将指向成员的指针从模板参数包传递给函数对象

Question

Bellow 是演示我正在尝试做的事情的示例代码。

基本上我想将多个指向成员的指针传递给 class 方法，该方法采用 std::function 对象和变量号或传递给目标函数对象的参数。

这些可变参数可以是任何东西：变量、指向成员的指针等。

我在使用普通变量时没有问题，但是如何扩展参数包，其中参数是指向成员的指针，但函数对象需要 POD 数据类型？

如何获取参数包中指向成员的指针的值并将其传递给函数对象？

#include <cstddef>
#include <functional>


class Test
{
public:
    inline Test(std::uint32_t max);

    template<typename ReturnType, typename... Args>
    using Algorithm = std::function<ReturnType(Args...)>;

    // pointer to member
    // not used but would like to make use of it in 'RunTargetAlgo'
    template<typename T>
    using ParamType = T Test::*;

    // exectutor
    template<typename ReturnType, typename... Args, typename... Params>
    inline double RunTargetAlgo(Algorithm<ReturnType, Args...> target, Params... params);

    std::uint32_t mLoop = 0;
    const std::size_t mLoopMax;
    double mPrecision = 0;
};

Test::Test(std::uint32_t max) :
    mLoopMax(max)
{
}

template<typename ReturnType, typename ...Args, typename ...Params>
double Test::RunTargetAlgo(Algorithm<ReturnType, Args...> target, Params ...params)
{
    while (++mLoop <= mLoopMax)
        mPrecision += target(params...);    // How if Params == Test::ParamType ??

    // ... in real code we measure bunch of other stuff here...

    return mPrecision;
}

template<typename RetrunType>
RetrunType TestAlgorithm(std::uint32_t x, std::uint32_t y)
{
    // ...
    return RetrunType();
}

int main()
{
    namespace pl = std::placeholders;

    Test::Algorithm<double, std::uint32_t, std::uint32_t> algo =
        std::bind(TestAlgorithm<double>, pl::_1, pl::_2);

    Test t(50);

    // this works just fine!
    t.RunTargetAlgo(algo, 3u, 4u);

    // but how to run this?
    // I need pass fresh data to target algorithm, value of
    // pointer to members may change anytime.
    // but main issue here is how to extract these pointer from
    // parameter pack into actual values, but inside Test::RunTargetAlgo?
    t.RunTargetAlgo(algo, &Test::mLoop, &Test::mLoopMax);

    return 0;
}

Answer 1

正如评论中所讨论的，假设所有参数都需要相同的操作，您可以简单地在参数包扩展中应用任何 dereferencing/member 访问操作。例如。对于应该访问 this 实例的成员指针：

target(this->*params...);

或者如果你直接传递对象指针（这对我来说似乎更直接）：

target(*params...);

//...

t.RunTargetAlgo(algo, &t.mLoop, &t.mLoopMax);

将指向成员的指针从模板参数包传递给函数对象

Passing pointers to member to function object from template parameter pack

c++

templates

member-pointers

parameter-pack