2.Lambda expressions

lambda表达式

规则

• [] - captures nothing.

• [=] - capture local objects (local variables, parameters) in scope by value.

• [&] - capture local objects (local variables, parameters) in scope by reference.

• [this] - capture this by reference.

• [a, &b] - capture objects a by value, b by reference.

int x = 1;

auto getX = [=] { return x; };
getX(); // == 1

auto addX = [=](int y) { return x + y; };
addX(1); // == 2

auto getXRef = [&]() -> int& { return x; };
getXRef(); // int& to `x`

1. 除非使用引用，否则不能在lambda里面修改传递的变量，默认是const的，除非加了mutable关键字，如下：

int x = 1;

auto f1 = [&x] { x = 2; }; // OK: x is a reference and modifies the original

auto f2 = [x] { x = 2; }; // ERROR: the lambda can only perform const-operations on the captured value
// vs.
auto f3 = [x]() mutable { x = 2; }; // OK: the lambda can perform any operations on the captured value

使用场景

stl sort 自定义排序规则

int main()
{
    vector<int> testdata = { 3, 4, 12, 2, 1, 6 };

    sort(testdata.begin(), testdata.end(), [](int a, int b){ return a > b; });

    return 0;
}

函数内部的简易函数调用

auto lambdaFun = [&](int param) -> void {
	// 这里是可以直接调用到TestFun中的变量
	// 使用a、b、c....一堆变量
}
if (条件1)
{
	lambdaFun(param1);
}
if (条件2）
{
	lambdaFun(param2);
}