why doesn't catch block share the scope of the try block?

Question

Surprisingly, I can't find an answer to that by googling and searching SO (there are lots of similar questions on SO but related to other languages).

I suspect that the answer is no. If so, there is an obvious inconvenience, e.g.

try
{
  std::string fname = constructFileName(); // can throw MyException
  ofstream f;
  f.exceptions(ofstream::failbit | ofstream::badbit);
  f.open(fname.c_str());
  // ...
}
catch (ofstream::failure &e)
{
  cout << "opening file " << fname << " failed\n"; // fname is not in the scope
}
catch (MyException &e)
{
  cout << "constructing file name failed\n";
}

If my assumption is correct, how do you deal with this? By moving the std::string fname; out of try, I guess?

I understand that a scope is defined by a {} block, but this seems as a reasonable case for, hmm, exception. Is the reason for that that objects can be not fully constructed if an exception is thrown?

Answer 1

Does catch block share the scope of the try block?

No.

How do you deal with this? By moving the std::string fname; out of try, I guess?

Yes.

I understand that a scope is defined by a {} block, but this seems as a reasonable case for, hmm, exception. Is the reason for that that objects can be not fully constructed if an exception is thrown?

The last thing C++ needs is more complex rules and exceptions to rules. :-)

Answer 2

There's an obvious reason: you cannot trust the state of the objects that were created inside the try block. The code there got interrupted by the exception, their constructors might not even have run yet.

Answer 3

While James' post correctly answers your questions, it doesn't supply the usual workaround: swap. Assuming constructFileName() returns std::string and not char const*, the following is idiomatic:

std::string fname;
try
{
    constructFileName().swap(fname); // can throw MyException
    std::ofstream f;
    f.exceptions(std::ios_base::failbit | std::ios_base::badbit);
    f.open(fname.c_str());
    // ...
}
catch (std::ios_base::failure &e)
{
    std::cout << "opening file " << fname << " failed\n";
}
catch (MyException &e)
{
    std::cout << "constructing file name failed\n";
}