c++ - 如何声明模板化函数以便可以在类构造函数/函数中传递

Question

我想将用户定义的函数传递给需要用户定义的匹配函数的类。在过去的 C 时代，我会使用带有 void* 参数的函数指针。但必须有更好的方法......

这大概是我想做的事情。我的一个限制是我所在的平台没有标准库。但是基本的核心语言C++11是可用的。

我需要做的:

#include <iostream>

using namespace std;

// TODO - replace this C construct with C++ equivalent
//typedef bool(*match_key)(const void* key1, const void* key2);

// somehow declare this as a typedef?  need a way to declare a signature in C++
typedef template<class T>
bool (*match_key)(const T& key1, const T& key2);


// *** User defined matching function
bool mymatcher(const int i, const int j) {
    return i == j;
}

template<class K>
class hashmap {
public:
    hashmap<K>(const K& key, match_key matchfunc) : key_(key), cmp(matchfunc) { }

    bool matched(const K& key) {
        return cmp(key_, key);
    }

private:
    const K key_;
    match_key cmp;
};


int main()
{
    int i = 3;
    int j = 4;

    hashmap<int> hm(i, mymatcher);
    cout << "i matches j? " << (hm.matched(j) ? "yes" : "no") << endl;

    return 0;
}

Answer 1

#include <iostream>

using namespace std;

// TODO - replace this C construct with C++ equivalent
//typedef bool(*match_key)(const void* key1, const void* key2);

// somehow declare this as a typedef?  need a way to declare a signature in C++
typedef template<class T>
using match_key = bool (*)(const T& key1, const T& key2);


// *** User defined matching function
bool mymatcher(const int i, const int j) {
    return i == j;
}

template<class K>
class hashmap{
public:
    hashmap(const K& key, match_key<K> matchfunc) : key_(key), cmp(matchfunc) { }

    bool matched(const K& key) {
        return cmp(key_, key);
    }

private:
    const K key_;
    match_key<K> cmp;
};


int main()
{
    int i = 3;
    int j = 4;

    hashmap<int> hm(i, mymatcher);
    cout << "i matches j? " << (hm.matched(j) ? "yes" : "no") << endl;

    return 0;
}