C++：11.C++中常用函数对象

源码阅读和追踪

仿函数

必须重载()运算符

class Equal{
private:
    int equal_number;
public:
    Equal(int equal_number):equal_number(equal_number){}

    bool operator()(const int& number){
        return equal_number == number;
    }
};

1. 预定义函数对象和函数适配器(bind2nd)

c/c++ 提供了很多定义好的函数对象,常见的几个 less, greater, plus, equal_to，这些都是二元函数

bind1st和bind2nd是将二元函数转换为一元函数，比如一个比较大小的函数是二元函数。 当在某些情况下我们想要固定第一个参数或者第二个参数时，就成了一元函数

• bind2nd(equal_to<string>(),"aaa")
• set<string> :: iterator find_it = find_if(set1.begin(),set1.end(),bind2nd(equal_to<string>(),"aaa"));// 查找是否包含

  int main(){
      // c/c++ 提供了很多定义好的函数对象
      // 常见的几个 less, greater , plus , equal_to
      plus<string> strAdd;
      string str = strAdd("aaa","bbb");
  
      // cout << str.c_str() << endl; // aaabbb
  
      set<string,greater<string>> set1;
      set1.insert("aaa");
      set1.insert("bbb");
      set1.insert("ccc");
  
      // 判断是不是包含 aaa
      // 怎么写仿函数，一定要确定好你的仿函数的参数
      // bind2nd 函数适配器， aaa 相当于 equal_to 中的 right
  
      set<string> :: iterator  find_it = find_if(set1.begin(),set1.end(),bind2nd(equal_to<string>(),"aaa"));
      if(find_it!=set1.end()){
          cout << "找到了" << (*find_it).c_str() << endl;
      }
  }

2.自定义仿函数（函数对象） 和 count_if

class Equal{
private:
    int equal_number;
public:
    Equal(int equal_number):equal_number(equal_number){}

    bool operator()(const int& number){
        return equal_number == number;
    }
};

// count_if
int main(){
    vector<int> vector1;
    vector1.push_back(1);
    vector1.push_back(2);
    vector1.push_back(3);
    vector1.push_back(2);
    vector1.push_back(4);
    vector1.push_back(2);

    // 找集合中 等于 2 的个数
    int count = count_if(vector1.begin(),vector1.end(),Equal(2));
    cout << "count = " << count << endl;

    // 预定义好的函数对象 +  函数适配器
    count = count_if(vector1.begin(),vector1.end(),bind2nd(equal_to<int>(),2));
    cout << "count = " << count << endl;
}

打印结果：

count = 3
count = 3

3. foreach, transform

• transform(vector1.begin(),vector1.end(),vector2.begin(),transform_print);：修改
• for_each(vector2.begin(),vector2.end(),print);：循环遍历元素

void print(int number){
    cout << number << endl;
}

// 进行修改
int transform_print(int number){
    // cout << number << endl;
    return number + 3;
}

int main(){
    vector<int> vector1;
    vector1.push_back(1);
    vector1.push_back(2);
    vector1.push_back(3);
    vector1.push_back(4);

    // for_each(vector1.begin(),vector1.end(),print);
    vector<int> vector2;
    vector2.resize(vector1.size());

    transform(vector1.begin(),vector1.end(),vector2.begin(),transform_print);

    for_each(vector2.begin(),vector2.end(),print);
}

打印结果：

4
5
6
7

4. find, find_if

• find(first, end, value);：返回区间[first,end)中第一个值等于value的元素的位置。如果没有找到匹配元素，则返回end。(只能判断等于)
• find_if (begin, end, func)：从begin开始 ，到end为止，返回第一个让 func这个函数返回true的iterator。（所有判断都可以）

vector<int> vector1;
vector1.push_back(1);
vector1.push_back(2);
vector1.push_back(3);
vector1.push_back(4);

vector<int>::iterator find_it = find(vector1.begin(),vector1.end(),2);
if(find_it!=vector1.end()){
    cout << "包含" << endl;
}else{
    cout << "不包含" << endl;
}

// 有没有大于2的，自定义函数对象，预定义函数对象+函数适配器，省略
vector<int>::iterator find_it1 = find_if(vector1.begin(),vector1.end(),bind2nd(less<int>(),4));
while(find_it1!=vector1.end()){
    if(*find_it1>=4){
        find_it1++;
        continue;
    }
    cout << *find_it1 << endl;
    find_it1++;
}

打印结果：

包含
1
3
2

5. count，count_if

• count(first, end, value);：返回区间[first,end)中值等于value的元素的个数。(只能判断等于)
• count_if(begin, end, func)：从begin开始 ，到end为止，返回让 func这个函数返回true的元素个数。（所有判断都可以）

vector<int> vector1;
    vector1.push_back(1);
    vector1.push_back(2);
    vector1.push_back(3);
    vector1.push_back(2);
    vector1.push_back(4);

    int number = count(vector1.begin(),vector1.end(),2);

    cout << "等于2的个数：" << number << endl;

    number = count_if(vector1.begin(),vector1.end(),bind2nd(less<int>(),2));

    cout << "小于2的个数：" << number << endl;

    number = count_if(vector1.begin(),vector1.end(),bind2nd(greater<int>(),2));

    cout << "大于2的个数：" << number << endl;

打印结果：

等于2的个数：2
小于2的个数：1
大于2的个数：2

6. merge

• merge(vector1.begin(),vector1.end(),vector2.begin(),vector2.end(),vector3.begin());：合并元素

class _merge{
public:
    bool operator()(int number1,int number2){
       return true;
    }
};

void print(int number){
    cout << number << endl;
}

// merge
int main(){
    // 两个有序数组进行合并 - 归并排序
    vector<int> vector1;
    vector1.push_back(1);
    vector1.push_back(2);
    vector1.push_back(3);

    vector<int> vector2;
    vector2.push_back(4);
    vector2.push_back(5);
    vector2.push_back(6);

    vector<int> vector3;
    vector3.resize(6);
    merge(vector1.begin(),vector1.end(),vector2.begin(),vector2.end(),vector3.begin());

    for_each(vector3.begin(),vector3.end(),print);
}

打印结果：

1
2
3
4
5
6

7. sort

• sort(vector1.begin(),vector1.end(),less<int>());：按照从小到大的方式排序
• random_shuffle(vector1.begin(),vector1.end());：打乱 vector1的顺序

vector<int> vector1;
    vector1.push_back(1);
    vector1.push_back(3);
    vector1.push_back(2);
    vector1.push_back(4);

    sort(vector1.begin(),vector1.end(),less<int>());
    for_each(vector1.begin(),vector1.end(),print);

    // 打乱顺序
    random_shuffle(vector1.begin(),vector1.end());
    for_each(vector1.begin(),vector1.end(),print);

8. copy，replace

• copy(vector1.begin(),vector1.begin()+2,vector2.begin());：将vector1的前两位拷贝到vector2中，从vector2的开始位置拷入。
• replace(vector1.begin(),vector1.end(),2,22);：将vector1中等于2的值替换成22

vector<int> vector1;
    vector1.push_back(1);
    vector1.push_back(2);
    vector1.push_back(3);
    vector1.push_back(4);

    vector<int> vector2(2);
    copy(vector1.begin(),vector1.begin()+2,vector2.begin());
    // for_each(vector2.begin(),vector2.end(),print);

    replace(vector1.begin(),vector1.end(),2,22);
    for_each(vector1.begin(),vector1.end(),print);