How to properly throw an exception which needs more than just a constructor?

Question

I have an Exception class on which I want to set more information before I throw it. Can I create the Exception object, call some of its functions and then throw it without any copies of it being made?

The only method I've found is throwing a pointer to the object:

class Exception : public std::runtime_error
{
public:
    Exception(const std::string& msg) : std::runtime_error(msg) {}
    void set_line(int line) {line_ = line;}
    int get_line() const {return line_;}
private:
    int line_ = 0;
};

std::unique_ptr<Exception> e(new Exception("message"));
e->set_line(__LINE__);
throw e;
...
catch (std::unique_ptr<Exception>& e) {...}

But throwing exceptions by pointer is generally avoided, so is there any other way?

There is also the option of setting all the options through the constructor, but this can quickly become unscalable if more fields are added to the class and you want to have fine-grained control over what fields to set:

throw Exception("message"); // or:
throw Exception("message", __LINE__); // or:
throw Exception("message", __FILE__); // or:
throw Exception("message", __LINE__, __FILE__); // etc.

Answer 1

C++ exception classes are expected to be copyable or at least movable. In your example, making your class copyable is a matter of adding a default copy constructor:

Exception(Exception const&) = default;

If you need to encapsulate some non-copyable and non-movable state in your exception class, wrap such state into std::shared_ptr.

Answer 2

You can create a data-holding class, like ExceptionData. Then create ExceptionData object and call it's methods. Then create Exception object using std::move in ctor like:

ExceptionData data;
data.method();
throw Exception(std::move(data));

Of course, ExceptionData needs to be movable and you have to have ctor that accepts ExceptionData && (rvalue reference).

It'll work if you really need to avoid copies, but to me it feels like preliminary optimization. Think how often exceptions are being thrown in your app and is it really worth it to complicate things for that matter.