How to throw std::exceptions with variable messages?

Question

This is an example of what I often do when I want to add some information to an exception:

std::stringstream errMsg;
errMsg << \"Could not load config          


        

Answer 1

A really nicer way would be creating a class (or classes) for the exceptions.

Something like:

class ConfigurationError : public std::exception {
public:
    ConfigurationError();
};

class ConfigurationLoadError : public ConfigurationError {
public:
    ConfigurationLoadError(std::string & filename);
};

The reason is that exceptions are much more preferable than just transferring a string. Providing different classes for the errors, you give developers a chance to handle a particular error in a corresponded way (not just display an error message). People catching your exception can be as specific as they need if you use a hierarchy.

a) One may need to know the specific reason

} catch (const ConfigurationLoadError & ex) {
// ...
} catch (const ConfigurationError & ex) {

a) another does not want to know details

} catch (const std::exception & ex) {

You can find some inspiration on this topic in https://books.google.ru/books?id=6tjfmnKhT24C Chapter 9

Also, you can provide a custom message too, but be careful - it is not safe to compose a message with either std::string or std::stringstream or any other way which can cause an exception.

Generally, there is no difference whether you allocate memory (work with strings in C++ manner) in the constructor of the exception or just before throwing - std::bad_alloc exception can be thrown before the one which you really want.

So, a buffer allocated on the stack (like in Maxim's answer) is a safer way.

It is explained very well at http://www.boost.org/community/error_handling.html

So, the nicer way would be a specific type of the exception and be avoiding composing the formatted string (at least when throwing).