C++ 如何提取和使用可变模板参数及其类型？

Question

我如何提取和使用 C++ 类的变量模板参数中定义的类型，而不是定义以下类的多个重复项，这些重复项仅按参数的数量和类型进行延迟：

template <typename T1, typename T2>
class SUTTestFixture
{
public:
    void SetUp(T1 expected, vector<T2> data)
    {
        _expected = expected;
        _data = data;
    }

protected:
    SUT _sutObj;
    vector<T2> _data;
    T1 _expected;
};

我希望能够仅使用带有变量模板参数的单个类：

template <typename... T>
class SUTTestFixture
{
public:
    void SetUp(T... args)
    {
        _expected = args...[0];
        _data = args...[1]; // expected ‘;’ before ‘...’ token
    }

protected:
    SUT _sutObj;
    vector<T...> _data; // XXX
    T... _expected; // error: expected unqualified-id before ‘...’ token
};

Answer 1

访问可变参数模板中的类型的一种方法是使用

std::tuple

。一旦定义了元组，您就可以访问其中的任何类型。示例：

#include <tuple>
#include <vector>

template <typename... T>
class SUTTestFixture
{
public:

    // Get the types of the variadic T as a tuple
    using types = std::tuple<T...>;
    
    // Then you can access one specific type
    using T1 = std::tuple_element_t<0,types>;
    using T2 = std::tuple_element_t<1,types>;

    void SetUp(T1 expected, std::vector<T2> data)
    {
        _expected = expected;
        _data = data;
    }

protected:
    std::vector<T2> _data;
    T1 _expected;
};

int main () {}