lambda表达式

初级部分

#include<iostream>
using namespace std;

//解决函数嵌套
void main1()
{
	[] {cout << "hello world"; }();//hello world   limbda表达式

	cin.get();
}

void main2()
{
	//auto fun = [] {cout << "hello world"; };//这里fun是一个函数指针
	//fun();//调用
	[] {cout << "hello world"; }();//匿名lambda表达式 最后的()是调用作用

	cin.get();
}

void main3()
{
	//[] {cout << "hello world", cout << "hello boy"; }();//{}是块语句，函数体，执行
	//[](char *str) {cout << str << endl; }("hahaha");
	char str[]{ "fangfang" };
	auto fun = [](char *str) {cout << str << endl; };
	cout << typeid(fun).name() << endl;//class <lambda_...>
	//lambda表达式不能直接取地址，无法当做函数指针
	fun(str);

	cin.get();
}

void main4()
{
	auto fun = [](double a, double b) {return a + b; };
	cout << fun(10, 19.6) << endl;//29.6
	//->在（）和{}之间，指定返回值类型
	auto fun1 = [](double a, double b)->int {return a + b; };
	cout << fun1(10, 19.6) << endl;//29
	//内联展开 无法取地址
	//decltype类型自动推理
	auto fun2 = [](double a, double b)->decltype(a+b) {return a + b; };
	cout << fun2(10, 19.6) << endl;

	cin.get();
}

void main()
{
	int num = 100;
	auto fun = [](int num) {num = 5, cout << num << endl; };
	fun(num);//5
	cout << "main=" << num << endl;//100  并未改变，lambda表达式遵循副本机制

	cin.get();
}

深入部分

#include<iostream>
#include<array>
#include<algorithm>//算法
using namespace std;
//主要解决代码内嵌
//[]()->type {}();//匿名表达式
//[],=引用，只读，=mutable,读原本改副本，&读写原本
//()参数，int a,int b
//{}语句的实现  ()调用
//->指定返回值类型

void mainA()
{
	int num1 = 111;
	int num2 = 222;
	//[]() {cout << num1 << num2 << endl; }();//报错
	[=]() {cout << num1 << num2 << endl; }();//[=]捕获外部变量
	//[=]() {num1=20,num2=30,cout << num1 << num2 << endl; }();//报错，[=]只可以读外部变量
	[=]()mutable {num1 = 10, num2 = 30, cout << num1 << num2 << endl; }();//mutable修改副本
	cout << "main= " << num1 << "   " << num2 << endl;//111   222

	[&]() {num1=10,num2=30,cout << num1 << num2 << endl; }();//[&]读写外部变量
	cout << "main1= " << num1 << "   " << num2 << endl;//10   30



	cin.get();
}

void mainB()
{
	//混合机制
	int a = 10;
	int b = 9;
	int c = 8;
	[&a,b,c]() {a=111,cout << a << b << c << endl; }();//只有a可读可写


	cin.get();
}

void mainC()
{
	[](auto a, auto b) {cout << a + b << endl; }(10, 11);
	[](auto a=0, auto b=0) {cout << a + b << endl; }(10, 11);//附初始值只能=不能()
	[](int a = 2, int b = 4) {cout << a + b << endl; }();//6  用auto的话不确定类型

	cin.get();
}

void main()
{
	array<int, 10>myint{ 0,1,2,3,4,5,6,7,8,9 };
	for_each(myint.begin(), myint.end(), [](int num) {cout << num << endl; });//显示
	for_each(myint.begin(), myint.end(), [](int &num) {num+=1,cout << num << endl; });//修改，所有元素+1
	for_each(myint.begin(), myint.end(), [](int num) {cout << num << endl; });//被改变

	cin.get();
}