When to use std::size_t?

A good rule of thumb is for anything that you need to compare in the loop condition against something that is naturally a std::size_t itself.

std::size_t is the type of any sizeof expression and as is guaranteed to be able to express the maximum size of any object (including any array) in C++. By extension it is also guaranteed to be big enough for any array index so it is a natural type for a loop by index over an array.

If you are just counting up to a number then it may be more natural to use either the type of the variable that holds that number or an int or unsigned int (if large enough) as these should be a natural size for the machine.

size_t is the result type of the sizeof operator.

Use size_t for variables that model size or index in an array. size_t conveys semantics: you immediately know it represents a size in bytes or an index, rather than just another integer.

Also, using size_t to represent a size in bytes helps making the code portable.

The size_t type is meant to specify the size of something so it's natural to use it, for example, getting the length of a string and then processing each character:

for (size_t i = 0, max = strlen (str); i < max; i++)
    doSomethingWith (str[i]);

You do have to watch out for boundary conditions of course, since it's an unsigned type. The boundary at the top end is not usually that important since the maximum is usually large (though it is possible to get there). Most people just use an int for that sort of thing because they rarely have structures or arrays that get big enough to exceed the capacity of that int.

But watch out for things like:

for (size_t i = strlen (str) - 1; i >= 0; i--)

which will cause an infinite loop due to the wrapping behaviour of unsigned values (although I've seen compilers warn against this). This can also be alleviated by the (slightly harder to understand but at least immune to wrapping problems):

for (size_t i = strlen (str); i-- > 0; )

By shifting the decrement into a post-check side-effect of the continuation condition, this does the check for continuation on the value before decrement, but still uses the decremented value inside the loop (which is why the loop runs from len .. 1 rather than len-1 .. 0).