const exceptions

[aspestrand]

Hi, just wondering if anybody can answer the following question as it
doesn't seem to be anywhere in the C++ spec.

What will print from the following statement? (note the order of the
catch statements):

void func(void)
{
try
{
const foo f1;
throw f1;
}
catch(foo &)
{
cout << "foo &" << endl;
}
catch(const foo &)
{
cout << "const foo &" << endl;
}
catch(...)
{
cout << "..." << endl;
}
}

[Thomas J. Gritzan]

as********@gmail.com schrieb:

Hi, just wondering if anybody can answer the following question as it
doesn't seem to be anywhere in the C++ spec.

The compiler can.

What will print from the following statement? (note the order of the
catch statements):

void func(void)
{
try
{
const foo f1;
throw f1;
}

Since classes are thrown by value (copied), the const qualifier doesn't
affect the thrown object. So the body of catch(foo&) will run.

g++ also gives this warning:

$ g++ throw.cpp
throw.cpp: In function 'int main()':
throw.cpp:20: warning: exception of type 'foo' will be caught
throw.cpp:16: warning: by earlier handler for 'foo'

And the output:

$ ./a.out
foo &

--
Thomas
http://www.netmeister.org/news/learn2quote.html

[mlimber]

Thomas J. Gritzan wrote:

as********@gmail.com schrieb:

Hi, just wondering if anybody can answer the following question as it
doesn't seem to be anywhere in the C++ spec.

The compiler can.

What will print from the following statement? (note the order of the
catch statements):

void func(void)

Abomination! (Cf.
<http://groups.google.com/group/comp.lang.c++/msg/895f1f98c4488dda>.)

{
try
{
const foo f1;
throw f1;
}

Since classes are thrown by value (copied), the const qualifier doesn't
affect the thrown object. So the body of catch(foo&) will run.

g++ also gives this warning:

$ g++ throw.cpp
throw.cpp: In function 'int main()':
throw.cpp:20: warning: exception of type 'foo' will be caught
throw.cpp:16: warning: by earlier handler for 'foo'

And the output:

$ ./a.out
foo &

I'd add to this that, when you can (which is almost always), you should
catch a *const* reference (see
<http://www.parashift.com/c++-faq-lite/exceptions.html#faq-17.7and
<http://www.parashift.com/c++-faq-lite/const-correctness.html>).

Cheers! --M