Re: Virtual Functions And Inline Definition
Marcelo De Brito wrote:
Hi!
Why is not possible to define a pure virtual inline function in C++?
For example:
class c1 {
virtual void f() = 0 {} // ERROR
virtual void g() = 0; // FINE
};
Why?
I appreciate your comments, suggestions, and etc.
Because a virtual function is secretly a pointer to a function, and {} inside a
class is the same as an explicit inline. 'inline' is a hint to the compiler it
can push a method's opcodes into its call sites, without the overhead of calling
a function and jumping into a different stack frame. Methods defined inside
classes are 'inline' by default.
Taking the address of an inline function, in turn, blows away that "hint", and
forces the function to appear out-of-line. And the compiler must take the
address to generate the secret pointer for virtual dispatch.
All these rules were invented when C was young. Pure virtual is a late addition,
so it does what other abuses of inline cannot - it puts up an error if you
attempt to mix the virtual and inline concepts together.
The C++ FAQ will have an answer here too, potentially more accurate!
--
Phlip