函数对象 与 count_if()结合使用 例子

例程一、

/*
|| Using a function object to help count things
*/
#include <string>
#include <list>
#include <algorithm>
#include <iostream>
using namespace std;
const string ToothbrushCode("0003");

class IsAToothbrush {
public: 
 bool operator() ( string& SalesRecord ) {
  return SalesRecord.substr(0,4)==ToothbrushCode;
 }    
};

int main (void) {
 list<string> SalesRecords;
#
 SalesRecords.push_back("0001 Soap");
 SalesRecords.push_back("0002 Shampoo");
 SalesRecords.push_back("0003 Toothbrush");
 SalesRecords.push_back("0004 Toothpaste");
 SalesRecords.push_back("0003 Toothbrush");
#
 int NumberOfToothbrushes(0); 
 NumberOfToothbrushes=count_if (SalesRecords.begin(), SalesRecords.end(),
  IsAToothbrush() ); //第三个参数IsAToothbrush()，它是由它的构造函数临时构造的一个对象
#
 cout << "There were "
  << NumberOfToothbrushes
  << " toothbrushes sold" << endl;
}

例程二、(与前面的例程一功能一致，改用函数来实现)

#include <string>
#include <list>
#include <algorithm>
#include <iostream>
using namespace std;
const string ToothbrushCode("0003");

bool IsAToothbrush(string x)
{
 return x.substr(0,4)==ToothbrushCode;
}
int main (void) {
 list<string> SalesRecords;
#
 SalesRecords.push_back("0001 Soap");
 SalesRecords.push_back("0002 Shampoo");
 SalesRecords.push_back("0003 Toothbrush");
 SalesRecords.push_back("0004 Toothpaste");
 SalesRecords.push_back("0003 Toothbrush");
#
 int NumberOfToothbrushes(0); 
 NumberOfToothbrushes=count_if (SalesRecords.begin(), SalesRecords.end(),
  IsAToothbrush );
#
 cout << "There were "
  << NumberOfToothbrushes
  << " toothbrushes sold" << endl;
}

例程三、(更加复杂的函数对象，传递的信息更多，使用更灵活)
*//*
|| Using a more complex function object
*/
#include <iostream>
#include <string>
#include <list>
#include <algorithm>
using namespace std;
#
class IsAToothbrush {
public:
 IsAToothbrush(string& InToothbrushCode) :
      ToothbrushCode(InToothbrushCode) {}
   bool operator() (string& SalesRecord) {
    return SalesRecord.substr(0,4)==ToothbrushCode;
   }      
private:
 string ToothbrushCode;       
};
#
int main (void) {
 list<string> SalesRecords;
#
 SalesRecords.push_back("0001 Soap");
 SalesRecords.push_back("0002 Shampoo");
 SalesRecords.push_back("0003 Toothbrush");
 SalesRecords.push_back("0004 Toothpaste");
 SalesRecords.push_back("0003 Toothbrush");
#
 string VariableToothbrushCode("0003");
#
 int NumberOfToothbrushes(0); 
 
 NumberOfToothbrushes=count_if (SalesRecords.begin(), SalesRecords.end(),
  IsAToothbrush(VariableToothbrushCode));
 cout << "There were  "
  << NumberOfToothbrushes
  << " toothbrushes matching code "
  << VariableToothbrushCode
  << " sold"
  << endl;
}

 

这个例子演示了如何向函数对象传递信息。你可以定义任意你想要的构造函数，你可以再函数对象中做任何你 想做的处理，都可以合法编译通过。
你可以看到函数对象真的扩展了基本记数算法