Why throw at derived class catches by base?

Question

For the code below, result is "EA Exception Finished", which means although we threw at derived class it caught by base class. Is it always? And if so, how can I make the derived class catches, thus "EB Exception Finished" appears?

Also I can't exactly get what does it mean by throw EB() and catch(EA&). And does catch(EA&) means the catch block gets a reference for EA object?

Sorry for my ignorance. If you recommend me a book or something to refer about exception structure, that'd be great help.

class EA {};
class EB: public EA {};

void F()
{
  throw EB();  // throw at EB().
}

int main()
{
  try
  {
    F();
  }
  catch(EA&) // caught here??
  {
    std::cout<<"EA Exception";
  }
  catch(EB&) // why not me? every time?
  {
    std::cout<<"EB Exception";
  }

  std::cout<<" Finished"<<std::endl;

  return 0;
}

An exception is always caught by the *first* catch block that *can* catch it, so your order is wrong. — Jesper Juhl, Sep 16 '16 at 09:53

score 3 · Answer 1 · answered Sep 16 '16 at 09:29

Change the order of the catch blocks to fix that behavior:

#include <iostream>

class EA {};
class EB: public EA {};

void F()
{
  throw EB();  // throw at EB().
}

int main()
{
  try
  {
    F();
  }
  catch(EB&) // why not me? every time?
  {
    std::cout<<"EB Exception";
  }
  catch(EA&) // caught here??
  {
    std::cout<<"EA Exception";
  }

  std::cout<<" Finished"<<std::endl;

  return 0;
}

The compiler even warns you about this:

main.cpp:21:3: warning: exception of type 'EB' will be caught
   catch(EB&) // why not me? every time?
   ^~~~~
main.cpp:17:3: warning:    by earlier handler for 'EA'
   catch(EA&) // caught here??
   ^~~~~

Thanks a lot! What does it mean by throw EB() in this context? What compiler do you use? — RyanC, Sep 16 '16 at 09:36
@DongkyuChoi I used GCC at [coliru](http://coliru.stacked-crooked.com/a/33ca04ff57d31fa6) — πάντα ῥεῖ, Sep 16 '16 at 09:37

score 2 · Answer 2 · answered Sep 16 '16 at 18:09

As mentioned by the standard in [except.handle] (working draft):

The handlers for a try block are tried in order of appearance. That makes it possible to write handlers that can never be executed, for example by placing a handler for a derived class after a handler for a corresponding base class.

That's exactly what you did. Interesting indeed.
Invert the handlers to solve the issue.

score 1 · Answer 3 · answered Sep 16 '16 at 09:28

1

Because the catch blocks check in the order you declare them.

you first catch by EA&. EB is derived from EA, so this is a valid catch and the second catch gets ignored.

You want to have the most "specialized" exception-catch first. So if you switch the catch blocks it should work the other way.

answered Sep 16 '16 at 09:28

Hayt

5,210
30
37

Thanks a lot! So throw to derived class can be also caught by base class, but it is not desirable because it cannot be a specialized treatment? – RyanC Sep 16 '16 at 09:34
It can be done if you change the order of the "catch" block. Also as mentioned in another answer. You should get warnings in those cases. – Hayt Sep 16 '16 at 09:35

score 1 · Answer 4 · answered Sep 16 '16 at 09:29

1

catch statements are inspected in order. EA& matches, so it is used. EB& can never be matched. You need to put the more specific catch first.

  catch(EB&) // Will catch
  {
    std::cout<<"EB Exception";
  }
  catch(EA&) // and this would catch EA objects that aren't EB.
  {
    std::cout<<"EA Exception";
  }

answered Sep 16 '16 at 09:29

Martin Bonner supports Monica

28,528
3
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Thanks a lot! Then EB catches first and then EA also catch consequently? – RyanC Sep 16 '16 at 09:39
1

No. Only the first matching catch block is executed. it's quite common to have a catch block for a specific exception, for `const std::exception&`, and for `...`. Each printing successively less detailed information. – Martin Bonner supports Monica Sep 16 '16 at 09:40

score 1 · Accepted Answer · answered Sep 19 '16 at 06:11

1

Reason:

Upcasting

of derived class to base. and hence always getting stuck on the first catch.

answered Sep 19 '16 at 06:11

Vishwadeep Singh

1,043
1
13
38

Why throw at derived class catches by base?

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