mexplus | C++ Matlab MEX development kit

You can safely ignore this.

Here's a nice description of why a loop might be written this way From http://en.cppreference.com/w/cpp/types/size_t:

std::size_t is commonly used for array indexing and loop counting. Programs that use other types, such as unsigned int, for array indexing may fail on, e.g. 64-bit systems when the index exceeds UINT_MAX or if it relies on 32-bit modular arithmetic.

The actual value of size_t, the maximum amount of memory allocatable, is going to be system dependent. The compiler looks to be casting the 0 (in i = 0) to an int and then throwing the warning, because it just reduced your usable range by half (i.e. instead of an unsigned int).

In practice though, i can probably still index up to a value of 2^32 (this is where it gets platform dependent), and you are probably not going to be dealing with structures having that many field names.

Might this warning actually point to an error in my configuration file?

I don't think so.

Can I safely ignore this?

Yes.