Question

I have a class which inherits from std::runtime_error like so:

#include <string>
#include <stdexcept>

class SomeEx : public std::runtime_error
{
public:
    SomeEx(const std::string& msg) : runtime_error(msg) { }
};

Said msg will always be something like “invalid type ID 43”. Is there any way to build that “what string” with another constructor (or another method) so that I provide only the integer type ID? Something like:

SomeEx(unsigned int id) {
    // set what string to ("invalid type ID " + id)
}

Answer 1

static std::string get_message(unsigned int id) {
    std::stringstream ss;
    ss << "invalid type ID " << id;
    return ss.str();
}
SomeEx(unsigned int id) 
    : runtime_error(get_message(id)) 
{}

unrelated: the reason we have string .what() is so that people STOP USING ERROR NUMBERS.