Re: exceptions in constructor
On 2008-10-18 18:29, vaclavpich@atlas.cz wrote:
Hi
I've a question about constructors and exceptions.
//************************************************************
class CObject
{
public:
// ctor
CObject();
// dtor
~ CObject();
// ... methods
};
CObject::CObject()
{
// 1) if ( FAILED( LoadLibrary(...) )) throw exception1;
// 2) if ( FAILED(CoInitialize( 0 )) throw exception2;
// 3) if ( FAILED( CoCreateInstance(....))) throw exception3;
}
CObject::~CObject()
{
// 1) release COM interface
// 2) CoUninitialize();
// 2) FreeLibrary
}
int main(int avgv, char** argc)
{
try{
CObject* ptrObj = new CObject;
}
catch(exception& exc)
{...}
return 0;
}
//****************************************************************
When an error occurs in constructor there is only one way how to say
that samething wrong happen.You can throw an exception but is then a
pointer to CObject valid ?
So can you call detructor ~CObject() to release resources. I mean
this : delete ptrObj;
No, there will be no object, so you can not get a pointer to it. On the
other hand you do not have to worry about destroying it either, it is
all taken care of by new.
Is really constructor good place to attach resources (load library or
to get COM interface) ?
I'm not sure.
The C++ programming RAII idiom relies on the constructor allocating all
the resources needed. You should, however, take care to ensure that you
do not leek resources if the constructor throws by doing some exception
handling inside the constructor too:
CObject::CObject()
{
// LoadLibrary(...), if this throws no harm done
try {
// Allocate(...), if this throws we need to clean up
}
catch(...) {
// Clean up, free allocated memory etc.
throw; // Re-throw the exception
}
try {
// Initialise(...), will require clean up too
}
catch(...) {
// Clean up from initialisation
// Free memory etc
throw; // Re-throw the exception
}
}
I've seen another way. Constructor and destructor are very simple
methods.
All resources are attached in method Initialize and released in
Uninitialize.
These methods can return error codes.
This is called two stage initialisation, and many consider it a bad thing.
--
Erik Wikstr??m