c ++向量emplace_back更快？ [英] c++ vector emplace_back is faster?

问题描述

如果我有一个类

class foo {
 public:
  foo() { // spend some time and do something. }
 private:
   // some data here
}

现在我有一个foo的向量，我想把这个向量放入另一个向量

Now I have a vector of foo, I want to put this vector into another vector

vector<foo> input; // assume it has 5 elements
vector<foo> output;

这两行有什么性能差异吗？

Is there ANY performance difference with these two lines?

output.push_back(input[0])
output.emplace_back(input[0])

推荐答案

这两行是否有性能差异？

Is there ANY performance difference with these two lines?

否，两者都将使用复制构造函数初始化新元素。

No, both will initialise the new element using the copy constructor.

emplace_back 可能会在构造多于（或少于）一个参数时产生效益：

emplace_back can potentially give a benefit when constructing with more (or less) than one argument:

output.push_back(foo{bar, wibble}); // Constructs and moves a temporary
output.emplace_back(bar, wibble);   // Initialises directly

emplace的真正好处在性能上不是那么多，而是允许在容器中创建不可复制（以及在某些情况下不可移动）的元素。

The true benefit of emplace is not so much in performance, but in allowing non-copyable (and in some cases non-movable) elements to be created in the container.

这篇关于c ++向量emplace_back更快？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！