c++ - What is the rationale for limitations on pointer arithmetic or comparison?

Question

In C/C++, addition or subtraction on pointer is defined only if the resulting pointer lies within the original pointed complete object. Moreover, comparison of two pointers can only be performed if the two pointed objects are subobjects of a unique complete object.

What are the reasons of such limitations?

I supposed that segmented memory model (see here §1.2.1) could be one of the reasons but since compilers can actually define a total order on all pointers as demonstrated by this answer, I am doubting this.

Answer 1

The reason is to keep the possibility to generate reasonable code. This applies to systems with a flat memory model as well as to systems with more complex memory models. If you forbid the (not very useful) corner cases like adding or subtracting out of arrays and demanding a total order on pointers between objects you can skip a lot of overhead in the generated code.

The limitations imposed by the standard allows the compiler to make assumptions on pointer arithmetic and use this to improve quality of the code. It covers both computing things statically in the compiler instead of at runtime and choosing which instrutions and addressing modes to use. As an example, consider a program with two pointers p1 and p2. If the compiler can derive that they point to different data objects it can safely assume that any no operation based on following p1 will ever affect the object pointed to by p2. This allows the compiler to reorder loads and stores based on p1 without consider loads and stores based on p2 and the other way around.