c++ - 映射中具有重载 () 运算符的仿函数 - 仅按需评估

Question

我想要一个具有重载的 () 运算符 的仿函数对象，并将这些重载版本中的每一个存储在映射中。这样我就可以将我的逻辑放在一个类中，这里是我试图完成的一小段代码:

#include <iostream>
#include <string>
#include <map>
using namespace std;

class functor{
public:
    void operator()(int i){
        cout << i << endl;
    }
    void operator()(string s){
        cout << s << endl;
    }
    void operator()(int i, int j){
        cout << i+j << endl;
    }
};

int main(){
    // i know i should put something here thats not "functor" 
    // but i have no idea what. 
    map<int,functor> hTable;

    // is there a way to add elements into a table somehow like this...
    hTable[0] = functor()(2);
    hTable[1] = functor()("foo and bar are overrated, boo.");
    hTable[2] = functor()(2,3);

    // and fire the function in the table simply like this?
    hTable[0];

    cin.get();
    return 0;
}

Answer 1

您想将某些参数“绑定(bind)”到一个函数吗？当然!

#include <functional>
int main(){
    // A map of integers to functions
    //each function takes no parameters () and returns nothing (void)
    map<int,std::function<void()>> hTable;

    // add elements into a table like this...
    hTable[0] = std::bind(functor(), 2);
    hTable[1] = std::bind(functor(), "foo and bar are overrated, boo.");
    hTable[2] = std::bind(functor(), 2, 3);

    // and fire the function in the table simply like this
    hTable[0]();

    cin.get();
    return 0;
}

现场观看:http://coliru.stacked-crooked.com/a/8042e98b19ccbf6b

此外，std::function 和 std::bind 可以在 functionoids(就像你的)、lambdas、函数指针、成员函数上工作......它们是惊人的。 std::bind 还有很棒的占位符:

double divide(int left, int right) {return (double)left/right;}

//brings _1 into the current scope
using std::placeholders; 
//bind the first input as the first parameter, bind 100 to the second parameter 
std::function<double(int)> percent = std::bind(divide, _1, 100); 
//the resulting function only has one input:
double half = percent(50);