[Bug c++/101850] Initialising a struct/class variable to itself does not fail at compile time (but throws std::bad_alloc at run time, as expected)

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101850

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Prasanta Behera from comment #0)
> Version info:
> =============
> ~$ g++ --version
> g++ (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0
> Copyright (C) 2018 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> 
> ~$ gcc --version
> gcc (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0
> Copyright (C) 2018 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> 
> Sample Program:
> ===============
> ~$ cat t.cpp
> #include <string>
> 
> int main() {
>   std::string s = s;
>   return 0;
> }
> 
> Problem Description:
> ====================
> The above program tries to initialise a string with itself which is wrong!

Yes, it's undefined behaviour.

> However g++ does not show any compile time error, but the resulting binary
> fails at run time throwing std::bad_alloc as shown below.

That's one way that undefined behaviour can manifest itself. You cannot expect
to get a compile time error for all cases of undefined behaviour, that's why
it's undefined.

> ~$ g++ t.cpp
> ~$ ./a.out
> terminate called after throwing an instance of 'std::bad_alloc'
>   what():  std::bad_alloc
> Aborted (core dumped)
> 
> gcc however shows a link time error which is slightly better.
> 
> ~$ gcc t.cpp
> /tmp/cc30YkgH.o: In function `main':
> t.cpp:(.text+0x27): undefined reference to `std::__cxx11::basic_string<char,
> std::char_traits<char>, std::allocator<char>
> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>,
> std::allocator<char> > const&)'
> t.cpp:(.text+0x38): undefined reference to `std::__cxx11::basic_string<char,
> std::char_traits<char>, std::allocator<char> >::~basic_string()'
> collect2: error: ld returned 1 exit status

No, this is just because gcc doesn't link to the C++ runtime which is needed to
use std::string. You will get the same link error for a correct program, it has
absolutely nothing to do with the bug in your code.

> I think the coding error should be caught at the compile time by g++. 

It is caught, by a warning.

In the general case, turning all such mistakes into errors is impossible:

const std::string& func(const std::string&);

std::string s = func(s);

The compiler cannot know whether func has undefined behaviour or not. It could
be defined like this, which would be OK:

const std::string& func(const std::string& /*unused*/) {
  static std::string str = "a string with static storage duration";
}

Or it could be defined like this, which would be equivalent to your example:

const std::string& func(const std::string& s) {
  return s;
}

tl;dr it is not reasonable to expect to always get a compile time error for
undefined behaviour, it's the programmer's responsibility to avoid undefined
behaviour. COmpiler warnings can help, if you enable them and pay attention to
them.