Is using string.length() in loop efficient?

For example, assuming a string s is this:

for(int x = 0; x < s.length(); x++)

better than this?:

int length = s.length();
for(int x = 0; x < length; x++)

In general, you should avoid function calls in the condition part of a loop, if the result does not change during the iteration.

The canonical form is therefore:

for (std::size_t x = 0, length = s.length(); x != length; ++x);

Note 3 things here:

• The initialization can initialize more than one variable
• The condition is expressed with != rather than <
• I use pre-increment rather than post-increment

(I also changed the type because is a negative length is non-sense and the string interface is defined in term of std::string::size_type, which is normally std::size_t on most implementations).

Though... I admit that it's not as much for performance than for readability:

• The double initialization means that both x and length scope is as tight as necessary
• By memoizing the result the reader is not left in the doubt of whether or not the length may vary during iteration
• Using pre-increment is usually better when you do not need to create a temporary with the "old" value

In short: use the best tool for the job at hand :)