Question

The following code works just fine:

#include <exception>

using namespace std;

class FileException : public exception { // error occurs here
    int _error;
    // string _error; <-- this would cause the error
public:
    FileException(int error);
    // FileException(string error);
    const char* what() const throw();
};

But as soon as I change the type of _error to string, the following compile error occurs:

Exception specification of overriding function is more lax than base version

Answer 1

The destructor of std::string is not no-throw, which causes the implicit destructor of FileException not no-throw either. But the destructor of std::exception is no-throw, thus there’s a compiler error.

You could declare an explicit no-throw destructor:

virtual ~FileException() throw() {}

or just inherit from std::runtime_error instead of std::exception, which has a constructor that takes std::string input.