这种自我指派是否做了明智的事情？

Question

I just found this line of code within a function, which puzzles me. Can this make sense in any context or it is undefined behavior?

char * acFilename = acFilename;

EDIT: The compiler complains with Warning C4700, that I am using an uninitialized variable.

Answer 1

At block scope, in C++, this is undefined behaviour, as the right-hand side reads the variable the variable before it has been initialized (C++14 [dcl.init]/12).

At block scope, in C11, this could either be undefined behaviour or behave as an uninitialized variable, depending on various details of the implementation and the rest of the function, see here for detailed analysis.

At namespace scope, in C++, it is OK well-defined and makes a null pointer. This is because all static variables are zero-initialized before their initializers are considered. (C++14 [basic.start.init]/2).

At file scope in C, it is a constraint violation; static variables must have a constant expression as initializer, and the value of a variable cannot be a constant expression.

Answer 2

No this code does not make any sense. It's probably a typo, maybe somebody meant to use

char* acFilename = ::acFilename;

or

char* acFilename = m_acFilename;

or something else.

As it stands, it's confusing and unhelpful at best, probably a bug because somebody meant to use a different variable.

Answer 3

I've seen this before. Since gcc is quite trigger happy with its uninitialized variable warnings this is a trick to silence those warnings. Not sure if it's intentional design choice by gcc or the compiler just being stupid, but I've seen people do this on purpose to make gcc shut up (and in the process break the warning that might be correct in the future).

I would just replace it with an initialization to NULL. This is a bad habit, doesn't work on other compilers and a NULL can't be less correct than an indeterminate value and undefined behavior.

Just to demonstrate how this works (and also because I wanted to know if newer gcc versions still do this):

$ cat foo.c
int
foobar(void)
{
    int foo;
    return foo + foo;
}
$ cc -c -Wall -O2 foo.c
foo.c: In function ‘foobar’:
foo.c:6:13: warning: ‘foo’ is used uninitialized in this function [-Wuninitialized]
  return foo + foo;
         ~~~~^~~~~
$ ed foo.c
[...]
$ cc -c -Wall -O2 foo.c
$ cat foo.c
int
foobar(void)
{
    int foo = foo;
    return foo + foo;
}
$ cc -c -Wall -O2 foo.c
$ cc -v
[...]
gcc version 6.2.0 20161005 (Ubuntu 6.2.0-5ubuntu12)
$

Answer 4

At some locations within a program, it may be possible for a variable to be in either of the following conditions:

1. Inputs that have been received have caused the variable to be written, and will also cause code to use its value in future.

2. Inputs that have been received have caused the variable not to be written, but will also cause code not to make any use of its value.

Some compilers will squawk if they see that there exist inputs which would cause a variable to not to get written, and there exist inputs which would cause the variable to be read. Even if the only input which would cause the variable to get read would also cause it to get written, the compilers may not realize that. While it would be possible to silence the compiler's complaints by unconditionally initializing the variable, machine code which initializes the variable with a value which can never actually get used would serve no functional purpose. A source code construct which would silence the compiler's complaints but not result in the compiler generating useless machine code may thus be preferable to one which would need to generate useless machine code. On some compilers, a self-initializing declaration may serve such a purpose.

If a type has trap representations, there may be no practical alternative to writing an initialization that a compiler will likely turn into useless machine code, but if a type doesn't have trap representations there is no particular reason why a quality compiler should force programmers to ask for useless code they don't want and don't need. Rather than try to have different rules for platforms where various types do and don't have trap representations, however, the authors of the Standard defer to implementers' judgments as to what sorts of behavior would be sensible on for a particular target platform and application field.

If on some platform a compiler couldn't allow programmers to omit initialization for variables which are copied but otherwise unused without having to generate likely-redundant initializations itself, it would make sense to require the programmer to include initializations in all situations that could actually occur. On another platform where copying a meaningless value would simply cause the destination to hold a meaningless value, however, it would generally make more sense to allow a programmer to omit initialization in cases where none of the copies would ever end up getting used for any real purpose. The authors of the Standard make no effort to specify what would make sense in any particular case, because they expect that people writing implementations should be capable of exercising good judgment.

The best approach for dealing with this situation would probably be to write a macro which accepts a variable, and whose expansion will initialize the variable to zero (for implementations whose authors judge that there is more value in allowing the compilers to jump the rails when an uninitialized variable is copied, even if none of the copies are really "used" for anything, than there would be in guaranteeing that the mere act of copying a variable will have no side-effects) or else do nothing (when using an implementation which will stay on the rails even without the useless initialization).