# V702. Classes should always be derived from std::exception (and alike) as 'public'.

The analyzer has detected a class derived from the std::exception class (or other similar classes) through the private or protected modifier. What is dangerous about such inheritance is that in case of nonpublic inheritance, the attempt to catch a std::exception will fail.

The error is often a result of the programmer forgetting to specify an inheritance type. According to the language rules, the default inheritance type is private inheritance. It results in exception handlers behaving other way than expected.

For example, see the following incorrect code:

    class my_exception_t : std::exception // <=
    {
    public:
      explicit my_exception_t() { }
      virtual const int getErrorCode() const throw() { return 42; }
    };
    ....
    try
      { throw my_exception_t(); }
    catch (const std::exception & error)
      { /* Can't get there */ }
    catch (...)
      { /* This code executed instead */ }

The code to catch all the standard and user exceptions "catch (const std::exception & error)" won't work properly because private inheritance does not allow for implicit type conversion.

To make the code run correctly, we need to add the public modifier before the base class std::exception in the list of base classes:

    class my_exception_t : public std::exception
    {
      ....
    }

This diagnostic is classified as:

CWE-544

You can look at examples of errors detected by the V702 diagnostic.