Re: Not what I expected from some exception code (throw/try/catch)
stevewilliams2004@comcast.net wrote:
I was wondering if someone could explain the output I am getting for
the program below. What I expected from the main program output was
"Cat" but instead I see "Mammal". The output is also included below.
I got the same results with GCC 3.4.4 under cygwin as with MSDev
studio 2003. Even stranger to me, if I change the catch statement to
catch a Cat instead of a Mammal, the program crashes in the catch
body, during the call to m.MyType(). Thanks for any explanations in
advance.
Program:
#include <iostream>
using namespace std;
class Mammal
{
public:
Mammal()
{
cout << "Constructing Mammal @ " << this << endl;
}
Mammal(const Mammal& source)
{
cout << "Copy Constructing Mammal @ " << this << " from " <<
&source << endl;
}
~Mammal()
{
cout << "Destructing Mammal @ " << this << endl;
}
virtual const char* MyType()
{
return "Mammal";
}
};
class Cat : public Mammal
{
public:
Cat()
{
cout << "Constructing Cat @ " << this << endl;
}
Cat(const Cat& source)
{
cout << "Copy Constructing Cat @ " << this << " from " <<
&source << endl;
}
~Cat()
{
cout << "Destructing Cat @ " << this << endl;
}
virtual const char* MyType()
{
return "Cat";
}
};
int main(int argc, char *argv[])
{
Cat fluffy;
Mammal &fluffyRef = fluffy;
try
{
throw fluffyRef;
You throw a Mamal. At this point, a temporary Mamal is created and
initialized from fluffyRef, which involves slicing. See [15.1/3]:
A throw-expression initializes a temporary object, called the exception
object, the type of which is determined by removing any top-level
cv-qualifiers from the static type of the operand of throw and adjusting
the type from ?array of T? or ?function returning T? to ?pointer to T?
or ?pointer to function returning T?, respectively.
Note that the _static type_ is used for the temporary.
Also note that a temporary copy is needed since the original is a local
object that has to go down during stack unwinding.
}
catch (Mammal &m)
At this point, the temporary implicitly created in the throw() is used to
initialize the Mamal& here. Since slicing already happened, you don't get a
Cat.
{
cout << "Caught a " << m.MyType() << endl;
return 0;
}
cout << "Nothing Caught" << endl;
return 0;
}
Output:
Steve@nessus /cygdrive/f/dev/src/TestWorkspace
$ ./TestWorkspace.exe
Constructing Mammal @ 0x22ccc0
Constructing Cat @ 0x22ccc0
Copy Constructing Mammal @ 0xc306d8 from 0x22ccc0
Caught a Mammal
Destructing Mammal @ 0xc306d8
Destructing Cat @ 0x22ccc0
Destructing Mammal @ 0x22ccc0
Best
Kai-Uwe Bux