Lambda in C++

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C++

按照对象的某个属性排序，是一种常见的排序需求。
在c++11之前，是这么做的:

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#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
using namespace std;
class dorian{
        public:
                int m_a;
                int m_b;
                dorian()=default;
                dorian(int a, int b):m_a(a),m_b(b){}
};
void printVec(vector<dorian> vec, string title){
        cout << left << setw(20) << title;
        for(auto i : vec)
                cout << "(" << i.m_a << ", " << i.m_b << ")    ";
        cout << endl;
}
int sort_m_a(dorian c1, dorian c2){
                return c1.m_a < c2.m_a;
}
int sort_m_b(dorian c1, dorian c2){
                return c1.m_b < c2.m_b;
}
int main(){
        vector<dorian> vec;
        vec.push_back(dorian(1,4));
        vec.push_back(dorian(3,2));
        vec.push_back(dorian(4,1));
        vec.push_back(dorian(2,3));
    
        printVec(vec,"[Original]");

        sort(vec.begin(),vec.end(),sort_m_a);
        printVec(vec,"[Sorted by m_a]");
        
        sort(vec.begin(),vec.end(),sort_m_b);
        printVec(vec,"[Sorted by m_b]");
}


//执行打印如下
[Original]          (1, 4)    (3, 2)    (4, 1)    (2, 3)    
[Sorted by m_a]     (1, 4)    (2, 3)    (3, 2)    (4, 1)    
[Sorted by m_b]     (4, 1)    (3, 2)    (2, 3)    (1, 4)

从C++11开始，我们可以使用Lambda表达式，消灭sort_m_a,sort_m_b这两个函数

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#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
using namespace std;
class dorian{
        public:
                int m_a;
                int m_b;
                dorian()=default;
                dorian(int a, int b):m_a(a),m_b(b){}
};
void printVec(vector<dorian> vec, string title){
        cout << left << setw(20) << title;
        for(auto i : vec)
                cout << "(" << i.m_a << ", " << i.m_b << ")    ";
        cout << endl;
}
int main(){
        vector<dorian> vec;
        vec.push_back(dorian(1,4));
        vec.push_back(dorian(3,2));
        vec.push_back(dorian(4,1));
        vec.push_back(dorian(2,3));
    
        printVec(vec,"[Original]");

        sort(vec.begin(),vec.end(),[](dorian c1,dorian c2)->int{return c1.m_a < c2.m_a;});
        printVec(vec,"[Sorted by m_a]");
    
        sort(vec.begin(),vec.end(),[](dorian c1,dorian c2)->int{return c1.m_b < c2.m_b;});
        printVec(vec,"[Sorted by m_b]");
}

//执行打印如下
[Original]          (1, 4)    (3, 2)    (4, 1)    (2, 3)    
[Sorted by m_a]     (1, 4)    (2, 3)    (3, 2)    (4, 1)    
[Sorted by m_b]     (4, 1)    (3, 2)    (2, 3)    (1, 4)

当然 lambda也可以先定义，方便多处使用

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sort(vec.begin(),vec.end(),[](dorian c1,dorian c2)->int{return c1.m_a < c2.m_a;});
可以改为：

auto func = [](dorian c1,dorian c2)->int{return c1.m_a < c2.m_a;};
sort(vec.begin(),vec.end(),func);

联合for_each和lambda，消灭传统for循环

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

int main(){
        vector<int> vec;
        vec.push_back(0); 
        vec.push_back(3); 
        vec.push_back(6);
        vec.push_back(8);
        int count = 0;
        for_each(vec.begin(), vec.end(), [&](int a){ if(a>5) { cout << a << endl; count++; } }); 
        cout << "Total count: "<< count <<endl;

}

以上。