Why would uint32_t be preferred rather than uint_fast32_t?

Question

It seems that uint32_t is much more prevalent than uint_fast32_t (I realise this is anecdotal evidence). That seems counter-intuitive to me, though.

Answer 1

Why do many people use uint32_t rather than uint32_fast_t?

Note: Mis-named uint32_fast_t should be uint_fast32_t.

uint32_t has a tighter specification than uint_fast32_t and so makes for more consistent functionality.

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uint32_t pros:

• Various algorithms specify this type. IMO - best reason to use.
• Exact width and range known.
• Arrays of this type incur no waste.
• unsigned integer math with its overflow is more predictable.
• Closer match in range and math of other languages' 32-bit types.
• Never padded.

uint32_t cons:

• Not always available (yet this is rare in 2018).
  E.g.: Platforms lacking 8/16/32-bit integers (9/18/36-bit, others).
  E.g.: Platforms using non-2's complement. old 2200

uint_fast32_t pros:

• Always available.
  This always allow all platforms, new and old, to use fast/minimum types.
• "Fastest" type that support 32-bit range.

uint_fast32_t cons:

• Range is only minimally known. Example, it could be a 64-bit type.
• Arrays of this type may be wasteful in memory.
• All answers (mine too at first), the post and comments used the wrong name uint32_fast_t. Looks like many just don't need and use this type. We didn't even use the right name!
• Padding possible - (rare).
• In select cases, the "fastest" type may really be another type. So uint_fast32_t is only a 1st order approximation.

In the end, what is best depends on the coding goal. Unless coding for very wide portability or some niched performance function, use uint32_t.

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There is another issue when using these types that comes into play: their rank compared to int/unsigned

Presumably uint_fastN_t could be the rank of unsigned. This is not specified, but a certain and testable condition.

Thus, uintN_t is more likely than uint_fastN_t to be narrower the unsigned. This means that code that uses uintN_t math is more likely subject to integer promotions than uint_fastN_t when concerning portability.

With this concern: portability advantage uint_fastN_t with select math operations.

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Side note about int32_t rather than int_fast32_t: On rare machines, INT_FAST32_MIN may be -2,147,483,647 and not -2,147,483,648. The larger point: (u)intN_t types are tightly specified and lead to portable code.