Cpp核心指南在这个例子中浪费了什么？

Question

What is wasted in the example from the Cpp Core Guidelines?

P.9: Don't waste time or space

[...]

 void lower(zstring s) { for (int i = 0; i < strlen(s); ++i) s[i] = tolower(s[i]); } 

Yes, this is an example from production code. We leave it to the reader to figure out what's wasted.

from https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rp-waste

Answer 1

strlen在循环的每次迭代中计算。

Answer 2

每次检查循环条件时都会调用strlen ，每次调用需要O（n）次，因此循环的总时间为O（n ^ 2）。

Answer 3

A lot of time is wasted and a segmentation fault may occur as the author of the code's increasing s , not i in the loop:

for (int i = 0; i < strlen(s); ++s)
                 //right here ^^^^

Answer 4

As other aswers have already stated, strlen(s) is called multiple times because it is in the condition, implying that it should be cached and reused instead.

But strlen(s) does not actually need to be called at all ! s is (or is implicitly convertible to) a nul-terminated char array, since that's what strlen expects. So we can just use this very property for our own loop.

void lower(zstring s) {
    for (char *p = s; *p; ++p)
        *p = std::tolower((unsigned char)*p);
}

Answer 5

Unless they have any very unintuitive semantics in the zstring class, the function in it's current form is a complete waste of both time and space, as its "result" can't be used after the function - it is passed in as value, and isn't returned.

So to avoid wasting the time to uselessly compute the lowercase which can't be used, and the space in copying the passed parameter, I would pass by reference!