c++ - What is the difference between throw and throw with arg of caught exception?

Question

Imagine two similar pieces of code:

try {
  [...]
} catch (myErr &err) {
  err.append("More info added to error...");
  throw err;
}

and

try {
  [...]
} catch (myErr &err) {
  err.append("More info added to error...");
  throw;
}

Are these effectively the same or do they differ in some subtle way? For example, does the first one cause a copy constructor to be run whereas perhaps the second reuses the same object to rethrow it?

Answer 1

Depending on how you have arranged your exception hierarchy, re-throwing an exception by naming the exception variable in the throw statement may slice the original exception object.

A no-argument throw expression will throw the current exception object preserving its dynamic type, whereas a throw expression with an argument will throw a new exception based on the static type of the argument to throw.

E.g.

int main()
{
    try
    {
        try
        {
            throw Derived();
        }
        catch (Base& b)
        {
            std::cout << "Caught a reference to base
";
            b.print(std::cout);
            throw b;
        }
    }
    catch (Base& b)
    {
        std::cout << "Caught a reference to base
";
        b.print(std::cout);
    }

    return 0;
}

As written above, the program will output:

Caught a reference to base
Derived
Caught a reference to base
Base

If the throw b is replace with a throw, then the outer catch will also catch the originally thrown Derived exception. This still holds if the inner class catches the Base exception by value instead of by reference - although naturally this would mean that the original exception object cannot be modified, so any changes to b would not be reflected in the Derived exception caught by the outer block.