Re: Virtual functions in constructors. Is it possible?
* Leo Equinox Gaspard, on 05.07.2011 19:55:
<code>
#include <iostream>
#include <stdexcept>
#include <cstdlib>
#include <memory>
using namespace std;
namespace g {
int nObjectsAllocated = 0;
} // namespace g
class VirtualBase {
protected:
class Initializer;
friend class Initializer;
class Initializer {
public:
Initializer() {
cout << "Initializer" << endl;
p = 0;
}
~Initializer() {
cout << "~Initializer" << endl;
try {
if (p)
p->Init(); // call the virtual function
} catch (...) {
p->InitFailed();
throw;
}
}
// might be private if VirtualBase is declared as friend...
void Register(VirtualBase* b) const {
p = b;
}
private:
mutable VirtualBase* p;
// private and not implemented
Initializer(const Initializer&);
void operator =(const Initializer&);
};
public:
VirtualBase(const Initializer& i) {
cout << "VirtualBase" << endl;
i.Register(this);
}
private:
virtual void Init() = 0;
// will be called immediately after the constructor
// of the most derived class
virtual void InitFailed() = 0;
// will be called if Init failed. Should leave the object
// in a destructable state.
};
// This is the actual hierarchy
class Base : public virtual VirtualBase {
public:
static void* operator new( size_t size )
{
cout << "Allocation" << endl;
++g::nObjectsAllocated;
return ::operator new( size );
}
static void operator delete( void* p )
{
cout << "Deallocation" << endl;
--g::nObjectsAllocated;
return ::operator delete( p );
}
Base(const VirtualBase::Initializer& i = Initializer()) : VirtualBase(i) {
cout << "Base" << endl;
}
~Base() {
cout << "~Base" << endl;
}
void Init() {
cout << "Base::Init()" << endl;
#ifdef FAIL_INIT
throw runtime_error( "Oops, `Init` failed." );
#endif
}
void InitFailed() {
cout << "Base::InitFailed() - could do something if needed" << endl;
}
};
int main()
{
int result = EXIT_FAILURE;
try
{
auto_ptr<Base> p(new Base); // calls Base::Init.
// We *need* an auto_ptr to ensure that the item will be
// deleted in case of exception.
result = EXIT_SUCCESS;
}
catch( exception const& x )
{
cerr << "!" << x.what() << endl;
}
if( g::nObjectsAllocated > 0 )
{
cerr << "!Memory leak." << endl;
}
cout << "Finished." << endl;
return result;
}
</code>
So I must repeat my question : Where am I mistaking ?
Your code
* has Undefined Behavior if a constructor throws, and
* leaks memory without the auto_ptr
(while there's no leak for an ordinary single stage construction, because the
new-expression then cleans up).
The UB is critical, so this technique is only applicable for guaranteed no-throw
constructors, which except for default constructors are pretty rare beasts nowadays.
I don't know what you think you're mistaken about, sorry, but hopefully it's one
of the two points above?
Cheers & hth.,
- Alf
--
blog at <url: http://alfps.wordpress.com>
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