Re: [boost] throw_exception rev. 44954 -> g++ warnings

From: Simonson, Lucanus J (lucanus.j.simonson_at_[hidden])
Date: 2008-05-03 15:37:52

Hartmut wrote:
>What's the problem with making the destructor virtual in the first
place?
>Since the class already has other virtual functions this won't generate
any
>(significant) additional overhead (the only thing what's added is
another
>function pointer to the already existing virtual function table, barely
>something to worry about).

This is not true.  In this case, the destructor is a no-op.  Virtual
functions incur significant overhead over non-virtual functions in the
case that the non-virtual would be inlined, where-as the virtual must
have a symbol for the pointer in the virtual function table to point to,
and cannot be called inline unless the exact type is known at compile
time (defeating the purpose of polymorphism.)

Consider the difference between this:

inline ~classname() {} //do nothing

inline virtual ~classname() {} //do nothing but pay function call
overhead anyway because the compiler usually can't inline the function.

There seems to be a widespread misconception that virtual functions have
low overhead.  This can be true, but when loss of potential inlining
happens it is a significant overhead.  This may seem like a small issue
until you profile code that spends double digit percentage of runtime
executing an out-of-line function call to a no-op destructor.

Luke