the way to compare string in an efficient way in C++

Question

Is it efficient to compare a string with another string or string literal like this?

string a;
string b;
if (a == "test")

or

if (a == b)

My coworker asked me to use memcmp

Any comments about this?

Thanks.

Answer 1

Yes use a == b, do not listen to your co-worker.

You should always prefer code readability and using STL over using C functions unless you have a specific bottleneck in your program that you need to optimize and you have proven that it is truly a bottleneck.

Answer 2

Obviously you should use a == b and rely on its implementation.

For the record, std::char_traits<char>::compare() in a popular implementation relies on memcmp(), so calling it directly would only be more painful and error-prone.

Answer 3

If you really need to know, you should write a test-application and see what the timing is.

That being said, you should rely on the provided implementation being quite efficient. It usually is.

Answer 4

I think your coworker is a bit hooked up on possible optimization.

• memcmp isn't intended to compare strings (that would be strcmp)
• to only compare upto the size of the shortest string, you would need strlen on both strings
• memcmp returns <0, =0, >0, which is a nuisance to always remember
• strcmp and strlen can cause weird behaviour with bad c-style strings (not ending with \0 or null)