C++ STL mem_fun / mem_fun_ref /

说明

这些函数都作用如其名称：member function，都是为了取得成员函数对象。

mem_fun和mem_fun_ref是C++98标准的。
mem_fn是C++11标准的，可以取代mem_fun和mem_fun_ref。

mem_fun是指针版的把成员函数转换成函数对象。
mem_fun_ref是引用版的把成员函数转换成函数对象。
mem_fn则是无论指针还是引用都可以把成员函数转换为函数对象。
下面详细说明。

头文件

#include <functional>

mem_fun_ref

例子如下：

#include <iostream>
#include <functional>
#include <vector>
#include <algorithm>
#include <string>

using namespace std;


int main(int argc, char **argv) 
{  
    vector<string> numbers;

    // populate vector:
    numbers.push_back("one");
    numbers.push_back("two");
    numbers.push_back("three");
    numbers.push_back("four");
    numbers.push_back("five");

    vector <int> lengths (numbers.size());

    transform (numbers.begin(), numbers.end(), lengths.begin(), mem_fun_ref(&string::length));
    
    for (int i=0; i<5; i++) {
        cout << numbers[i] << " has " << lengths[i] << " letters.\n";
    }
  
    return 0;
}

运行结果如下：

one has 3 letters.
two has 3 letters.
three has 5 letters.
four has 4 letters.
five has 4 letters.

代码 transform (numbers.begin(), numbers.end(), lengths.begin(), mem_fun_ref(&string::length));中mem_fun_ref取得string的成员函数length()，从而取得numbers中每个string对象的长度，存入lengths容器中

mem_fun

如果上述numbers容器存储的不是string对象，而是string指针，则不能使用mem_fun_ref，而应该使用mem_fun，代码如下：

#include <iostream>
#include <functional>
#include <vector>
#include <algorithm>
#include <string>

using namespace std;


int main(int argc, char **argv) 
{  
    vector <string*> numbers;

    // populate vector of pointers:
    numbers.push_back ( new string ("one") );
    numbers.push_back ( new string ("two") );
    numbers.push_back ( new string ("three") );
    numbers.push_back ( new string ("four") );
    numbers.push_back ( new string ("five") );

    vector <int> lengths ( numbers.size() );

    transform (numbers.begin(), numbers.end(), lengths.begin(), mem_fun(&string::length));
    
    for (int i=0; i<5; i++) {
      cout << *numbers[i] << " has " << lengths[i] << " letters.\n";
    }

    // deallocate strings:
    for (vector<string*>::iterator it = numbers.begin();   it!=numbers.end(); ++it)
      delete *it;

    return 0;
}

结果如下：

one has 3 letters.
two has 3 letters.
three has 5 letters.
four has 4 letters.
five has 4 letters.

mem_fn

mem_fn则无论是指针还是引用都可以使用，是C++11标准的，上面的例子可以使用mem_fn替代如下：
只要把mem_fun_ref或者mem_fun改成mem_fn即可。

指针

#include <iostream>
#include <functional>
#include <vector>
#include <algorithm>
#include <string>

using namespace std;


int main(int argc, char **argv) 
{  
    vector <string*> numbers;

    // populate vector of pointers:
    numbers.push_back ( new string ("one") );
    numbers.push_back ( new string ("two") );
    numbers.push_back ( new string ("three") );
    numbers.push_back ( new string ("four") );
    numbers.push_back ( new string ("five") );

    vector <int> lengths ( numbers.size() );

    transform (numbers.begin(), numbers.end(), lengths.begin(), mem_fn(&string::length));
    
    for (int i=0; i<5; i++) {
      cout << *numbers[i] << " has " << lengths[i] << " letters.\n";
    }

    // deallocate strings:
    for (vector<string*>::iterator it = numbers.begin();   it!=numbers.end(); ++it)
      delete *it;

    return 0;
}

引用

#include <iostream>
#include <functional>
#include <vector>
#include <algorithm>
#include <string>

using namespace std;


int main(int argc, char **argv) 
{  
    vector<string> numbers;

    // populate vector:
    numbers.push_back("one");
    numbers.push_back("two");
    numbers.push_back("three");
    numbers.push_back("four");
    numbers.push_back("five");

    vector <int> lengths (numbers.size());

    transform (numbers.begin(), numbers.end(), lengths.begin(), mem_fn(&string::length));
    
    for (int i=0; i<5; i++) {
        cout << numbers[i] << " has " << lengths[i] << " letters.\n";
    }
  
    return 0;
}

结果都是：

one has 3 letters.
two has 3 letters.
three has 5 letters.
four has 4 letters.
five has 4 letters.

扩展

mem_fun_ref mem_fun mem_fn都是针对无参数函数的。
bind可替代它们，且还可以使用占位符来转换有参数的函数。
具体使用可参考：
https://zh.cppreference.com/w/cpp/utility/functional/bind

参考

http://www.cplusplus.com/reference/functional/mem_fun/
http://www.cplusplus.com/reference/functional/mem_fun_ref/
http://www.cplusplus.com/reference/functional/mem_fn/