如何从 C++11 匿名函数内部访问局部变量? [英] How can I access local variables from inside a C++11 anonymous function?

问题描述

我正在对向量(权重)进行简单的归一化，试图利用 STL 算法使代码尽可能干净(我意识到这对于 for 循环来说非常简单):

I'm doing a simple normalization on a vector (weights), trying to make use of STL algorithms to make the code as clean as possible (I realize this is pretty trivial with for loops):

float tot = std::accumulate(weights.begin(), weights.end(), 0.0);
std::transform(weights.begin(), weights.end(), [](float x)->float{return(x/tot);});

目前，匿名函数对 tot 不可见，因此无法编译.使局部变量对匿名函数可见的最佳方法是什么?

At present, tot is not visible to the anonymous function, so this doesn't compile. What's the best way of making a local variable visible to the anonymous function?

推荐答案

你需要一个闭包.

float tot = std::accumulate(weights.begin(), weights.end(), 0);
std::transform(weights.begin(), weights.end(), [tot](float x)->float{return(x/tot);});

在这种情况下，tot 是按值捕获的.C++11 lambdas 支持捕获:

In this case tot is captured by value. C++11 lambdas support capturing by:

1. 值 [x]
2. 参考[&x]
3. 当前在引用范围内的任何变量[&]
4. 与 3 相同，但按值 [=]

您可以在逗号分隔列表[x, &y]中混合上述任何内容.

You can mix any of the above in a comma separated list [x, &y].

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