Question

这有点难以解释，在我的类中，我有一个带有指针和参数的create函数我希望能够检查传递给该函数的函数是否具有ClassPointer作为其第一个参数，如果这样自动将其添加到函数内部＆＃34; create＆＃34;如果不是只是添加正常的函数参数（如果有的话），比如im doing

       template <typename Function, typename... Arguments>
        void SomeClass::create(HWND hWnd, Function func, Arguments&&... args)
        {
            // check if function's 1st argument = class pointer

            std::function<void()> function = std::function<void()>(std::bind(std::forward<Function>(func), this, std::forward<Arguments>(args)...)); 

            // else

            std::function<void()> function = std::function<void()>(std::bind(std::forward<Function>(func), std::forward<Arguments>(args)...)); 
        }

        void ThreadProc1(SomeClass* pThis, HWND hwnd)
        {
             // do some stuff in here
        }

        void ThreadProc2(HWND hwnd)
        {
             // do some stuff in here
        }

test.Create(hwnd, ThreadProc1, hwnd);
test.Create(hwnd, ThreadProc2, hwnd);

Answer 1

问题可以更加普遍。您可以询问如何确定函数的第N个参数是否属于给定类型。要在这个问题上静态获得答案，您可以创建SFINAE helper struct nth_param_is，如下所示：

#include <type_traits>
#include <functional>
#include <iostream>

using namespace std;

typedef int HWND;


template<int N, class T, class... Args>
struct nth_template_param_is: false_type { };

template<int N, class T, class First, class... Others>
struct nth_template_param_is<N, T, First, Others...>: nth_template_param_is<N-1, T, Others...> { };

template<class T, class First, class... Others>
struct nth_template_param_is<0, T, First, Others...>: is_same<T, First> { };

template<int N, class T, class Foo>
struct nth_param_is: false_type {};

template<int N, class T, class Result, class... Args>
struct nth_param_is<N, T, Result(*)(Args...)>: nth_template_param_is<N, T, Args...> { };

struct SomeClass {

      template <typename Function, typename... Arguments>
        typename enable_if<nth_param_is<0, SomeClass *, Function>::value >::type create(HWND hWnd, Function func, Arguments&&... args)
        {
            // check if function's 1st argument = class pointer

            std::function<void()> function = std::function<void()>(std::bind(std::forward<Function>(func), this, std::forward<Arguments>(args)...)); 
            cout << "function with 1st argument = class pointer" << endl;
            function();
        }
      template <typename Function, typename... Arguments>
        typename enable_if<!nth_param_is<0, SomeClass *, Function>::value >::type create(HWND hWnd, Function func, Arguments&&... args)
        {
            // else
            cout << "function with 1st argument = non-class pointer" << endl;
            std::function<void()> function = std::function<void()>(std::bind(std::forward<Function>(func), std::forward<Arguments>(args)...)); 
            function();
        }
};
        void ThreadProc1(SomeClass* pThis, HWND hwnd)
        {
             cout << "ThreadProc1 invoked" << endl;
             // do some stuff in here
        }

        void ThreadProc2(HWND hwnd)
        {

             cout << "ThreadProc2 invoked" << endl;
             // do some stuff in here
        }



int main() {
HWND hwnd;
SomeClass test;
test.create(hwnd, ThreadProc1, hwnd);
test.create(hwnd, ThreadProc2, hwnd);
}

修改

我在头文件之间移动了功能，一切正常例如：

my_traits.h：

#ifndef MY_TRAITS_H #define MY_TRAITS_H #include <type_traits> template<int N, class T, class... Args> struct nth_template_param_is: std::false_type { }; template<int N, class T, class First, class... Others> struct nth_template_param_is<N, T, First, Others...>: nth_template_param_is<N-1, T, Others...> { }; template<class T, class First, class... Others> struct nth_template_param_is<0, T, First, Others...>: std::is_same<T, First> { }; template<int N, class T, class Foo> struct nth_param_is: std::false_type {}; template<int N, class T, class Result, class... Args> struct nth_param_is<N, T, Result(*)(Args...)>: nth_template_param_is<N, T, Args...> { }; #endif

some_class.h：

#ifndef SOME_CLASS_H #define SOME_CLASS_H #include <type_traits> #include <iostream> #include <functional> #include "my_traits.h" typedef int HWND; struct SomeClass { // check if function's 1st argument = class pointer template <typename Function, typename... Arguments> typename std::enable_if<nth_param_is<0, SomeClass *, Function>::value >::type create(HWND hWnd, Function func, Arguments&&... args) { std::function<void()> function = std::function<void()>(std::bind(std::forward<Function>(func), this, std::forward<Arguments>(args)...)); std::cout << "function with 1st argument = class pointer" << std::endl; function(); } template <typename Function, typename... Arguments> typename std::enable_if<!nth_param_is<0, SomeClass *, Function>::value >::type create(HWND hWnd, Function func, Arguments&&... args) { std::cout << "function with 1st argument = non-class pointer" << std::endl; std::function<void()> function = std::function<void()>(std::bind(std::forward<Function>(func), std::forward<Arguments>(args)...)); function(); } }; #endif

usage.cc：

#include <iostream> #include "some_class.h" void ThreadProc1(SomeClass* pThis, HWND hwnd) { std::cout << "ThreadProc1 invoked" << std::endl; // do some stuff in here } void ThreadProc2(HWND hwnd) { std::cout << "ThreadProc2 invoked" << std::endl; // do some stuff in here } void ThreadProc3(SomeClass* pThis, int arg1, int arg2) { std::cout << "ThreadProc3 invoked" << std::endl; // do some stuff in here } int main() { HWND hwnd; SomeClass test; test.create(hwnd, ThreadProc1, hwnd); test.create(hwnd, ThreadProc2, hwnd); test.create(hwnd, ThreadProc3, 1, 2); }

汇编：

g++ -std=c++11 usage.cc

用法：

./a.out

输出：

function with 1st argument = class pointer ThreadProc1 invoked function with 1st argument = non-class pointer ThreadProc2 invoked function with 1st argument = class pointer ThreadProc3 invoked

Answer 2

我相信std :: is_same可能对你有用https://jsfiddle.net/pz1yd11u/1/

传递给模板的check函数在参数中有class

2 个答案: