17.7.2 Arithmetic Type Conversion ¶

Here are some ways in C to convert an arithmetic expression e to a possibly different arithmetic type t.

• An explicit conversion like ((t) e) is powerful and can therefore be dangerous, because the conversion can succeed even if neither e nor t happens to have an arithmetic type. For example, if e is a pointer, ((long int) e) will do a conversion even if that was not the intent.
• An implicit conversion like t v = e; is less powerful, as it does not convert from pointers. However, it can lose information and sign, as in for example int v = -0.9; which sets v to zero.
• A no-op arithmetic expression like +e is even less powerful, as it preserves value (including sign) because it does only integer promotions. That is, it converts to int if that can represent all values of e’s underlying type, otherwise to unsigned int if that can represent all values, and otherwise it does no conversion.

INT_PROMOTE (e) is an expression with the same value as the arithmetic expression e but with e’s type after any integer promotion. It behaves like +e.

In the following example, using INT_PROMOTE pacifies GCC’s -Wswitch-enum option, and may help human readers see what is going on even if they are not expert in C’s integer promotion rules and might be confused by the simpler switch (+v).

enum { A = 1, B, C, D, E } v = ...;
switch (INT_PROMOTE (v))
  {
    case A: case C:
      return true;
    default:
      /* Handle all other cases,
         even cases like v == 0.  */
      return false;
  }