I created a custom exception class that derives from std::exception.
#include <iostream>
class Exception : std::exception {
public:
const char* what() const noexcept override {
return "test";
}
};
int main() {
try {
throw Exception();
} catch (std::exception& e) {
std::cout << e.what() << std::endl;
}
}
This program, when compiled by g++ -stdc++=17 on Ubuntu, causes the exception to not get caught by the catch block, even though catching by reference is supposed to catch derived exceptions too. It calls std::terminate, even though it happens in a try block that catches its base class by reference. Same thing happens if Exception inherits from std::runtime_error and passes "test" to the std::runtime_error constructor in its own constructor. Normally the solution would be to only catch using Exception, but in my original code I need to catch different types of exceptions, all of which inherit from std::exception. Why does this happen? Does catching by reference to base not work? How can I catch all exceptions deriving from std::exception using one catch block?
When you inherit from a base class during the definition of a class, the default access modifier for the inheritance is private. This means that the two following definitions are equivalent:
class derived : base { /* ... */ };
class derived : private base { /* ... */ };
The language doesn't allow1 you to refer to a derived class from a private base2. As an example, the following code does not compile:
int main()
{
derived d;
base& b = d; // <== compilation error
}
error: 'base' is an inaccessible base of 'derived' base& b = d; ^
This is the reason why your catch block cannot handle Exception. Change your inheritance to public...
class Exception : public std::exception
...and your original code will work.
1 See [dcl.init.ref] and [conv.ptr].
2 Unless you're in the scope of derived itself. See this live example on wandbox.org.