C++ equivalent for VB TypeOf operator?

[Bob Altman]

Hi all,

What's the C++ equivalent for the VB TypeOf operator. For example:

void ReportError(Sys tem::Exception^ ex)
{
// Psuedo-VB syntax
if (TypeOf ex Is System::Operati onCanceledExcep tion) return;

// Decode the exception and tell the user about it...
<...>
}

TIA - Bob

[Cholo Lennon]

Bob Altman wrote:

Hi all,

What's the C++ equivalent for the VB TypeOf operator. For example:

dynamic_cast (http://msdn.microsoft.com/en-us/library/cby9kycs.aspx)

The operator returns 0 (or nullptr in C++ CLI) if the convertion fails.

void ReportError(Sys tem::Exception^ ex)
{
// Psuedo-VB syntax
if (TypeOf ex Is System::Operati onCanceledExcep tion) return;

// Decode the exception and tell the user about it...
<...>
}

The general pattern to use dynamic_cast is:

if (System::Operat ionCanceledExce ption^ opCanEx = dynamic_cast<Sy stem::Operation CanceledExcepti on^>(ex)) {
// Use opCanEx
...
}

Regards

--
Cholo Lennon
Bs.As.
ARG

[Jialiang Ge [MSFT]]

Good morning Bob and Cholo

If you would like to handle System::Operati onCanceledExcep tion and all the
subclasses of System::Operati onCanceledExcep tion in the if statement, we
can use Cholo's suggestion of dynamic_cast:

if (dynamic_cast<S ystem::Operatio nCanceledExcept ion^>(ex))
{
// if ex belongs to a subclass of System::Operati onCanceledExcep tion, the
above if statement will still return true.
// Decode the exception and tell the user about it
}

If you would like to only handle System::Operati onCanceledExcep tion and
disregard the subclasses of OperationCancel edException, the 'typeid'
keyword will be proper:
http://msdn.microsoft.com/en-us/library/kwd9abya.aspx

void ReportError(Sys tem::Exception^ ex)
{
if (ex->GetType() == System::Operati onCanceledExcep tion::typeid)
{
// Decode the exception and tell the user about it
}
}

Please let me know whether the above information is helpful to you or not.
If you have any other questions/concerns, feel free to tell me.

Have a very nice day!

Regards,
Jialiang Ge (ji****@online. microsoft.com, remove 'online.')
Microsoft Online Community Support

Delighting our customers is our #1 priority. We welcome your comments and
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[Bob Altman]

Thanks a million!

[Paavo Helde]

"Cholo Lennon" <ch*********@ho tmail.comkirjut as:

Bob Altman wrote:

>Hi all,

What's the C++ equivalent for the VB TypeOf operator. For example:

dynamic_cast (http://msdn.microsoft.com/en-us/library/cby9kycs.aspx)

The operator returns 0 (or nullptr in C++ CLI) if the convertion
fails.

>void ReportError(Sys tem::Exception^ ex)
{
// Psuedo-VB syntax
if (TypeOf ex Is System::Operati onCanceledExcep tion) return;

// Decode the exception and tell the user about it...
<...>

dynamic_cast only works on polymorphic pointers/references (i.e. having
at least one virtual function). While it's true that std::exception is
polymorphic, there might be other exception objects which are not. In C++
it is possible to distinguish them, but only if you know the types in
advance.

In such a situation, it is often the case that one does not have an idea
which exceptions might arise, so I give an example with a catch(...)
clause. This by itself does not give any information about the exception
type so it has to be "resurrecte d" by another function:

std::string ResurrectExcept ion();

int f() {
try {
// actual code ...
} catch(...) {
std::string msg = ResurrectExcept ion();
std::cerr << "Exception: " << msg << "\n";
}
}
std::string ResurrectExcept ion() {
try {
throw; // valid only when called from inside a catch block
} catch(const std::exception& e) {
return e.what();
} catch(int e) {
std::ostringstr eam os;
os << e;
return "Int exception: " + os.str();
} catch(const my_private_exce ption& e) {
return e.ToString();
} catch(...) {
return "Unknown exception";
}
}

[Cholo Lennon]

Paavo Helde wrote:

"Cholo Lennon" <ch*********@ho tmail.comkirjut as:

>Bob Altman wrote:

>>Hi all,

What's the C++ equivalent for the VB TypeOf operator. For example:

dynamic_cast (http://msdn.microsoft.com/en-us/library/cby9kycs.aspx)

The operator returns 0 (or nullptr in C++ CLI) if the convertion
fails.

>>void ReportError(Sys tem::Exception^ ex)
{
// Psuedo-VB syntax
if (TypeOf ex Is System::Operati onCanceledExcep tion) return;

// Decode the exception and tell the user about it...
<...>

dynamic_cast only works on polymorphic pointers/references (i.e.
having at least one virtual function). While it's true that
std::exception is polymorphic, there might be other exception objects
which are not. In C++ it is possible to distinguish them, but only if
you know the types in advance.

You're right, but just a clarification: the OP asked about "TypeOf" with a .Net exception (not std::exception) . BTW all .Net
exceptions are polymorphics.

Regards

>
In such a situation, it is often the case that one does not have an
idea which exceptions might arise, so I give an example with a
catch(...) clause. This by itself does not give any information about
the exception type so it has to be "resurrecte d" by another function:

std::string ResurrectExcept ion();

int f() {
try {
// actual code ...
} catch(...) {
std::string msg = ResurrectExcept ion();
std::cerr << "Exception: " << msg << "\n";
}
}
std::string ResurrectExcept ion() {
try {
throw; // valid only when called from inside a catch block
} catch(const std::exception& e) {
return e.what();
} catch(int e) {
std::ostringstr eam os;
os << e;
return "Int exception: " + os.str();
} catch(const my_private_exce ption& e) {
return e.ToString();
} catch(...) {
return "Unknown exception";
}
}

--
Cholo Lennon
Bs.As.
ARG

[Paavo Helde]

"Cholo Lennon" <ch*********@ho tmail.comkirjut as:

>
You're right, but just a clarification: the OP asked about "TypeOf"
with a .Net exception (not std::exception) .

Yes sorry, I have to check more carefully in which ng I am posting!

Cheers
Paavo