我应该将std :: string与“ string”进行比较吗？或“字符串”？ [英] Should I compare a std::string to "string" or "string"s?

问题描述

考虑以下代码段：

bool foo(const std::string& s) {
    return s == "hello"; // comparing against a const char* literal
}

bool bar(const std::string& s) {
    return s == "hello"s; // comparing against a std::string literal
}

在第一眼，看起来与 const char * 相比汇编指令 ¹ ，因为使用字符串文字将导致 std :: string 的就地构造。

At first sight, it looks like comparing against a const char* needs less assembly instructions¹, as using a string literal will lead to an in-place construction of the std::string.

（编辑：正如答案中指出的那样，我忘记了有效地 s.compare（const char *）将在 foo（）中调用，因此在这种情况下当然不会进行就地构造。因此，请在下面删除一些行。）

( As pointed out in the answers, I forgot about the fact that effectively s.compare(const char*) will be called in foo(), so of course no in-place construction takes place in this case. Therefore striking out some lines below.)

但是，请查看 operator ==（const char *，const std :: string&）参考：

所有比较都是通过 compare（） mem完成的ber函数。

All comparisons are done via the compare() member function.

根据我的理解，这意味着我们将需要构建 std :: string 还是为了进行比较，所以我怀疑最终的开销是一样的（尽管对 operator == 的调用隐藏了）

From my understanding, this means that we will need to construct a std::string anyway in order to perform the comparison, so I suspect the overhead will be the same in the end (although hidden by the call to operator==).

• 我应该选择哪个比较？


• 一个版本是否有相对于其他优点（可能在特定情况下）？

¹ 较少的汇编指令并不一定意味着更快的代码，但我不想在这里进行微基准测试。

¹ I'm aware that less assembly instructions doesn't neccessarily mean faster code, but I don't want to go into micro benchmarking here.

推荐答案

都不是。

如果您想变得聪明一点，请与 字符串 sv ，它返回 std :: string_view 。

与类似的文字进行比较字符串 不会导致任何分配开销，它被视为以null结尾的字符串，具有所有伴随的缺点：不能容忍嵌入的null，用户必须注意

While comparing against a literal like "string" does not result in any allocation-overhead, it's treated as a null terminated string, with all the concomittant disadvantages: No tolerance for embedded nulls, and users must heed the null terminator.

string s 进行分配，除非小型字符串优化或分配选择。而且，运算符可以传递文字的长度，无需计数，并且允许嵌入空值。

"string"s does an allocation, barring small-string-optimisation or allocation elision. Also, the operator gets passed the length of the literal, no need to count, and it allows for embedded nulls.

最后使用 string sv 结合了其他两种方法的优点，避免了它们各自的缺点。另外， std :: string_view 是比 std :: string 简单得多的野兽，尤其是如果后者使用

And finally using "string"sv combines the advantages of both other approaches, avoiding their individual disadvantages. Also, a std::string_view is a far simpler beast than a std::string, especially if the latter uses SSO as all modern ones do.

至少自C ++ 14（通常允许省略分配）以来，编译器理论上可以优化该选项的所有选项。最后一个，在视情况而定。但是我们还没有。

At least since C++14 (which generally allowed eliding allocations), compilers could in theory optimise all options to the last one, given sufficient information (generally available for the example) and effort, under the as-if rule. We aren't there yet though.

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