Use sizeof(char) in place for sizeof(void)

[hidmic]

The latter is invalid C++, even though gcc accepts it as an extension. Connected to ros2/rclcpp#1647.

CI up to rclcpp and demo_nodes_cpp:

• Linux
• Linux-aarch64
• macOS
• Windows

[clalancette]

The latter is invalid C++, even though gcc accepts it as an extension. Connected to ros2/rclcpp#1647.

I'm not against making this change, but this can't just be a GCC extension, right? This same code is compiling with clang and MSVC as well.

[wjwwood]

Yeah, I dunno, libcxx (which works on windows afaik) just straight does it:

https://github.com/llvm/llvm-project/blob/5bf2ef9d869ba882480232a4ed87728af74dac3b/libcxx/include/__memory/allocator.h#L74-L84

There is a specialization for void, but it doesn't affect that method, afaict:

https://github.com/llvm/llvm-project/blob/aaf026d9da3885a951dcdc5edd64c8e7d23b6285/libcxx/include/__memory/allocator.h#L31-L53

[ivanpauno]

I'm not against making this change, but this can't just be a GCC extension, right? This same code is compiling with clang and MSVC as well.

This seems to be a broadly accepted extension 😂.

But AFAIU it's not supposed to be correct.
According to cppreference:

sizeof cannot be used with function types, incomplete types (including void), or bit field lvalues.

[hidmic]

FWIW, besides sizeof(void) being accepted by some compilers, I suspect templates' lazy instantiation has a role here. IIUC if the allocate method in an allocator bound to void is never called or even addressed, its definition won't ever be instantiated (referred to as partial instantiation in C++ Templates - The Complete Guide, section 14.2). That changed with ros2/rclcpp#1647.