C-String和C ++字符串的效率 [英] Efficiency of C-String vs C++Strings

问题描述

C ++ Primer说

对于大多数应用程序，除了更安全之外，字符串而不是C风格的字符串

安全被理解。为什么C ++字符串库更高效？毕竟，下面的所有，不是字符串仍然表示为字符数组？

为了澄清，作者谈论程序员的效率（理解）或处理效率？ C>字符串通常更快，因为它们不调用malloc / new。但是有些情况下 std :: string 更快。函数 strlen（）是O（N），但 std :: string :: size（） ）。

当你搜索substring时，在C字符串中你需要检查'\0'在每个周期，在 std :: string - 你不会。在一个朴素的子字符串搜索算法没有什么关系，因为不是检查'\0'你需要检查 i 。但是现代高性能子串搜索算法以多字节步长遍历字符串。并且每个字节需要一个'\0'检查会减慢它们。这就是为什么GLIBC memmem 比 strstr 快x2倍的原因。我对子串算法进行了很多基准测试。

这不仅适用于子字符串搜索算法。许多其他字符串处理算法对于零终止字符串较慢。

C++ Primer says

For most applications, in addition to being safer, it is also more efficient to use library strings rather then C-style strings

Safety is understood. Why is C++ strings library more efficient? After all, underneath it all, aren't strings still represented as character arrays?

To clarify, does author talk about programmer efficiency (understood) or processing efficiency?

解决方案

C-strings are usually faster, because they do not call malloc/new. But there are cases where std::string is faster. Function strlen() is O(N), but std::string::size() is O(1).

Also when you search for substring, in C strings you need to check for '\0' on every cycle, in std::string - you don't. In a naive substring search algorithm it does not matter much, because instead of checking for '\0' you need to check for i<s.size(). But modern high-performance substring search algorithms traverse strings in multibyte steps. And the need for a '\0' check in every byte slows them down. This is the reason why GLIBC memmem is x2 times faster than strstr. I did a lot of benchmarking of substring algorithms.

This is true not only for substring search algorithm. Many other string processing algorithms are slower for zero-terminated strings.

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