virtual destructor needs an implementation.Declaring a destructor pure still requires you to define it (unlike a regular function):
struct X
{
virtual ~X() = 0;
};
struct Y : X
{
~Y() {}
};
int main()
{
Y y;
}
//X::~X(){} //uncomment this line for successful definition
This happens because base class destructors are called when the object is destroyed implicitly, so a definition is required.
virtual methods must either be implemented or defined as pure.This is similar to non-virtual methods with no definition, with the added reasoning that
the pure declaration generates a dummy vtable and you might get the linker error without using the function:
struct X
{
virtual void foo();
};
struct Y : X
{
void foo() {}
};
int main()
{
Y y; //linker error although there was no call to X::foo
}
For this to work, declare X::foo() as pure:
struct X
{
virtual void foo() = 0;
};
virtual class membersSome members need to be defined even if not used explicitly:
struct A
{
~A();
};
The following would yield the error:
A a; //destructor undefined
The implementation can be inline, in the class definition itself:
struct A
{
~A() {}
};
or outside:
A::~A() {}
If the implementation is outside the class definition, but in a header, the methods have to be marked as inline to prevent a multiple definition.
All used member methods need to be defined if used.
struct A
{
void foo();
};
void foo() {}
int main()
{
A a;
a.foo();
}
The definition should be
void A::foo() {}
static data members must be defined outside the class in a single translation unit:struct X
{
static int x;
};
int main()
{
int x = X::x;
}
//int X::x; //uncomment this line to define X::x
An initializer can be provided for a static const data member of integral or enumeration type within the class definition; however, odr-use of this member will still require a namespace scope definition as described above. C++11 allows initialization inside the class for all static const data members.