On 10/22/2014 05:30 PM, Jonathan Wakely wrote:
On 22 October 2014 16:15, Martin Liška wrote:
Hello.
I've been playing with following example:
#include <stdlib.h>
class Base
{
public:
virtual ~Base() {}
};
class Derived: public Base
{
};
#define N 1000
int main()
{
Base **b = (Base **)malloc (sizeof(Base *) * N);
for (unsigned i = 0; i < N; i++)
b[i] = new Derived();
for (unsigned i = 0; i < N; i++)
delete b[i];
return 0;
}
Where I would like to somehow give an advice to devirtualize machinery. My
motivation is to inline destruction in 'delete b[i]'.
'final' keyword does not solve my problem:
a.c:9:7: error: virtual function ‘virtual Derived::~Derived()’
class Derived: public Base
^
a.c:6:11: error: overriding final function ‘virtual Base::~Base()’
virtual ~Base() final {}
Virtual destructors are special, derived classes *must* override them,
so you can't make it final in the base class.
If I enclose my classes to anonymous namespace:
Procesing function int main()/172
Targets of polymorphic call of type 0:struct Base token 2
Contained in type:struct Base at offset 0
This is a complete list. (derived types included)
virtual {anonymous}::Base::~Base()/164 virtual
{anonymous}::Derived::~Derived()/183
More than one likely target
Presumably because the dynamic type of *b[i] could be Base or Derived,
and the compiler can't know until it checks it. Does it make any
difference if you make Base an abstract class? In that case the
compiler should know that the dynamic type of *b[i] cannot be Base, so
must be a Derived.
Looks that addition of an abstract method to Base class really helps:
for (unsigned i = 0; i < N; i++)
delete b[i];
4006a0: 48 8b 3b mov (%rbx),%rdi
4006a3: 48 85 ff test %rdi,%rdi
4006a6: 74 05 je 4006ad <main+0x4d>
public:
virtual ~Base() {}
virtual void Foo() = 0;
};
class Derived final: public Base
4006a8: e8 83 ff ff ff callq 400630 <_ZdlPv@plt>
4006ad: 48 83 c3 08 add $0x8,%rbx
Thank you,
Martin
My question is how can one help compiler if he knows that a class hierarchy
is complete and there's no destructor except the virtual for base class?
