unreadVariable with RAII classes

[Daniel Bengtsson]

Hi,

I am having issues like: style: Variable 'raii' is assigned a value that is never used. [unreadVariable] with our custom raii classes.

What information is cppcheck missing such that it would know not to warn for such cases? I did use --check-config to see if there were any missing includes. But it only lists various system includes not from my code.

I am using cppcheck 2.7.3.

[Daniel Marjamäki]

can you please create a small example that reproduce the false positive.

I would spontanously think that if cppcheck see that raii is a class that might have global side effects during destruction we should not write this warning.

[Daniel Bengtsson]

I worked on minimizing it and ended up with this:

namespace test {

class RAII {
public:
  RAII(){}
  ~RAII(){}
};

}

using namespace test;

int main() {
  auto raii = RAII();
  return 0;
}

The problem seem to be the using namespace test; , if I remove that and explicitly use test::RAII() the warning goes away.

I ran it like this:

cppcheck --std=c++17 --enable=all raii.cpp

(The compiler are happy with both versions.)

It also reproduces at https://cppcheck.sourceforge.io/demo/

[CHR]

There are several issues at play here.
Cppcheck emits a known false positive with auto, this is tracked here: https://trac.cppcheck.net/ticket/10505
Declaring the variable as RAII raii; in your example still yields a warning, because cppcheck sees the empty con/destructors. If you then just declare class RAII; (or the con/destructor) inside test, the warning disappears. I did not see any effect of using namespace, but I might be missing something.

[Daniel Bengtsson]

I see, so I guess ticket 10505 is the main issue.

Regarding the namespace the original code above warns, but this one does not:

namespace test {

class RAII {
public:
  RAII(){}
  ~RAII(){}
};

}

int main() {
  auto raii = test::RAII();
  return 0;
}

[CHR]

Hmm, with current head I do get a warning:

bar.cpp:12:8: style: Variable 'raii' is assigned a value that is never used. [unreadVariable]
  auto raii = test::RAII();
       ^