lambda与函数调用的转换

#include<iostream>
#include<vector>
#include<string>
#include<algorithm>
#include<fstream>
using namespace std;

class Length
{
public:
    Length(size_t n):sz(n) {}
    bool operator()(string &s){ return s.size()==sz;}
private:
    size_t sz;
};

int main()
{
    ifstream in("4.txt");
    string str;
    vector<string> vec;
    int arr[10];
    while(in>>str)
        vec.push_back(str);
    for(size_t i=0;i<10;++i)
        arr[i]=count_if(vec.begin(),vec.end(),Length(i+1));
    for(size_t i=0;i<10;++i)
        cout<<arr[i]<<" ";
    cout<<endl;
    return 0;
}