返回空字符串：C++ 中的有效方法
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Returning an empty string : efficient way in c++
提问by Abhishek Chandel
I have 2 ways of returning an empty string from a function.
我有两种从函数返回空字符串的方法。
1)
1)
std::string get_string()
{
return "";
}
2)
2)
std::string get_string()
{
return std::string();
}
which one is more efficient and why?
哪个更有效，为什么？
回答by Baldrickk
Gcc 7.1 -O3 these are all identical, godbolt.org/z/a-hc1d – jterm Apr 25 at 3:27
Gcc 7.1 -O3 这些都是相同的，godbolt.org/z/a-hc1d – jterm 4 月 25 日 3:27
Original answer:
原答案：
Did some digging. Below is an example program and the relevant assembly:
做了一些挖掘。下面是一个示例程序和相关程序集：
Code:
代码：
#include <string>
std::string get_string1(){ return ""; }
std::string get_string2(){ return std::string(); }
std::string get_string3(){ return {}; } //thanks Kerrek SB
int main()
{
get_string1();
get_string2();
get_string3();
}
Assembly:
集会：
__Z11get_string1v:
LFB737:
.cfi_startproc
pushl %ebx
.cfi_def_cfa_offset 8
.cfi_offset 3, -8
subl , %esp
.cfi_def_cfa_offset 48
movl 48(%esp), %ebx
leal 31(%esp), %eax
movl %eax, 8(%esp)
movl $LC0, 4(%esp)
movl %ebx, (%esp)
call __ZNSsC1EPKcRKSaIcE
addl , %esp
.cfi_def_cfa_offset 8
movl %ebx, %eax
popl %ebx
.cfi_restore 3
.cfi_def_cfa_offset 4
ret
.cfi_endproc
__Z11get_string2v:
LFB738:
.cfi_startproc
movl 4(%esp), %eax
movl $__ZNSs4_Rep20_S_empty_rep_storageE+12, (%eax)
ret
.cfi_endproc
__Z11get_string3v:
LFB739:
.cfi_startproc
movl 4(%esp), %eax
movl $__ZNSs4_Rep20_S_empty_rep_storageE+12, (%eax)
ret
.cfi_endproc
This was compiled with -std=c++11 -O2.
这是用-std=c++11 -O2.
You can see that there is quite a lot more work for the return "";statement and comparably little for return std::stringand return {};(these two are identical).
你可以看到，有相当多的大量的工作return "";说明和相当小的return std::string和return {};（这两个是相同的）。
As Frerich Raabe said, when passing an empty C_string, it still needs to do processing on it, instead of just allocating memory. It seems that this can't be optimised away (at least not by GCC)
正如 Frerich Raabe 所说，当传递一个 empty 时C_string，它仍然需要对其进行处理，而不仅仅是分配内存。似乎这无法优化（至少不是 GCC）
So the answer is to definitely use:
所以答案是肯定使用：
return std::string();
or
或者
return {}; //(c++11)
Although unless you are returning a lotof empty strings in performance critical code (logging I guess?), the difference is going to still be insignificant.
尽管除非您在性能关键代码中返回大量空字符串（我猜是日志记录？），但差异仍然是微不足道的。
回答by Frerich Raabe
The latter version is never slower than the first. The first version calls the std::stringconstructor taking a C string, which then has to compute the length of the string first. Even though that's fast to do for an empty string, it's certainly not faster than not doing it at all.
后一个版本永远不会比第一个慢。第一个版本调用std::string带有 C 字符串的构造函数，然后它必须首先计算字符串的长度。尽管这对于空字符串来说很快，但它肯定不会比根本不这样做快。
