How to raise warning if return value is disregarded?

Question

I\'d like to see all the places in my code (C++) which disregard return value of a function. How can I do it - with gcc or static code analysis tool?

Bad code exampl

Answer 1

As far as I'm aware there is no GCC option to give this warning. However, if you are interested in specific functions, you can tag them with an attribute:

int fn() __attribute__((warn_unused_result));

which would give a warning if the return value of fn() was not used. Caveat: I've never used this feature myself.

Answer 2

You can use this handy template to do it at run-time.

Instead of returning an error code (e.g. HRESULT) you return a return_code<HRESULT>, which asserts if it goes out of scope without the value being read. It's not a static analysis tool, but it's useful none the less.

class return_value
{
public:
  explicit return_value(T value)
    :value(value), checked(false)
  {
  }

  return_value(const return_value& other)
    :value(other.value), checked(other.checked)
  {
    other.checked = true;
  }

  return_value& operator=(const return_value& other)
  {
    if( this != &other ) 
    {
      assert(checked);
      value = other.value;
      checked = other.checked;
      other.checked = true;
    }
  }

  ~return_value(const return_value& other)
  {
    assert(checked);
  }

  T get_value()const {
    checked = true;
    return value;
  }

private:
  mutable bool checked;
  T value;
};