从类中存储函数并在数组中调用它们c ++

Question

我尝试从数组中的类存储函数(方法)并使用它们.错误句柄是

在函数'int main()'中:| '范围'未在此范围内声明

这个我的代码(我删除不必要的代码)class.h:

class Calculator
{
    public:
        int num1,num2;
        void (Calculator::*actions[4])();
        void  add();
        void  minuz();
        void  multi();
        void  div();
        Calculator();
};

class.cpp:

void  Calculator::add()
{}
void  Calculator::minuz()
{}
void  Calculator::div()
{ }
void Calculator::multi()
{}
Calculator::Calculator()
{
    actions[0]=add;
    actions[1]=minuz;
    actions[2]=div;
    actions[3]=multi;
}

主要:

Calculator cal;
.....
.....
cal.*actions[num]();

Answer 1

使用typedef通常会有所帮助:(c ++ 03)

Live On Coliru

class Calculator
{
    public:
        int num1,num2;
        typedef void (Calculator::*Action)();
        Action actions[4];
        Calculator() {
            actions[0]=&Calculator::add;
            actions[1]=&Calculator::minuz;
            actions[2]=&Calculator::div;
            actions[3]=&Calculator::multi;
        }
    private:
        void add()   {}
        void minuz() {}
        void multi() {}
        void div()   {}
};

int main() {
    Calculator cal;
    (cal.*cal.actions[1])();
}

C++ 11别名

C++ 11使它更容易:

    using Action = void (Calculator::*)();
    Action actions[4];

另见https://isocpp.org/wiki/faq/pointers-to-members#fnptr-vs-memfnptr-types

Live On Coliru

std::function<>

同样在c ++ 11中(如果你想在c ++ 03中使用它,还可以提升):

    using Action = std::function<void(Calculator&)>;
    Action actions[4];

您仍然可以称之为

cal.actions[1](cal);

我预先绑定到Calculator实例:

Live On Coliru

#include <functional>

class Calculator
{
    public:
        int num1,num2;
        using Action = std::function<void()>;
        Action actions[4];
        Calculator() {
            actions[0] = [this]() { add(); };
            actions[1] = [this]() { minuz(); };
            actions[2] = [this]() { multi(); };
            actions[3] = [this]() { div(); };
        }
    private:
        void add()   {}
        void minuz() {}
        void multi() {}
        void div()   {}
};

int main() {
    Calculator cal;
    cal.actions[1]();
}

Answer 2

函数指针声明的 C++ 语法非常复杂，因此最好使用typedefs 通过指针调用函数，您需要额外的()解引用函数指针。最后会是：

class Calculator
{
    public:
        typedef void (Calculator::*action)();
        int num1,num2;
        action actions[4];
        void  add();
        void  minuz();
        void  multi();
        void  div();
        Calculator();
};

void  Calculator::add()
{}
void  Calculator::minuz()
{}
void  Calculator::div()
{ }
void Calculator::multi()
{}
Calculator::Calculator()
{
    actions[0]=&Calculator::add;
    actions[1]=&Calculator::minuz;
    actions[2]=&Calculator::div;
    actions[3]=&Calculator::multi;
}

int main(int, char**) {
    Calculator cal;
    int num = 0;
    (cal.*cal.actions[num])();
    return 0;
}

为了更好的可读性，我建议添加功能Calculator::call_by_index(int)：

void Calculator::call_by_index(int index)
{
    (this->*actions[index])();
}

并以这种方式调用它：

cal.call_by_index(num);