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c++macrosnumeric-limits

definition of UINT_MAX macro

I would like to know if there is a particular reason to define the macro UINT_MAX as (2147483647 * 2U + 1U) and not directly its true value (4294967295U) in the climits header file.

Thank you all.

Solution

  • As far as the compiled code is concerned, there would be no difference, because the compiler would evaluate both constant expressions to produce the same value at compile time.

    Defining UINT_MAX in terms of INT_MAX lets you reuse a constant that you have already defined:

    #define UINT_MAX (INT_MAX * 2U + 1U)

    In fact, this is very much what clang's header does, reusing an internal constant __INT_MAX__ for both INT_MAX and UINT_MAX:

    #define INT_MAX   __INT_MAX__
#define UINT_MAX  (__INT_MAX__  *2U +1U)