C++ what happens if an exception occurs inside catch block?

Question

What happens if an exception occurs inside a catch block?

try {}
catch(...)
{
    stream.close(); // IO exception here
}

What's the default behaviour?

That exception will propagate outside the `catch(...)` block. — Evg, Apr 14 '22 at 11:32
Does this answer your question? [How can I handle exception in catch block in c++](https://stackoverflow.com/questions/26989879/how-can-i-handle-exception-in-catch-block-in-c) — codeling, Apr 14 '22 at 11:53

score 8 · Accepted Answer · answered Apr 14 '22 at 11:34

8

Nothing special will happen. A new exception object will be initialized by the throw expression inside the call and a search for a matching catch handler will start, which on escaping the function call will continue on the nearest enclosing try block (enclosing the shown try/catch pair). The old exception object will be destroyed during stack unwinding when leaving the old handler, since it wasn't rethrown.

answered Apr 14 '22 at 11:34

user17732522

53,019
2
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If the exception isn’t caught, is stack unwinding still guaranteed? – viraltaco_ Apr 14 '22 at 12:26
@ViralTaco_ No, stack unwinding is never guaranteed if an exception isn't caught at all (meaning if the search I mentioned doesn't find any matching handler). – user17732522 Apr 14 '22 at 13:28

Deepak Patankar · Answer 2 · 2022-04-14T11:33:36.807

1

If you encounter an exception in the catch block, then that exception will be thrown.

If you don't want to throw the exception, you can add one more catch block for this exception.

try {

}catch(...){
  try{
    stream.close(); // IO exception here
  }catch(IOException ioException){

  }
}

edited Apr 14 '22 at 11:33

answered Apr 14 '22 at 11:22

Deepak Patankar

3,076
3
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That block should be a nested one. – Evg Apr 14 '22 at 11:30
`try { } catch() { try { } catch() { } }`! – Aconcagua Apr 14 '22 at 11:32
Thanks for pointing it out, fixing it – Deepak Patankar Apr 14 '22 at 11:33

score 1 · Answer 3 · answered Apr 14 '22 at 12:24

To the best of my recollection:

#include <cstdio>
#include <fstream>
#include <stdexcept>

static auto foo(char const* path) try {
  if (not path) 
    throw std::runtime_error("path was nullptr");
  else
    std::printf("Opening file '%s'\n", path);

  auto f = std::ifstream(path);
  if (not f)
    throw std::runtime_error("Failed to open file in foo(char const*)");
  
  char x[5] {};
  f.read(x, 4); // 4 chars + '\0'
  std::printf("File type: %s\n", x + 1);
  
  return x[1]; // 'E' on linux
} catch (std::runtime_error const& e) {
  // If an exception occurs here, unless the call to foo(char const*)
  // was itself in a try-catch, the exception will be uncaught and std::terminate will be called
  // Whether std::terminate causes any stack unwinding to occur is implementation defined.
  // (That's not great.)
  // If the function was marked `noexcept (true)` then std::terminate would get immediately called.
  
  // f is destructed at the end of the scope (before the catch). 
  // This is called "RAII": https://en.wikipedia.org/wiki/Resource_acquisition_is_initialization
  std::fprintf(stderr, "%s: An exception occured:\n\t'%s'\n", __func__, e.what());
  throw; // rethrows the caught exception
}

int main(int, char** argv) {
  std::printf("%c", foo(argv[0]));
}

Live example

In a function try-catch block: the file object gets destructed when it goes out of scope; Regardless of the exception state.

The question is asking what happens if an exception is thrown within a `catch`. I don't see where in the answer answer or the code shown that is addressed. — François Andrieux, Apr 14 '22 at 13:16

C++ what happens if an exception occurs inside catch block?

3 Answers3