Question

在尝试使用可变参数模板实现 Delegate - 类时，我遇到了一个我无法解决的问题：

/// --------------------------------------
/// @thanks     God
///             Steve Reinalter
/// @author     Henri Korpela aka Helixirr
/// --------------------------------------
#include <cstdio>

template<typename>
class Delegate;

template<typename Return, typename Param, typename... ParamsOther>
class Delegate<Return (Param, ParamsOther...)>{
public:

    /// Constructors & destructors:
    Delegate(void) = default;
    Delegate(Delegate const& delegate_) = default;
    Delegate(Delegate&& delegate_) = default;

    /// Member functions:
    Delegate& bind(Return (*function_)(Param, ParamsOther...));
    template<class C>
    Delegate& bind(C& c_, Return (C::*function_)(Param, ParamsOther...));

    /// Member functions (overloaded operators, assignment):
    Delegate& operator=(Delegate const& delegate_) = default;
    Delegate& operator=(Delegate&& delegate_) = default;

    /// Member functions (overloaded operators, function call):
    inline Return operator()(Param param_, ParamsOther... params_other_) const;

private:
    /// Member data:
    Return (*_m_opFunction)(Param, ParamsOther...) = nullptr;
    void* _m_opInstance = nullptr;
    /// Static member functions:
    template<class C, Return (C::*Function)(Param, ParamsOther...)>
    static inline Return _wrap_function_member(void* instance_, Param param_, ParamsOther... params_other_);
    template<Return (*Function)(Param, ParamsOther...)>
    static inline Return _wrap_function_static(void*, Param param_, ParamsOther... params_other_);
};

/// Member functions:
template<typename Return, typename Param, typename... ParamsOther>
Delegate<Return (Param, ParamsOther...)>& Delegate<Return (Param, ParamsOther...)>::bind(Return (*function_)(Param, ParamsOther...)){
    _m_opFunction = &_wrap_function_static<decltype(function_)>;
    _m_opInstance = nullptr;
    return *this;
}
template<typename Return, typename Param, typename... ParamsOther>
template<class C>
Delegate<Return (Param, ParamsOther...)>& Delegate<Return (Param, ParamsOther...)>::bind(C& c_, Return (C::*function_)(Param, ParamsOther...)){
    _m_opFunction = &_wrap_function_member<C, decltype(function_)>;
    _m_opInstance = &c_;
    return *this;
}
/// Member functions (overloaded operators, function call):
template<typename Return, typename Param, typename... ParamsOther>
Return Delegate<Return (Param, ParamsOther...)>::operator()(Param param_, ParamsOther... params_other_) const{
    return _m_opFunction(_m_opInstance, param_, params_other_...);
}
/// Static member functions:
template<typename Return, typename Param, typename... ParamsOther>
template<class C, Return (C::*Function)(Param, ParamsOther...)>
Return Delegate<Return (Param, ParamsOther...)>::_wrap_function_member(void* instance_, Param param_, ParamsOther... params_other_){
    return (static_cast<C*>(instance_)->*Function)(param_, params_other_...);
}
template<typename Return, typename Param, typename... ParamsOther>
template<Return (*Function)(Param, ParamsOther...)>
Return Delegate<Return (Param, ParamsOther...)>::_wrap_function_static(void*, Param param_, ParamsOther... params_other_){
    return (Function)(param_, params_other_...);
}

int f(int i_){
    return i_ * 2;
}

int main(void){
    Delegate<int (int)> delegate__;
    delegate__.bind(&f);
    printf("Result: %i\n", delegate__(8));
    return 0;
}

我尝试使用C ++ 11编译器（ GCC 4.7.2 ）在 Ideone 上编译它，但它似乎失败了：

prog.cpp：实例化'代表＆amp; Delegate :: bind（Return（*）（Param，ParamsOther ...））[with Return = int; Param = int; ParamsOther = {}]'： prog.cpp：79：23：从这里要求 prog.cpp：45：5：错误：没有匹配将函数'_wrap_function_static'转换为'int（*）（int）'类型 prog.cpp：39：26：错误：候选者是：template static返回Delegate :: _ wrap_function_static（void *，Param，ParamsOther ...）[with Return（* Function）（Param，ParamsOther ...）= Function; Return = int; Param = int; ParamsOther = {}] prog.cpp：实例化'Return Delegate :: operator（）（Param，ParamsOther ...）const [with Return = int; Param = int; ParamsOther = {}]'： prog.cpp：80：40：从这里需要 prog.cpp：59：65：错误：从'void *'无效转换为'int'[-fpermissive] prog.cpp：59：65：错误：函数的参数太多

根据我的理解， decltype 和函数指针在这里

template<typename Return, typename Param, typename... ParamsOther>
Delegate<Return (Param, ParamsOther...)>& Delegate<Return (Param, ParamsOther...)>::bind(Return (*function_)(Param, ParamsOther...)){
    _m_opFunction = &_wrap_function_static<decltype(function_)>;
    _m_opInstance = nullptr;
    return *this;
}

似乎导致了这个问题。当我尝试将成员函数绑定到委托时，也会发生同样的情况。为什么会这样？我究竟做错了什么？对我来说，获取函数指针的类型并使用该类型作为模板参数似乎很自然，但由于某种原因，它在这里不起作用。 这个decltype和函数指针场景有什么问题？

Answer 1

以下是来自clang ++ 3.2的错误消息的第一部分：

temp.cpp:41:19: error: assigning to 'int (*)(int)' from incompatible type
      '<overloaded function type>'
    _m_opFunction = &_wrap_function_static<decltype(function_)>;
                  ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
temp.cpp:75:16: note: in instantiation of member function 'Delegate<int
      (int)>::bind' requested here
    delegate__.bind(&f);
               ^
temp.cpp:35:26: note: candidate function has different number of parameters
      (expected 1 but has 2)
    static inline Return _wrap_function_static(void*, Param param_,...
                         ^


temp.cpp:55:41: error: too many arguments to function call, expected 1, have 2
    return _m_opFunction(_m_opInstance, param_, params_other_...);
           ~~~~~~~~~~~~~                ^~~~~~
temp.cpp:76:38: note: in instantiation of member function 'Delegate<int
      (int)>::operator()' requested here
    printf("Result: %i\n", delegate__(8));

这是因为_m_opFunction：

的声明

Return (*_m_opFunction)(Param, ParamsOther...) = nullptr;

template<Return (*Function)(Param, ParamsOther...)>
static inline Return _wrap_function_static(void*, Param param_, ParamsOther... params_other_);

也就是说，_wrap_function_static期望void*然后参数转发到函数调用，而_m_opFunction只需要函数调用的参数。

标准库解决方案：

#include <iostream>
#include <functional>

int f(int i_){
    return i_ * 2;
}

struct foo
{
    int m;
    int f(int i) { return i * m; }
};

int main()
{
    std::function<int (int)> delegate__;

    delegate__ = f;
    std::cout << "Result: " << delegate__(8) << std::endl;

    foo o;
    o.m = 21;
    delegate__ = std::bind(&foo::f, std::ref(o), std::placeholders::_1);
    std::cout << "Result: " << delegate__(2) << std::endl;
}

尝试修复您的方法：注意：您不能将成员函数指针转换为“普通”函数指针（它可以使用联合或复制原始数据... UB）。更好的方法是使用多态（即虚函数和调用者对象的动态分配）。

#include <cstdio>

template<typename>
class Delegate;

template<typename Return, typename Param, typename... ParamsOther>
class Delegate<Return (Param, ParamsOther...)>{
public:

    /// Constructors & destructors:
    Delegate(void) = default;
    Delegate(Delegate const& delegate_) = default;
    Delegate(Delegate&& delegate_) = default;

    /// Member functions:
    Delegate& bind(Return (*function_)(Param, ParamsOther...));
    template<class C>
    Delegate& bind(C& c_, Return (C::*function_)(Param, ParamsOther...));

    /// Member functions (overloaded operators, assignment):
    Delegate& operator=(Delegate const& delegate_) = default;
    Delegate& operator=(Delegate&& delegate_) = default;

    /// Member functions (overloaded operators, function call):
    inline Return operator()(Param param_, ParamsOther... params_other_) const;

private:
    /// Member data:
    Return (*_m_opFunction)(Param, ParamsOther...) = nullptr;
    Return (Delegate::*_m_opMemFunction)(Param, ParamsOther...) = nullptr;
    void* _m_opInstance = nullptr;

    /// function wrappers:
    template<class C>
    static inline Return _wrap_member_function(Delegate const&, Param param_, ParamsOther... params_other_);
};

/// Member functions:
template<typename Return, typename Param, typename... ParamsOther>
Delegate<Return (Param, ParamsOther...)>& Delegate<Return (Param, ParamsOther...)>::bind(Return (*function_)(Param, ParamsOther...)){
    _m_opFunction = function_;
    _m_opMemFunction = nullptr;
    _m_opInstance = nullptr;
    return *this;
}
template<typename Return, typename Param, typename... ParamsOther>
template<class C>
Delegate<Return (Param, ParamsOther...)>& Delegate<Return (Param, ParamsOther...)>::bind(C& c_, Return (C::*function_)(Param, ParamsOther...)){
    _m_opFunction = reinterpret_cast<decltype(_m_opFunction)>( &_wrap_member_function<C> );
    _m_opMemFunction = reinterpret_cast<decltype(_m_opMemFunction)>( function_ );
    _m_opInstance = &c_;
    return *this;
}
/// Member functions (overloaded operators, function call):
template<typename Return, typename Param, typename... ParamsOther>
Return Delegate<Return (Param, ParamsOther...)>::operator()(Param param_, ParamsOther... params_other_) const{
    if(nullptr == _m_opMemFunction)
    {
        return _m_opFunction(param_, params_other_...);
    }else
    {
        auto f = reinterpret_cast<Return (*)(Delegate const&, Param, ParamsOther...)>( _m_opFunction );
        return f(*this, param_, params_other_...);
    }
}
/// function wrappers:
template<typename Return, typename Param, typename... ParamsOther>
template<class C>
Return Delegate<Return (Param, ParamsOther...)>::_wrap_member_function(Delegate<Return (Param, ParamsOther...)> const& instance_, Param param_, ParamsOther... params_other_){
    Return (C::*memFuncPtr)(Param, ParamsOther...) = reinterpret_cast<decltype(memFuncPtr)>( instance_._m_opMemFunction );
    return (reinterpret_cast<C*>(instance_._m_opInstance)->*memFuncPtr)(param_, params_other_...);
}

int f(int i_){
    return i_ * 2;
}

struct foo
{
    int m;
    int f(int i) { return i * m; }
};

int main(void){
    Delegate<int (int)> delegate__;
    delegate__.bind(&f);
    printf("Result: %i\n", delegate__(8));

    foo o;
    o.m = 21;
    delegate__.bind(o, &foo::f);
    printf("Resilt: %i\n", delegate__(2));
    return 0;
}

为什么我说“设计存在缺陷”：

标准库已包含“委托”类型（std::function）
- StdLib绑定中的成员函数的参数顺序不同（std::bind(&A::func, instance) vs delegate__.bind(instance, &A::func)）
- StdLib绑定复制实例，您必须显式使用std::ref（或指针）来传递引用＆lt; - 更不容易出错，因为很明显您必须保持实例处于活动状态，直到调用该函数为止< / LI>
函数调用复制参数（更好：完美转发）
为什么要在Param和ParamOthers？
与仿函数不兼容

C ++ 11 - 这种使用decltype函数指针有什么问题？

1 个答案: