string.empty（）或string.size（）== 0哪个更快？ [英] Which is faster, string.empty() or string.size() == 0?

问题描述

最近，在一次讨论中，一位同行程序员要求我进行一些代码更改。我有类似的东西：

  if（mystring.size（）== 0）
 //做点
 else 
 //做其他事情
  

讨论是关于<$的使用c $ c> mystring.empty（）来验证字符串是否为空。现在，我同意可以争论的是， string.empty（）是更冗长和可读的代码，但是它对性能有好处吗？

我做了一些挖掘，发现了与我的问题有关的以下两个答案：

• 从basic_string.h实现


• 指向ISO标准的答案-此处

这两个答案都支持了我的观点，即与 string相比， string.empty（）更具可读性，并且不提供任何性能优势。 size（）== 0 。

我仍然想确定是否有 string 的实现保留了内部布尔值标记以验证字符串是否为空？

或者某些实现使用其他方式可以使我的主张无效？

解决方案

标准如下定义 empty（）：

bool empty（）const noexcept;

返回：size（）== 0。

您很难找到无法做到这一点的东西，由于两者都是恒定时间操作，因此任何性能差异都可以忽略不计。我希望它们都能在任何合理的实现上编译为完全相同的程序集。

也就是说， empty（）清晰明了。为了提高可读性，您最好使用 size（）== 0 （或！size（））。

Recently, during a discussion I was asked by a fellow programmer to do some code changes. I had something like:

if( mystring.size() == 0)
    // do something
else
    // do something else 

The discussion was regarding the use of mystring.empty() to validate if the string is empty. Now, I agree that it can be argued that string.empty() is more verbose and readable code, but are there any performance benefits to it?

I did some digging and found these 2 answers pertaining to my question:

• Implementation from basic_string.h
• SO Answer that points to ISO Standard - here

Both the answers buttress my claim that the string.empty() is just more readable and doesn't offer any performance benefits, compared to string.size() == 0.

I still want to be sure, if there are any implementations of string that keep an internal boolean flag to validate if a string is empty or not?

Or there are other ways that some implementations use, that would nullify my claim??

解决方案

The standard defines empty() like this:

bool empty() const noexcept;
Returns: size() == 0.

You'd be hard-pressed to find something that doesn't do that, and any performance difference would be negligible due to both being constant time operations. I would expect both to compile to the exact same assembly on any reasonable implementation.

That said, empty() is clear and explicit. You should prefer it over size() == 0 (or !size()) for readability.

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