c++ - 模仿 std::function 模板参数

Question

我想模仿std::function模板参数，但我不知道它是如何工作的。 例如考虑以下代码:

std::function<int(int)> p;

如何编写模仿此模板参数的类模板 <int(int)> ？

template<typename ...>   <-- what should be here instead of `...`
MyClass

我真正想要实现的是我希望能够 typedef <int(int)>作为函数指针，我希望它不仅对于 int (*func)(int) 是通用的功能。

我正在努力实现这样的目标:

SomeSmartStruct<MyClass, int(int)>::MemFuncPointerType pMemFunc;

我要MemFuncPointerType属于以下类型:

int (__thiscall MyClass::* )(int)

还有:

SomeSmartStruct<MyClass, int(int)>::FunctionPointer pFunc;

应该是这种类型:

int (__cdecl *)(int)

我使用的是 VS2010，因此并非支持所有 C++11 功能，但它确实实现了 std::function .

Answer 1

使用可变参数模板:

template <typename C, typename T>
struct make_member_function_pointer;

template <typename C, typename R, typename... Args>
struct make_member_function_pointer<C,R(Args...)>
{
    using type = R(C::*)(Args...);
};

DEMO 1

---

没有可变参数模板:

template <typename T> struct identity { typedef T type; };

template <typename C, typename T>
struct make_member_function_pointer;
template <typename C, typename R, typename Arg1>
struct make_member_function_pointer<C,R(Arg1)> : identity<R(C::*)(Arg1)> {};
template <typename C, typename R, typename Arg1, typename Arg2>
struct make_member_function_pointer<C,R(Arg1,Arg2)> : identity<R(C::*)(Arg1,Arg2)> {};
template <typename C, typename R, typename Arg1, typename Arg2, typename Arg3>
struct make_member_function_pointer<C,R(Arg1,Arg2,Arg3)> : identity<R(C::*)(Arg1,Arg2,Arg3)> {};

DEMO 2

---

用法:

template <typename T, typename F>
struct SomeSmartStruct
{
    typedef typename make_member_function_pointer<T,F>::type MemFuncPointerType;
    typedef F* FunctionPointer;
};

测试:

struct MyClass
{
    int foo(int) {return 0;}
};
int bar(int) {return 0;}

int main()
{
    SomeSmartStruct<MyClass, int(int)>::MemFuncPointerType pMemFunc = &MyClass::foo;
    SomeSmartStruct<MyClass, int(int)>::FunctionPointer pFunc = &bar;
}

---

更新

Can I somehow utilize the preprocessor to auto generate partial specializations of make_member_function_pointer? I've seen something similar is done using BOOST_PP_ITERATION but I don't know how it works.

当然:

#include <boost/preprocessor.hpp>

template <typename T> struct identity { typedef T type; };

template <typename C, typename T>
struct make_member_function_pointer;

#define BOOST_PP_LOCAL_MACRO(n)\
    template <typename C, typename R BOOST_PP_COMMA_IF(n) BOOST_PP_ENUM_PARAMS(n, typename Arg)>\
    struct make_member_function_pointer<C,R(BOOST_PP_ENUM_PARAMS(n, Arg))> : identity<R(C::*)(BOOST_PP_ENUM_PARAMS(n, Arg))> {};
#define BOOST_PP_LOCAL_LIMITS (0, 20) // 20 is the limit of params
#include BOOST_PP_LOCAL_ITERATE()

测试:

int bar10(int,int,int,int,int,int,int,int,int,int) {return 0;}

SomeSmartStruct<MyClass, int(int,int,int,int,int,int,int,int,int,int)>::FunctionPointer pFunc10 = &bar10;

DEMO 3