Is there ever a good reason to not declare a virtual destructor for a class? When should you specifically avoid writing one?
To answer the question explicitly, i.e. when should you not declare a virtual destructor.
C++ '98/'03
Adding a virtual destructor might change your class from being POD (plain old data)* or aggregate to non-POD. This can stop your project from compiling if your class type is aggregate initialized somewhere.
struct A {
// virtual ~A ();
int i;
int j;
};
void foo () {
A a = { 0, 1 }; // Will fail if virtual dtor declared
}
In an extreme case, such a change can also cause undefined behaviour where the class is being used in a way that requires a POD, e.g. passing it via an ellipsis parameter, or using it with memcpy.
void bar (...);
void foo (A & a) {
bar (a); // Undefined behavior if virtual dtor declared
}
[* A POD type is a type that has specific guarantees about its memory layout. The standard really only says that if you were to copy from an object with POD type into an array of chars (or unsigned chars) and back again, then the result will be the same as the original object.]
Modern C++
In recent versions of C++, the concept of POD was split between the class layout and its construction, copying and destruction.
For the ellipsis case, it is no longer undefined behavior it is now conditionally-supported with implementation-defined semantics (N3937 - ~C++ '14 - 5.2.2/7):
...Passing a potentially-evaluated argument of class type (Clause 9) having a non-trivial copy constructor, a non-trivial move constructor, or a on-trivial destructor, with no corresponding parameter, is conditionally-supported with implementation-defined semantics.
Declaring a destructor other than =default will mean it's not trivial (12.4/5)
... A destructor is trivial if it is not user-provided ...
Other changes to Modern C++ reduce the impact of the aggregate initialization problem as a constructor can be added:
struct A {
A(int i, int j);
virtual ~A ();
int i;
int j;
};
void foo () {
A a = { 0, 1 }; // OK
}