在 GTEST (Googletest) 中使用模板

Question

I know. The headline is not perfekt. Perhaps I fund a better Headline later. I have a Problem with GTEST with template. (Yes I know. The code make no sense. It is a sample). I have the following template:

template<typename T>
   struct MY_VALUE{};

template<>
    struct MY_VALUE<uint8_t>{
        static const uint8_t val = 8;
    };

template<>
    struct MY_VALUE<uint16_t>{
        static const uint16_t val = 16;
};

I can test the code with (test sucess):

TEST(MY_VALUE_test, test) {
    uint32_t var8 = MY_VALUE<uint8_t>::val;
    ASSERT_EQ(8, var8);

    uint32_t var16 = MY_VALUE<uint16_t>::val;
    ASSERT_EQ(16, var16);
}

But when i try to test this, the LINKER give me an error:

TEST(MY_VALUE_test, test1) {
    ASSERT_EQ(8, MY_VALUE<uint8_t>::val);
    ASSERT_EQ(16, MY_VALUE<uint16_t>::val);
}

Linker error:

undefined reference to `MY_VALUE<unsigned char>::val
undefined reference to `MY_VALUE<unsigned short>::val

anyone an idea:) Thanks

Answer 1

The problem is with the fact that assertion engine of GTEST requires reference. To have reference - variable needs to be defined:

template<>
struct MY_VALUE<uint8_t>{
        static const uint8_t val = 8;
};

// this line is missing 
// Note: do not put it in header - if you have multiple files project
const uint8_t MY_VALUE<uint8_t>::val;

Do the same for uint16_t.

IF you have compiler supporting C++17 - you might try to just add inline or constexpr to val:

template<>
struct MY_VALUE<uint8_t>{
        static constexpr uint8_t val = 8;
};

Answer 2

Thanks, your answer helps. Adding:

const uint8_t MY_VALUE<uint8_t>::val;

works. I have no c++17 support. I cant try this.