Why does the comparison with integer works for nullptr?

Question

The following works for g++

assert(nullptr == 0);

I need to know if there is any implicit type conversion that is happening. From what I know, nullptr can be compared with pointers only and not with integers, and also that it is more type-safe. Then why the comparison with integer works?

Basile Starynkevitch · Accepted Answer · 2020-12-06 10:23:37Z

2

Then why the comparison with integer works?

Because, in most implementations, the nullptr is a 0 machine address. In other words (intptr_t)nullptr is 0. This is the case on Linux/x86-64 for example. Check by inspecting the generated assembler code obtained with g++ -S -O2 -fverbose-asm

I even believe that this is guaranteed by the C++ standard (read e.g. n3337)

However, if you compile your code with a recent GCC as gcc -Wall -Wextra you could get a warning.

Read also assert(3). In some cases (with NDEBUG) it is expanded to a no-op at compilation time.

edited Dec 6, 2020 at 10:23

answered Dec 6, 2020 at 10:18

Basile Starynkevitch

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