求助这个代码是什么意思?

[jdynnwxs]

#include "algorithm"
#include "iostream"
#include "vector"
#include "list"
using  namespace std;
template <typename elementType>
struct DisplayElementKeepCount
{
    int m_nCount;
    DisplayElementKeepCount() // 为什么这个会输出2次后才执行下面的operator？？
    {
        cout<<"hello world"<<endl;
        m_nCount = 0;
    }
    void operator()(const elementType & element) //这样写DisplayElementKeepCount<int>()，为什么就会指向该operator?
    {
        ++m_nCount;
        cout<<element<<' ';
    }
};
int main()
{
    vector<int> vecIntegers;
    for(int nCount =0; nCount<10; ++nCount)
        vecIntegers.push_back(nCount);
    cout<<"Displaying the vector of integers: " <<endl;
    DisplayElementKeepCount<int> mResult;
    mResult = for_each(vecIntegers.begin(), vecIntegers.end(), DisplayElementKeepCount<int>());
    cout<<endl<<endl;
    cout<<"'" << mResult.m_nCount<< "' elements were displayed!"<<endl;
    return 0;

}

[fefe82]

    DisplayElementKeepCount<int> mResult; // 这里构造了一个
    mResult = for_each(vecIntegers.begin(), vecIntegers.end(), DisplayElementKeepCount<int>()); 这里有构造了一个

可以改成

    DisplayElementKeepCount<int> mResult;
    mResult = for_each(vecIntegers.begin(), vecIntegers.end(), mResult);

[mujiok2003]

DisplayElementKeepCount<int> mResult; //1
    mResult = for_each(vecIntegers.begin(), vecIntegers.end(), DisplayElementKeepCount<int>() //2
);

因为你构造了两次呀

[mujiok2003]

引用

void operator()(const elementType & element) //这样写DisplayElementKeepCount<int>()，为什么就会指向该operator? { ++m_nCount; cout<<element<<' '; }

不是这个原因。 这是个函数调用运算符的重载.

DisplayElementKeepCount<int> x;
x(); //x.m_count = 1;
x();  //x.m_cout = 2;