std::for_each 和仿函数 - operator() 可以有哪些签名？

Question

我想使用std::for_each 调用循环一些值并调用函子将值映射到字符串，然后将该值返回给for_each，如下所示：

#include <iostream>
#include <map>
#include <string>
using std::string;
using std::map;
using std::cout;
using std::endl;

struct myfunctor
{
    myfunctor(map<int, string>& x) : mymap(x) {
        cout << "Inside myfunctor ctor" << endl;
    };

    map<int, string>& mymap;

    string operator()(int mapthis)
    {
        cout << "Inside myfunctor operator(" << mapthis << ")" << endl;
        return mymap[mapthis];
    }
};

int main()
{
    map<int, string> codes { {1, "abel"}, {2, "baker"}, {3, "charlie"} };

    cout << "Main() - construct myfunctor" << endl;
    myfunctor t(codes);

    int arr[] = {1, 2, 3};

    string blah;
    cout << "Main() - begin for_each" << endl;
    std::for_each(arr, arr+2, blah.append(t));
    cout << blah << endl;
}

它无法编译，因为它无法从 myfunctor 转换为 string。但即使它确实如此，从operator() 返回一些东西是否合法，以便由for_each 应用，就像我正在尝试做的那样？除了for_each的隐含范围变量之外，是否可以将其他参数传递给operator()？

如果operator()可以有返回值，我该如何写一个myfunctor-to-string的转换方法？

我从未见过除void operator()(SingleArgument)以外的任何例子

Answer 1

首先，您必须将函子作为第三个参数传递给std::for_each。您正在传递函数调用blah.append(t) 的结果。此外，函子应该在for_each 期间完成所有工作，因此您必须将对append 的调用转移到operator() 内部。最简单的方法：只需将您的字符串作为参考传递。

struct myfunctor
{
    myfunctor( string& toAppend, map<int, string>& x ) : appendage(toAppend), mymap( x ) {
        cout << "Inside myfunctor ctor" << endl;
    };

    map<int, string>& mymap;
    string& appendage;

    void operator()(int mapthis)
    {
        cout << "Inside myfunctor operator(" << mapthis << ")" << endl;
        appendage.append( mymap[mapthis] );
            //std::for_each will not use a return value of this function for anything
    }
};

void main2()
{
    map<int, string> codes { {1, "abel"}, { 2, "baker" }, { 3, "charlie" } };

    cout << "Main() - construct myfunctor" << endl;


    int arr[] = { 1, 2, 3 };

    string blah;
    myfunctor t( blah, codes ); //Pass in blah so the functor can append to it.
    cout << "Main() - begin for_each" << endl;
    std::for_each( arr, arr + 3, t ); //arr+3 to loop over all 3 elems of arr
    cout << blah << endl;
}