No, actually in C++, there are
/* multi-line
comments */

as well as
// single-line comments

the latter of which happen to also be valid in C, but only if you compile against the C99 specification (C programming has two specifications - C89 (1989) and C99 (1999)).

If I used "#" for something like this:
#ifndef _SOMETHING_
#define _SOMETHING_
#endif

then those are actually not comments. #include, #ifndef, #define and #endif are preprocessor directives. That part comes from C and is used in case someone does something stupid like #include <stdio.h> in two different files that are supposed to work together, things like #ifndef serve a very important purpose to prevent clashes with duplicate definitions of functions. It would be like creating a program that uses two Main() methods in C# or two main() methods in Java.

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