C++ Primer Plus 学习笔记第十六章函数符

朱雀 2022-01-12 09:11 296阅读 0赞

啥是函数符

![20190621152748135.png][]

有定义了operator()()的类

函数符概念

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70][]

这些都是概念 就是一个称呼

程序示例

#include <iostream>
    #include <list>
    #include <iterator>
    // STL的算法库
    #include <algorithm>
    
    template<class T>
    class TooBig
    {
      private:
        T cutoff;
      public:
        TooBig(const T & t) : cutoff(t) {}
    // 定义了operator()()
        bool operator()(const T & v) {return v > cutoff;}
    };
    
    void outint(int n) {std::cout << n << " ";}
    
    int main()
    {
      using std::list;
      using std::cout;
      using std::endl;
    
      TooBig<int> f100(100);
      int vals[10] = {50, 100, 90, 180, 60, 210, 415, 88, 188, 201};
      list<int> yadayada(vals, vals + 10);
      list<int> etcetera(vals, vals + 10);
      cout << "Original lists:\n";
      for_each(yadayada.begin(), yadayada.end(), outint);
      cout << endl;
      for_each(etcetera.begin(), etcetera.end(), outint);
      cout << endl;
    // list容器的内置函数 remove_if() 会将容器中的每个元素丢给f100 将f100这个类对象当做函数来调用（类名后面加括号 ）这样就调用了operator()函数 返回true的话 就删除元素
      yadayada.remove_if(f100);
      etcetera.remove_if(TooBig<int>(200));
      cout << "Trimmed lists:\n";
      for_each(yadayada.begin(), yadayada.end(), outint);
      cout << endl;
      for_each(etcetera.begin(), etcetera.end(), outint);
      cout << endl;
      return 0;
    }

运行结果

![20190621154943937.png][]

函数适配器

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 1][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 2][]

预定义的函数符

就是C++ 内置的函数符

在<functional>中

C++ 针对内置的算术运算符、关系运算符和逻辑运算符 都提供了预定义的函数符

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 3][]

自适应函数符

自适应生成器，自适应一元函数，自适应二元函数，自适应谓词， 自适应二院谓词

自适应函数符中有标识参数类型，返回类型的成员变量result\_type, first\_argument\_type, second\_argument\_type(二元函数的第二个参数类型名称）

STL内置binder1st和binder2nd类（函数适配器）将二元函数转成一元函数

![20190621161757637.png][]

另一个适配器：

bind1st() 其实就是将binder1st做成匿名函数而已

用法：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 4][]

#include <iostream>
    #include <vector>
    #include <iterator>
    #include <algorithm>
    #include <functional>
    
    void Show(double);
    const int LIM = 6;
    int main()
    {
      using namespace std;
      double arr1[LIM] = {28, 29, 30, 35, 38, 59};
      double arr2[LIM] = {63, 65, 69, 75, 80, 99};
      vector<double> gr8(arr1, arr1 + LIM);
      vector<double> m8(arr2, arr2 + LIM);
      cout.setf(ios_base::fixed);
      cout.precision(2);
      cout << "gr8:\t";
      for_each(gr8.begin(), gr8.end(), Show);
      cout <<endl;
      cout << "m8: \t";
      for_each(m8.begin(), m8.end(), Show);
      cout <<endl;
    
      vector<double> sum(LIM);
      transform(gr8.begin(), gr8.end(), m8.begin(), sum.begin(),
          plus<double>());
      cout << "sum:\t";
      for_each(sum.begin(), sum.end(), Show);
      cout << endl;
    
      vector<double> prod(LIM);
      transform(gr8.begin(), gr8.end(), prod.begin(),
    // 这里使用了bind1st 其实就是建立了类似binder1st()的匿名函数 然后将gr8的每个元素和2.5丢给multiplies对象的operator()
                bind1st(multiplies<double>(), 2.5));
      cout << "prod:\t";
      for_each(prod.begin(), prod.end(), Show);
      cout << endl;
    
      return 0;
    }
    
    void Show(double v)
    {
      std::cout.width(6);
      std::cout << v << ' ';
    }

运行结果

![20190621170817319.png][]

c++11提供了lambda表达式（python的匿名函数标识符么？ 不知道 18章会说到）

函数对象完结

[20190621152748135.png]: /images/20220109/640acf47f6af41ee81db6e875f521b17.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70]: /images/20220109/e3f71aab4db949f5966a3a84ce7c9b6e.png
[20190621154943937.png]: /images/20220109/a8929e75dbe046169182f05f1aa888dd.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 1]: /images/20220109/b04176d460ff451d8b7b39ed76a0ab26.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 2]: /images/20220109/fe644ddad2cb4531a092f72b67745cb6.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 3]: /images/20220109/4561a771918440dc9b9108f67103a83a.png
[20190621161757637.png]: /images/20220109/dde4c5c6cec54f828cc5f175b1c0d719.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 4]: /images/20220109/5cc988b6f2084b16ac0a4b93264f973e.png
[20190621170817319.png]: /images/20220109/b7d325f6ae034341ab052a787ac21ca6.png