, (FRUIT)1>::find_fruit(FRUIT, int)':
fruit.cpp:(.text._ZN9TestClassIJL5FRUIT0ELS0_1EEE10find_fruitES0_i[_ZN9TestClassIJL5FRUIT0ELS0_1EEE10find_fruitES0_i]+0xf): undefined reference to `TestClass<(FRUIT)0, (FRUIT)1>::pv_mem_'
fruit.cpp:(.text._ZN9TestClassIJL5FRUIT0ELS0_1EEE10find_fruitES0_i[_ZN9TestClassIJL5FRUIT0ELS0_1EEE10find_fruitES0_i]+0x1d): undefined reference to `TestClass<(FRUIT)0, (FRUIT)1>::pv_mem_'
collect2: error: ld returned 1 exit status

I have tried defining pv_mem_ as:

constexpr static std::array<TestClass::fruitGroup, sizeof...(TotalFruits)> pv_mem_;

but that resulted in the following error:

$ g++ -std=c++11 fruit.cpp -o foo
fruit.cpp:44:74: error: wrong number of template arguments (1, should be 2)
 constexpr static std::array<TestClass::fruitGroup, sizeof...(TotalFruits)> pv_mem_;
                                                                          ^
In file included from fruit.cpp:2:0:
/usr/include/c++/5/array:89:12: note: provided for ‘template<class _Tp, long unsigned int _Nm> struct std::array’
     struct array
            ^
fruit.cpp:44:76: error: uninitialized const ‘pv_mem_’ [-fpermissive]
 constexpr static std::array<TestClass::fruitGroup, sizeof...(TotalFruits)> pv_mem_;
                                                                            ^

What would be the right way to initialize this variable?

康桓瑋 · Accepted Answer · 2022-04-18 02:43:56Z

1

pv_mem_ is defined as follows

constexpr static std::array<fruitGroup, sizeof...(TotalFruits)> pv_mem_ 
{
  fruitGroup{TotalFruits, &FruitFunction<TotalFruits>}...
};

which uses & to take the address of FruitFunction<TotalFruits>, but since FruitFunction is only declared and not defined, it will generate an undefined reference error at runtime.

Adding the definition for the template function FruitFunction will solve the problem in C++17

template <FRUIT T>
void FruitFunction() { /* */ }

In C++11, constexpr static member variables still need to be defined outside the class, so you also need to add

template <FRUIT...TotalFruits>
constexpr std::array<
  typename TestClass<TotalFruits...>::fruitGroup, 
  sizeof...(TotalFruits)> TestClass<TotalFruits...>::pv_mem_;

Demo

edited Apr 18, 2022 at 2:43

answered Apr 18, 2022 at 2:24

康桓瑋

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4 Comments

rookie Over a year ago

Shouldn't pv_mem_ be initialized? Unfortunately, defining FruitFunction doesn't resolve the compile error as it still complains of undefined reference to TestClass<(FRUIT)0, (FRUIT)1>::pv_mem_

rookie Over a year ago

Wow, that worked! Would you mind elaborating/explaining that definition, it looks quite involved. Thank you very much _/\_

康桓瑋 Over a year ago

This definition is basically the same as the one defined inside the class. Since it is defined outside, we can't use fruitGroup directly like inside the class, which is why it looks verbose.

rookie Over a year ago

Correct, I did realize that. To be specific, I was curious how the class template which is defined as template <FRUIT...TotalFruits> class TestClass, can also be used as TestClass<TotalFruits...> as per the definition above. I realized I also totally missed the keyword typename is seems like it is required

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