Question

I need the opinion of stack overflow to settle something that has been sitting in the back of my head about for loop efficiency. Now, when I began programming, “getting it to work” was a big priority and most of my for loops looked like this.

for (int i = 0; i < N; i++) {;}

Then I was tought, especially with C++ development that pre-increment saves you some calls, and considering that would be N calls and it doesn’t alter the readability I got into the habit of.

for (int i = 0; i < N; ++i) {;}

This was good enough for a while, but focusing on the readability, and after reading some of Code Complete by Steve McConnell, I arrived at this.

for (int loop_index = 0; loop_index < loop_count; ++loop_index) {;}

And these variables change based on the context of the code. Then I read some of Effective C++ about built in types constructors and assignment. And basically that the difference between

int i = 42;

and

int i(42); 

Is that the former one calls the constructor and the assignment operator while the latter only the constructor. So I got that into my routine whilst coding. So my question, is this the most efficient and readable way of writing a for loop:

for (int loop_index(0); loop_index < loop_counter; ++loop_index) {;}

Answer 1

Actually,

for (int i = 0; i < N; i++) {;}

is fine. i is an int, so the compiler will optimise the post-increment away anyway. Whether you initialise an int with int i(42); or with int i = 42; is also a question of taste.

I would also call the iterator i, instead of loop_index. The former is ubiquitously understood, while the latter is strange.

If you are dealing with iterators, however, the picture changes. Chances are it++ will still be optimised to ++it, but here, I’d rather use pre-increment.