c++ overriding exception::what()

Question

I have a custom exception which is

class RenterLimitException : public std::exception
{
public:
    const char* what();
};

What is the proper way to override what()? For now, I created this custom in a header file and want to override what() in my cpp file. My function is defined as following:

const char* RenterLimitException::what(){
    return "No available copies.";
}

However, when I use my try/catch block, it doesn't print the message that I gave to my function what(), instead, it prints std::exception My try/catch block is this:

try{
   if (something){
            throw RenterLimitException();}
}
catch(std::exception& myCustomException){
        std::cout << myCustomException.what() << std::endl;
    }

Is it because my try/catch block or it is my what() function? Thanks in advance

Your `what` method has the [wrong signature](https://en.cppreference.com/w/cpp/error/exception/what). — G.M., Feb 22 '21 at 10:07
@G.M. So it should be ```virtual const char* RenterLimitException::what() const throw(){ return "No available copies."; }```? — Alex, Feb 22 '21 at 10:11
In general, you should the `override` keyword, it would have warned you that your implementation does not override the `what()` method in the base class. — Mansoor, Feb 22 '21 at 10:11
In modern C++, use `noexcept` specifier instead of the deprecated `throw()` exception specification. — heap underrun, Feb 22 '21 at 10:19

paxdiablo · Accepted Answer · 2021-02-22T12:15:48.300

9

That is not the correct signature for the what method, you should be using const char * what() const noexcept override as per the following complete program:

#include <iostream>

class RenterLimitException : public std::exception {
public:
    const char * what() const noexcept override {
        return "No available copies.";
    }
};

int main() {
    try {
        throw RenterLimitException();
    } catch (const std::exception& myCustomException) {
        std::cout << myCustomException.what() << std::endl;
    }
}

Notice the specific signature used for the override and the catching of a const exception in main (not absolutely necessary but a good idea nonetheless). In particular, note the override specifier, which will cause compilers to actually check that the function you're specifying is a virtual one provided by a base class.

Said program prints, as expected:

No available copies.

edited Feb 22 '21 at 12:15

answered Feb 22 '21 at 10:11

paxdiablo

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2

you should probably what to catch by const reference https://stackoverflow.com/questions/2145147/why-catch-an-exception-as-reference-to-const – Alessandro Teruzzi Feb 22 '21 at 10:12
3

Use `override` instead of `virtual`. – Aykhan Hagverdili Feb 22 '21 at 10:13
1

Done and done. Both good points, bods, appreciate the input. – paxdiablo Feb 22 '21 at 10:18
1

I also want to ask, if I have 10 custom exceptions like this, one catch could catch all thrown? – Alex Feb 22 '21 at 10:20
1

@Alex if they all publicly derive from `std::exception`, yes, this will catch them all. Read about dynamic polymorphism for more. – Aykhan Hagverdili Feb 22 '21 at 10:21
1

@AyxanHaqverdili Thank y'all, this is really helpful. – Alex Feb 22 '21 at 10:22
1

It is important to explicitly stress in your answer that the most important is "override" in it. I mean - adding `override` causes that compilers check the correctness of all other parts – PiotrNycz Feb 22 '21 at 10:27

john · Answer 2 · 2021-02-22T10:11:29.637

Try this

class RenterLimitException : public std::exception
{
public:
    const char* what() const noexcept;
};

const is part of a function signature. To avoid this kind of error in the future you could also get into the habit of using override

class RenterLimitException : public std::exception
{
public:
    const char* what() const noexcept override;
};

override will give you an error if you get the signature of a virtual function wrong.

c++ overriding exception::what()

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