Don't mix signed and unsigned arithmetic

Reason

Avoid wrong results.

Example

int x = -3;
unsigned int y = 7;

cout << x - y << '\n';  // unsigned result, possibly 4294967286
cout << x + y << '\n';  // unsigned result: 4
cout << x * y << '\n';  // unsigned result, possibly 4294967275

It is harder to spot the problem in more realistic examples.

Note

Unfortunately, C++ uses signed integers for array subscripts and the standard library uses unsigned integers for container subscripts. This precludes consistency. Use gsl::index for subscripts; see ES.107.

Enforcement

• Compilers already know and sometimes warn.
• (To avoid noise) Do not flag on a mixed signed/unsigned comparison where one of the arguments is sizeof or a call to container .size() and the other is ptrdiff_t.