C++ Lambda表达式基本用法

创建一个匿名函数并执行。Objective-C采用的是上尖号^,而C++ 11采用的是配对的方括号[]。实例如下:

#include <iostream>
using namespace std;

int main()
{
	[]{
		cout << "Hello,World\n"; 
	}();
}

我们也可以方便的将这个创建的匿名函数赋值出来调用:

#include <iostream>
using namespace std;

int main()
{
	int i = 1024;
	auto func = [](int i) { // (int i) 是指传入改匿名函数的参数
		cout << i;
	};
	func(i);
}

捕获选项

  • [] Capture nothing (or, a scorched earth strategy?)
  • [&] Capture any referenced variable by reference
  • [=] Capture any referenced variable by making a copy
  • [=, &foo] Capture any referenced variable by making a copy, but capture variable foo by reference
  • [bar] Capture bar by making a copy; don't copy anything else
  • [this] Capture the this pointer of the enclosing class

[] 不捕获任何变量

#include <iostream>
using namespace std;

int main()
{
	int i = 1024;
	auto func = [] { cout << i; };
	func();
}

vs 报错
error C3493: 无法隐式捕获“i”,因为尚未指定默认捕获模式
error C2064: 项不会计算为接受 0 个参数的函数

g++ 报错:
error: 'i' is not captured

要直接沿用外部的变量需要在 [] 中指名捕获。

[=] 拷贝捕获

#include <iostream>
using namespace std;

int main()
{
	int i = 1024;
	auto func = [=]{  // [=] 表明将外部的所有变量拷贝一份到该函数内部
		cout << i;
	};
	func();
}

结果:
1024

#include <iostream>
using namespace std;

int main()
{
	int i = 1024;
	auto fun1 = [=]{
		// fun1 内存在 i
		cout << i; // 1024
		auto fun2 = []{ // 未指名捕获, i 不存在
			cout << i;
		};
		fun2();
	};
	fun1();
}

[&] 引用捕获

#include <iostream>
using namespace std;

int main()
{
	int i = 1024;
	cout << &i << endl;
	auto fun1 = [&]{
		cout << &i << endl;
	};
	fun1();
}

结果:
0x28ff0c
0x28ff0c

[=, &] 拷贝与引用混合

#include <iostream>
using namespace std;

int main()
{
	int i = 1024, j = 2048;

	cout << "i:" << &i << endl;
	cout << "j:" << &j << endl;

	auto fun1 = [=, &i]{ // 默认拷贝外部所有变量,但引用变量 i
		cout << "i:" << &i << endl;
		cout << "j:" << &j << endl;
	};
	fun1();
}

结果
outside i:0x28ff0c
outside j:0x28ff08
inside i:0x28ff0c
inside j:0x28ff04

[bar] 指定引用或拷贝

#include <iostream>
using namespace std;

int main()
{
	int i = 1024, j = 2048;

	cout << "outside i value:" << i << " addr:" << &i << endl;

	auto fun1 = [i]{
		cout << "inside  i value:" << i << " addr:" << &i << endl;
		// cout << j << endl; // j 未捕获
	};
	fun1();
}

结果:
outside i value:1024 addr:0x28ff08
inside i value:1024 addr:0x28ff04

#include <iostream>
using namespace std;

int main()
{
	int i = 1024, j = 2048;

	cout << "outside i value:" << i << " addr:" << &i << endl;

	auto fun1 = [&i]{
		cout << "inside  i value:" << i << " addr:" << &i << endl;
		// cout << j << endl; // j 未捕获
	};
	fun1();
}

结果:
outside i value:1024 addr:0x28ff08
inside i value:1024 addr:0x28ff08

#include <iostream>
using namespace std;

int main()
{
	int i = 1024, j = 2048, k;

	cout << "outside i:" << &i << endl;
	cout << "outside j:" << &j << endl;

	auto fun1 = [i, &j]{
		cout << "inside  i:" << &i << endl;
		cout << "inside  j:" << &j << endl;
		// cout << k; // k 未捕获
	};
	fun1();
}

结果:
outside i:0x28ff0c
outside j:0x28ff08
inside i:0x28ff00
inside j:0x28ff08

[this] 捕获 this 指针

#include <iostream>
using namespace std;

class test
{
public:
	void hello() {
		cout << "test hello!\n";
	};
	void lambda() {
		auto fun = [this]{ // 捕获了 this 指针
			this->hello(); // 这里 this 调用的就是 class test 的对象了
		};
		fun();
	}
};

int main()
{
	test t;
	t.lambda();
}

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