The context: I wrote some tools for archiving data, in a similar way of archives from boost. Then, as example, I can write this kind of code :
class A
{
private:
double a;
public:
A() : a(3.14159)
{}
A(const A& a_) : a(a_.a) {}
virtual ~A()
{}
virtual A* clone() const = 0; // Then, A is virtual
virtual void save(O_Archive& oa) const //
{ //
oa << this->a; // INTERESTING
} // PART OF THE
virtual void load(I_archive& ia) // CLASS
{ //
ia >> this->a; //
} //
};
O_Archive& operator << (O_Archive& oa, const A& a)
{
a.save(oa);
return oa;
}
I_Archive& operator >> (I_Archive& ia, A& a)
{
a.load(ia);
return ia;
}
class B : public A
{
private:
double b;
public:
B() : A(), b(1.0) {}
B(const B& b_) : A(b_), b(b_.b) {}
virtual ~B() {}
virtual A* clone() const
{
return new B(*this);
}
void save(O_Archive& oa) const //
{ //
this->A::save(oa); //
oa << this->b; // INTERESTING
} // PART OF THE
void load(I_Archive& ia) // CLASS
{ //
this->A::load(ia); //
ia >> this->b; //
} //
};
// Consider classes 'C' and 'D' similar to 'B'
void example_save(O_Archive& oa)
{
A* p1 = new B;
A* p2 = new C;
D* p3 = new D;
oa << Archive::declare_derived<A,B,C,D>();
oa << p1 << p2; // Automatically detect the inheritance
oa << p3; // Store the instance as a usual pointer
}
void example_load(I_Archive& ia)
{
A* p1 = 0;
A* p2 = 0;
B* p3 = 0;
ia << Archive::declare_derived<A,B,C,D>();
ia >> p1 >> p2;
ia >> p3;
}
Where is the problem ? This works with several functions like the following load_pointer function in the class I_Archive in charge of checking if the pointer was allocated, if it was an instance with a derived type, or simply a usual pointer.
template <typename T>
void I_Archive::load_pointer(T*& p)
{
delete p;
bool allocated;
this->load_bool(allocated);
if(allocated)
{
bool inheriance;
this->load_bool(inheriance);
if(inheriance)
{
unsigned long int i;
this->load_unsigned_long_int(i);
p = boost::static_pointer_cast< const Archive::allocator<T> >(this->alloc[&typeid(T)][i])->allocate();
}
else
p = new T; // ERROR AT THIS LINE
*this >> *p;
}
else
p = 0;
}
My problem: Actually, my code doesn't compile with the following error on the line p = new T; :
error: cannot allocate an object of abstract type ‘A’.
I was first surprised, but I really well understand why I have this error : when the function load_pointer is called on p1, the instruction new T becomes new A which is forbidden, even if the instruction is never run if the type is abstract.
My question: I can't find a way to correctly use templates to avoid my error. Is there a possible workaround to do that or to say to the compiler "I know what I'm doing, you'll never have to instanciate an abstract type" ?
Important note: I can't work with C++11 for compatibility reason.
The trait you're looking for is std::is_abstract. As you mentioned, you cannot use C++11, but you can use its implementation from boost.
You can then use is_abstract together with std::enable_if (again, due to your restriction of not using C++11, you can just take the example implementation from here) to implement it similarly to this:
#include <iostream>
#include <type_traits>
struct A {
virtual void f() = 0;
};
struct B : A {
void f() override {}
};
template<typename T>
std::enable_if_t<std::is_abstract<T>::value, T*> allocate()
{
return nullptr;
}
template<typename T>
std::enable_if_t<!std::is_abstract<T>::value, T*> allocate()
{
return new T;
}
// Test
template<typename T>
T* test_alloc()
{
return allocate<T>();
}
int main()
{
std::cout << test_alloc<A>() << "\n"; // Outputs nullptr
std::cout << test_alloc<B>() << "\n"; // Outputs an address
}