Using typecasting to remove gcc compiler warnings

Question

I am doing embedded ARM programming with gcc 4.9. I've been using the -Wconversion switch because it's in my company's default dev tool configuration. I'm using the stdint.h types (uint8_t, uint32_t, etc).

The compiler creates warnings every time I perform a compound assignment or even simple addition. For example:

uint8_t u8 = 0;
uint16_t u16;

// These cause warnings:
u8 += 2;
u8 = u16 >> 8;

The "common method" to fix this is to use casts, as discussed here and here:

u8 = (uint8_t)(u8 + 2);
u8 = (uint8_t)(u16 >> 8);

In addition to this being ugly, I keep running into convincing evidence that casting is generally bad practice.

My questions:

1. Why is it bad to use typecasts in this way?
2. Do I lose anything by simply omitting -Wconversion and letting the compiler do implicit conversions for me?

Answer 1

The compiler is showing you potential problem spots, that you should consider changing, but not by casts. The problem in the code that you have is that in C, arithmetic is never done on type that are narrower than int: the compiler always performs an implicit conversion to int for them.

So effectively your code is converting the narrow unsigned types to int, does the operation and then converts them back to the unsigned type.

Generally doing arithmetic with narrow types is not a good idea. Only use them if storage size is a problem (usually that would be larger structures such as arrays). For local, more or less temporary variables these narrow types make no sense.