c++中的 lambda函数

一开始的变量捕获不太懂,写一遍就好理解了

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

class Test {
public:
    void hello() {
        cout << "test hello" << endl;
    }


    void lambda() {
        auto fun = [this] { //捕获 this
            this->hello();
            };
        fun();
    }
};

int main() {


    std::vector<int> numbers = { 1,2,3,4,5 };
    std::for_each(numbers.begin(), numbers.end(), [](int x) {
        std::cout << "下标x is " << x << " ;" << std::endl;
        });



    auto sum = [](int a, int b) -> int {
        return a + b;
        };


    std::cout << sum(1, 3) << std::endl;


    ///捕获外部变量
    /**
     * 捕获外部变量,2 种方式
     * 1. 值捕获: 通过值拷贝外部变量
     * 2. 引用捕获: 通过引用捕获外部变量
     */

    int x = 10;

    auto lambda = [x]() {
        std::cout << "x ====" << x << std::endl;
        };
    lambda();//应该输出 10


    auto lambda_ref = [&x]() {
        x = 20;
        };
    lambda_ref();

    std::cout << x << std::endl;//输出 20

    ///经典的 lambda 排序
    std::vector<int> my_numbers = { 1,2,77,43,400,5,98,1 };
    std::sort(my_numbers.begin(), my_numbers.end(), [](int a, int b) {
        return a < b;
        });

    for (int i = 0; i < my_numbers.size(); i++)
    {
        std::cout << "value is " << my_numbers[i] << std::endl;
    }



    ///使用=来捕获外部的所有变量,会把外部的所有变量拷贝一份到 lambda内部
    ///格式大概像这样
    int my_i = 100;
    auto fun_my_i = [=] {
        std::cout << "fun_my_i i is " << my_i << std::endl;
        };


    ///引用的用[&],所有外部变量皆可引用
    int my_i_2 = 100;
    auto fun_my_i_2 = [&] {
        std::cout << "my_i_2 is " << &my_i_2 << std::endl;
        };

    ///复制并引用捕获

    int a1 = 100, a2 = 200;
    std::cout << "a1=" << a1 << std::endl;
    cout << "a2=" << a1 << endl;

    //这里是所有变量拷贝一份,a2则使用引用
    auto fun_a = [=, &a2] {
        std::cout << "a1=" << &a1 << std::endl;
        cout << "a2=" << &a1 << endl;
        };

    fun_a();

    //引用或者复制
    int b1 = 100, b2 = 200;
    cout << "b1 =" << b1 << endl;
    cout << "b2 = " << b2 << endl;
    auto fun_b = [b1] {
        cout << "b1==" << b1 << ",b1=(引用)" << &b1 << endl;
        };
    fun_b();


    /// this类型的捕获
    Test test;
    test.lambda();


    return 0;
}

输出

$ 函数
下标x is 1 ;
下标x is 2 ;
下标x is 3 ;
下标x is 4 ;
下标x is 5 ;
4
x ====10
20
value is 1
value is 1
value is 2
value is 5
value is 43
value is 77
value is 98
value is 400
a1=100
a2=100
a1=0x16bb08de0
a2=0x16bb08de0
b1 =100
b2 = 200
b1==100,b1=(引用)0x16bb08dcc
test hello