Using auto in loops c++

Question

I get warning signed/unsigned mismatch for the following code:

auto n = a.size(); for (auto i = 0; i < n; i++) { } 

The problem is that by assigning 0 to i it becomes int rather than size_t. So what is better:

size_t n = a.size(); for (size_t i = 0; i < n; i++) { } 

or this:

auto n = a.size(); for (size_t i = 0; i < n; i++) { } 

or maybe you have a better solution? I like the first one more because it is bit more consistent, it just uses size_t rather than both size_t and auto for the same purpose.

Answer 1

A range based loop could be a cleaner solution:

for (const auto& i : a) {  } 

Here, i is a const reference to an element of container a.

Otherwise, if you need the index, or if you don't want to loop over the entire range, you can get the type with decltype(a.size()).

for (decltype(a.size()) i = 0; i < a.size(); ++i) { } 

Answer 2

Depending on what you want to do inside the loop and the capabilities of your compiler, range-based for loop might be a better solution.

All of your presented solutions are not bad in most situations, with minor differences Your first solution is actually worse choice and that's exactly what your compiler tells you. Second solution is better but if you want to avoid directly defining types for simplicity or some future changes, you can do the following:

auto n = a.size(); for (decltype(n) i = 0; i < n; i++) { } 

This way you bind the i and n types to always match each other.