Correct printf format specifier for size_t: %zu or %Iu?

MS Visual Studio didn't support %zu printf specifier before VS2013. Starting from VS2013 (e.g. _MSC_VER >= 1800) %zu is available.

As an alternative, for previous versions of Visual Studio if you are printing small values (like number of elements from std containers) you can simply cast to an int and use %d:

printf("count: %d\n", (int)str.size()); // less digital ink spent
// or:
printf("count: %u\n", (unsigned)str.size());

The Microsoft documentation states:

The hh, j, z, and t length prefixes are not supported.

And therefore %zu is not supported.

It also states that the correct prefix to use for size_t is I – so you'd use %Iu.

Microsoft's C compiler does not catch up with the latest C standards. It's basically a C89 compiler with some cherry-picked features from C99 (e.g. long long). So, there should be no surprise that something isn't supported (%zu appeared in C99).