没有用于调用for_each的匹配函数

Question

这是代码:

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

map<string, int> g_map;
void read_item(const pair<string, int>& p) {
   g_map[p.first] += p.second;
}

void myprint(std::pair<const string, int> ci) {
   cout << "first : " << ci.first << "seconde : " << ci.second << endl;
}

void myprint(int ci) {
   cout << ci << endl;
}

int main()
{
    string a = string("nail");
    string b = string("hammer");
    read_item(make_pair(a, 100));
    read_item(make_pair(b, 2));
    read_item(make_pair(b, 10));
    read_item(make_pair(a, 200));

    std::for_each(g_map.begin(), g_map.end(), myprint); // can't find the matching function here
    vector<int> vec;
    vec.push_back(3);
    vec.push_back(3);
    std::for_each(vec.begin(), vec.end(), myprint);  // and here
    return 0;
}

我重载了这个功能myprint,我认为应该可以工作,但事实并非如此.当我改变了第一myprint至myprint1第二要myprint2,它的工作原理.任何身体都可以帮忙？编译错误是:

funcTemOverload.cpp: In function 'int main()':
funcTemOverload.cpp:29:54: error: no matching function for call to 'for_each(std::map<std::basic_string<char>, int>::iterator, std::map<std::basic_string<char>, int>::iterator, <unresolved overloaded function type>)'
funcTemOverload.cpp:29:54: note: candidate is:
In file included from d:\mingw\bin\../lib/gcc/x86_64-w64-mingw32/4.7.2/include/c++/algorithm:63:0,
         from funcTemOverload.cpp:2:
d:\mingw\bin\../lib/gcc/x86_64-w64-mingw32/4.7.2/include/c++/bits/stl_algo.h:4436:5: note: template<class _IIter, class _Funct> _Funct std::for_each(_IIter, _IIter, _Funct)
d:\mingw\bin\../lib/gcc/x86_64-w64-mingw32/4.7.2/include/c++/bits/stl_algo.h:4436:5: note:   template argument deduction/substitution failed:
funcTemOverload.cpp:29:54: note:   couldn't deduce template parameter '_Funct'
funcTemOverload.cpp:35:50: error: no matching function for call to 'for_each(std::vector<int>::iterator, std::vector<int>::iterator, <unresolved overloaded function type>)'
funcTemOverload.cpp:35:50: note: candidate is:
In file included from d:\mingw\bin\../lib/gcc/x86_64-w64-mingw32/4.7.2/include/c++/algorithm:63:0,
         from funcTemOverload.cpp:2:
d:\mingw\bin\../lib/gcc/x86_64-w64-mingw32/4.7.2/include/c++/bits/stl_algo.h:4436:5: note: template<class _IIter, class _Funct> _Funct std::for_each(_IIter, _IIter, _Funct)
d:\mingw\bin\../lib/gcc/x86_64-w64-mingw32/4.7.2/include/c++/bits/stl_algo.h:4436:5: note:   template argument deduction/substitution failed:
funcTemOverload.cpp:35:50: note:   couldn't deduce template parameter '_Funct'

Answer 1

根据要求,原始代码失败的原因是UnaryFunction模板参数std::for_each仅用于"非推断的上下文",并且未明确指定.[temp.deduct.type]/4:

...如果模板参数仅在非推导的上下文中使用且未明确指定,则模板参数推断将失败.

UnaryFunction 被视为非推断的上下文因为 [temp.deduct.call]/6

当P [UnaryFunction]是一个函数类型,指向函数类型的指针,或指向成员函数类型的指针...如果参数[myprint]是一个重载集(不包含函数模板),则尝试使用每个函数模板进行试验推论.成员.如果仅对其中一个重载集成员进行推导成功,则该成员将用作推导的参数值.如果对重载集的多个成员进行推导成功,则将该参数视为非推导上下文.

一种选择是添加强制转换:

std::for_each(
    g_map.begin(), g_map.end(),
    static_cast<void (*)(std::pair<const string, int>)>(myprint));

std::for_each(
    vec.begin(), vec.end(),
    static_cast<void (*)(int)>(myprint));

另一种选择是使其myprint成为一个函数对象,以便在内部发生重载解析std::for_each:

struct myprint {
    void operator()(std::pair<const string, int> ci) const {
        cout << "first : " << ci.first << "seconde : " << ci.second << endl;
    }
    void operator()(int ci) const {
        cout << ci << endl;
    }
};

std::for_each(g_map.begin(), g_map.end(), myprint());
std::for_each(vec.begin(), vec.end(), myprint());