does std::runtime_error copy the string passed in the constructor?

Question

I wonder if this line creates a dangling pointer:

string arg="derp";
throw std::runtime_error("Unknown argument "+arg);

Does std::runtime_error copy the string, or does it store the reference?

Answer 1

std::exception (which std::runtime_error inherits from) must make a copy of the message, even if that's not called out in the standard explicitly (that I can find). There's no reason for the exception class to expect that the source of the what() message will outlive it in the general case.