Notional operations Rule 10.1

[George Brown]

On a 16 bit integer machine, the following happens. (Assume number represents the number of bits used for storage.)

Evaluation is left to right.

I have also changed the variables to u16a, u16b amd u32c.

u32c + u16a + u16b

u32c + u16a is evaluated in 32 bits unsigned.
x + u16b is added in 32bits unsigned.

Therefore, all operations have occured in one type.

But.

u16a + u16b + u32c

u16a + u16b is evaluated in 16 bits unsigned. Any overflow is handled by defined mod 65536.

x + u32c is evaluated in 32 bits unsigned.

There are therefore values of u16a, u16b and u32c for which

u32c + u16a + u16b is not equal to u16a + u16b + u32c

While the compiler's behaviour IS defined, the rule is intended to avoid such subtle differences.

Note that
(uint32_t) u16a + u16b + u32c is ok.

(uint32_t) u16a puts u16a into a 32 unsigned value.

x + u16b x is uint32_t, so is evaluated in 32 bit unsigned.

y + u32c is evaluated in 32 bit unsigned.

u16a + (uint32_t) u16b + u32c
(uint32_t) u16a + (uint32_t) u16b + u32c

would also work. The last line contains more than the minimum casting necessary to achieve consistant type.

I.e. consider

(uint32_t) u16a + u16b + u16c + ... + u16z + u32c

There is no need to clutter the code with a cast on every operator.

George