C++ Lambda表达式基本用法（言简意赅，非常清楚）
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：
|
1
2
3
4
5
6
7
8
9
|
#include <iostream>using namespace std;int main(){ []{ cout << "Hello,Worldn"; }();} |
我们也可以方便的将这个创建的匿名函数赋值出来调用：
|
1
2
3
4
5
6
7
8
9
10
11
|
#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
|
1
2
3
4
5
6
7
8
9
|
#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
|
1
2
3
4
5
6
7
8
9
10
11
|
#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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
|
#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
|
1
2
3
4
5
6
7
8
9
10
11
12
|
#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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
|
#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();} |
|
1
|
|
结果
outside i:0x28ff0c
outside j:0x28ff08
inside i:0x28ff0c
inside j:0x28ff04
[bar] 指定引用或拷贝
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
|
#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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
|
#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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
|
#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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
|
#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();} |
https://lellansin.wordpress.com/2014/01/02/c-lambda%E8%A1%A8%E8%BE%BE%E5%BC%8F%E5%9F%BA%E6%9C%AC%E7%94%A8%E6%B3%95/
