Re: Exception Propagation / Copy Constructors

From:

Richard Herring <junk@[127.0.0.1]>

Newsgroups:

comp.lang.c++

Date:

Thu, 18 Jun 2009 18:19:36 +0100

Message-ID:

<5rUmnEJocnOKFwX0@baesystems.com>

In message
<f44b3a24-54c2-46f7-b33b-8373a2485014@r13g2000vbr.googlegroups.com>,
"better_cs_now@yahoo.com" <better_cs_now@yahoo.com> writes

Hello All,

Please consider the following:

#include <iostream>

using namespace std;

class Foo
{
  public:
     Foo()
     {
        cout << "Foo::Foo()" << endl;
     }

     ~Foo()
     {
        cout << "Foo::~Foo()" << endl;
     }

     Foo(const Foo &)
     {
        cout << "Foo::Foo(const Foo &)" << endl;
     }
};

void func3()
{
  throw Foo();
}

void func2()
{
  func3();
}

void func1()
{
  func2();
}

int main()
{
  try
  {
     func1();
  }
  catch(const Foo &)
  {
     cout << "Caught Foo" << endl;
  }
}

This results in the following output:

Foo::Foo()
Caught Foo
Foo::~Foo()

This shows that the copy constructor does not run. Yet, if I make the
copy constructor private, the compiler balks with the following:

main.cpp: In function `void func3()':
main.cpp:20: error: `Foo::Foo(const Foo&)' is private
main.cpp:27: error: within this context
main.cpp:20: error: `Foo::Foo(const Foo&)' is private
main.cpp:27: error: within this context

This leads to two questions:

1. Why does the compiler balk about inaccessibility of a function (the
copy constructor) it does not use?

Because, among other things, deliberately making it inaccessible is a
common way of preventing an object being copied. Not enforcing the
accessibility rule merely because of a compiler optimization would break
that.

2. Why *is* it not using the copy constructor? I would have expected
to see the exception object copy constructed / destructed at each
level of the call stack as that stack is unwound.

Why would you expect that, and what use would it be? Notionally, throw
Foo() initializes a temporary (15.1/3) (which is why an accessible copy
ctor is required) which gets passed directly to the catch block. It's
only automatic objects which get destroyed as the stack unwinds.

--
Richard Herring