What is the meaning of this header (virtual const char* what() const throw())?

Question

class myexception: public exception
{
  virtual const char* what() const throw()
  {
    return "My exception happened";
  }
};

Sorry, this question may sound dumb, but I have trouble parsing the header. Can someone describe in English what the header actually means? The first thing that seems odd to me is the keyword virtual. The myexception class is not a base class and inherits from the already implemented exception class, so why use virtual here? I guess const is for the return type which is a c-style string that is const, and the other const is to make sure nothing that the this object cannot be modified (can someone tell me what that object could be?). I have no idea what throw() does exactly, never seen this syntax before.

virtual const char* what() allows the what() method to be overridden in classes derived from myexception — Jay, Mar 18 '14 at 23:36
This is a base class as soon as you derive another class from it. — David Heffernan, Mar 18 '14 at 23:37
what does throw() do and generally when can you put another method in the header? — user3435009, Mar 18 '14 at 23:57
throw() says it throws no exceptions. It's not a method, it's an empty throw-list. — user207421, Mar 18 '14 at 23:58
@LightnessRacesinOrbit Ah, as I see from @EJP's comment: `throw()` without further specification effectively means nothing throwed, right? — πάντα ῥεῖ, Mar 19 '14 at 00:20
@πάνταῥεῖ: Yes... `noexcept` means `throw()`. From cppreference: "noexcept is an improved version of throw(), which is deprecated in C++11. Unlike throw(), noexcept will not call std::unexpected and may or may not unwind the stack, which potentially allows the compiler to implement noexcept without the runtime overhead of throw()." — Lightness Races in Orbit, Mar 19 '14 at 00:23
Downvoted the irrelevant stuff, and left place for a **real** answer (just too tired/lazy myself for now) ... — πάντα ῥεῖ, Mar 19 '14 at 00:29
http://stackoverflow.com/questions/5230463/what-does-this-function-declaration-mean-in-c — silentNinJa, Apr 29 '14 at 10:35

score 32 · Accepted Answer · answered Mar 19 '14 at 01:14

virtual

Adds nothing, as the method being overridden is already virtual. You are correct: it can be omitted.

const char* what()

A member function named what() that takes no arguments and returns a pointer to const char.

const

The member function can be called via a const pointer or reference to an instance of this class or a derived class.

throw()

Throws no exceptions.

score 1 · Answer 2 · answered Mar 18 '14 at 23:39

1

The virtual keyword is optional (you can skip it or explicitly write down - no difference) when you override an already virtual method from a base class (like in this case). Your remarks about the two const keywords are almost correct. It's basic C++.

answered Mar 18 '14 at 23:39

user3353219

29
1

The virtual keyword actually does a difference. – ciamej Mar 18 '14 at 23:41
3

@ciamej Not in the case specified. – user207421 Mar 18 '14 at 23:44
@EJP but, we don't know if there exists a class that derives from myexception, the code snippet might be just a small excerpt of the whole code. If so the virtual keyword is important. – ciamej Mar 18 '14 at 23:45
Or more precisely, we don't know if there are, somewhere in the code, pointers to myexception which may point to instances of myexception subclasses – ciamej Mar 18 '14 at 23:46
3

@ciamej You've missed the point completely. The method is already virtual in the base class, so adding virtual here does nothing. – user207421 Mar 18 '14 at 23:53
but what does throw() do and generally when can you put another method in the header? – user3435009 Mar 18 '14 at 23:57
@ciamej As I said, you've missed the point completely. Several times now. – user207421 Mar 19 '14 at 00:00
2

@ciamej: It makes no difference because `std::exception` already has a `what()` that is virtual. This virtualness is already being inherited. It is redundant to write it again here. – Lightness Races in Orbit Mar 19 '14 at 00:11

What is the meaning of this header (virtual const char* what() const throw())?

2 Answers2