Re: sizeof

[Russell Shaw <rjshaw@netspace.net.au>]

On 9/5/22 3:29 am, Simon Marchi wrote:
> 
> 
> On 2022-05-08 11:22, Russell Shaw wrote:
>> When inspecting a C++ file, i get:
>>
>> (gdb) p sizeof(int())
>> $82 = 1
>>
>> (gdb) p sizeof(int)
>> $83 = 4
>>
>> Not right ?
>>
>> Package: gdb
>> Version: 11.2-1
>> on debian
> 
> The compiler seems to agree with GDB:
> 
>      $ cat test.cpp
>      #include <stdio.h>
> 
>      int main()
>      {
>        printf("%zu\n", sizeof(int));
>        printf("%zu\n", sizeof(int()));
>      }
>      $ g++ test.cpp
>      test.cpp: In function ‘int main()’:
>      test.cpp:6:19: warning: invalid application of ‘sizeof’ to a function type [-Wpointer-arith]
>          6 |   printf("%zu\n", sizeof(int()));
>            |                   ^~~~~~~~~~~~~
>      $ ./a.out
>      4
>      1
> 
> I don't really know what sizeof(int()) means anyway.  clang just rejects
> it:
> 
>      $ clang++ test.cpp
>      test.cpp:6:19: error: invalid application of 'sizeof' to a function type
>        printf("%zu\n", sizeof(int()));
>                        ^     ~~~~~~~

sizeof(int()) should transform to sizeof(int (*)()) according to the 
C++20 standard.

**********************************************************************
cat main.cpp

void f(int()) { }

int a = f;

int main() { }

g++ -DHAVE_CONFIG_H -I.    -std=c++20 -Wall -Wno-unused -g -O0 -MT 
main.o -MD -MP -MF .deps/main.Tpo -c -o main.o main.cpp
main.cpp:8:9: error: invalid conversion from ‘void (*)(int (*)())’ to 
‘int’ [-fpermissive]
     8 | int a = f;
       |         ^
       |         |
       |         void (*)(int (*)())

**********************************************************************