C++ nullpointer dereference when adding instance to std::map within the constructor

Question

I am trying to get an error-code list for my C++ application, so that errors are shared across multiple projects. Each error contains simply a unique code (to give to exit() and similars) and a message to display. Other than a constructor and some casting methods, I also need some function to get an error by its code, so that when program A exits I can read the return code from program B and display it to the user/log/whatever. This implies, obviously, keeping track of the whole error list. To do so, i keep a static std::unordered_map inside the Error class, so that the Error constructor adds the being-created instance to it. However, for some reason I can't quite understand, I keep getting a nullpointer dereference error when adding the Error to the map. My code is as follows:

error_codes.h

namespace ErrorCodes{
    class Error{
    private:
        int code;
        std::string message;

        static int counter;

        static std::unordered_map<int, Error*> allErrors;

        Error(int code, std::string message);

    public:
        Error(): code(1), message(""){}

        explicit Error(std::string message);

        Error getByCode(int code);

        operator int() const{
            return code;
        }

        operator std::string() const{
            return message;
        }

        std::ostream& operator<<(std::ostream& os)
        {
            os << std::string("ERR_")+std::to_string(code)+std::string(": ") + message;
            return os;
        }

        operator QString() const{
            return QString::fromStdString(message);
        }

        bool operator == (Error other){
            return other.code == code;
        }


    };

    //List all errors here
    const Error ERR_ERROR_CODE_NOT_FOUND("Couldn\'t find error with code %1"); //Follow QString formatting style for more complex messages
    const Error ERR_GLOBAL_ID_NOT_FOUND("Problem with the global Id, identifier not found.");
    const Error ERR_DIR_NOT_FOUND_OR_CREATED("Dir not found or created, check configuration.");
}

and then my error_codes.cpp:

using namespace ErrorCodes;

int Error::counter = 0;
std::unordered_map<int, Error*> Error::allErrors = std::unordered_map<int, Error*>();

Error::Error(int code, std::string message) : code(code), message(message){
    std::pair pair = std::make_pair(code, this);
    allErrors.emplace(pair); //It fails here
}

Error::Error(std::string message){
    int code = --counter;
    Error(code, message);
}

Error Error::getByCode(int code){
    auto it = allErrors.find(code);
    if(it!=allErrors.end()){
        return *(it->second);
    }else{
        qCritical() << QString(ERR_ERROR_CODE_NOT_FOUND).arg(code);
        exit(ERR_ERROR_CODE_NOT_FOUND);
    }
}

PS: Ideally, I wouldn't make Error a class, but rather a struct, and have some kind of collection (such as an array) that contains all the errors and will allow me to retrieve them by the error code. However, I did not find a way of having the errors available within an array AND as a "constant" (so that anywhere in my code I can just write ErrorCodes::XXX) without having to manually create an array with all the Errors (for example, an enum would be perfect for this purpose). I would be grateful if anyone else knew a better way to do this.

Answer 1

Currently, you create Error instances before the unordered_map is initialized. Initialize the static variables before using them. Example:

Header:

namespace ErrorCodes {
// class definition

// List all errors here
extern const Error ERR_ERROR_CODE_NOT_FOUND;
extern const Error ERR_GLOBAL_ID_NOT_FOUND;
extern const Error ERR_DIR_NOT_FOUND_OR_CREATED;
}  // namespace ErrorCodes

.cpp file:

namespace ErrorCodes {

// Initialize the static variables:
int Error::counter = 0;
std::unordered_map<int, Error*> Error::allErrors{};

// Create all the errors afterwards:
const Error ERR_ERROR_CODE_NOT_FOUND("Couldn\'t find error with code %1");
const Error ERR_GLOBAL_ID_NOT_FOUND("Problem with the global Id, identifier not found.");
const Error ERR_DIR_NOT_FOUND_OR_CREATED("Dir not found or created, check configuration.");
//...
}  // namespace ErrorCodes

Also note that constructor delegation needs to use the member initializer list.

Example:

Error::Error(int code, std::string message) : code(code), message(std::move(message)) {
    allErrors.emplace(code, this);  // It doesn't fail here anymore
}

Error::Error(std::string message) : Error(--counter, std::move(message)) {}

---

Alternatively, define both the static variables and constants inline.

Header:

namespace ErrorCodes {
class Error {
private:
    inline static int counter = 0;
    inline static std::unordered_map<int, Error*> allErrors{};

    //...
};

// List all errors here
inline const Error ERR_ERROR_CODE_NOT_FOUND("Couldn\'t find error with code %1");
inline const Error ERR_GLOBAL_ID_NOT_FOUND("Problem with the global Id, identifier not found.");
inline const Error ERR_DIR_NOT_FOUND_OR_CREATED("Dir not found or created, check configuration.");

}  // namespace ErrorCodes