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berkus wrote:hi!
class need library sometimes.
pleas

berkus wrote:1) they are pure and therefore are not implemented

Griwes wrote:

berkus wrote:1) they are pure and therefore are not implemented

You know that pure virtual function can have implementation, don't you? ;D

virtual void virtualfunctioname() = 0;

Solar wrote:

Code: Select all

virtual void virtualfunctioname() = 0;

The "= 0" indicates that this function does not have an implementation, which - at least in C++ - is the definition of "pure virtual". Not only can the function not have an implementation, the whole class cannot be instantiated.

virtual void func() = 0
{ std::cout << "HELLO WORLD"; }

berkus wrote: Are you trying to approach C++ programming without actually understanding what C++ is and how it works? This approach is bound to repetitive fail.

cyr1x wrote: I think you are wrong here.

solar@valkyrie ~ $ cat test.cpp 
#include <iostream>

class AbstractClass
{
    public:
        virtual void func() = 0
        {
            std::cout << "Hello World" << std::endl;
        }
};
solar@valkyrie ~ $ g++ -c test.cpp -o test.o
test.cpp:6:29: error: pure-specifier on function-definition

hojjatrakhshani wrote:

berkus wrote: Are you trying to approach C++ programming without actually understanding what C++ is and how it works? This approach is bound to repetitive fail.

can you give me book or some resource aboute this?

The use case I can think of off the top of my head is when there's a more-or-less reasonable default behavior, but the class designed wants that sort-of-default behavior to be invoked only explicitly. It can also be the case what you want derived classes to always perform their own work but also be able to call a common set of functionality.

Note that even though it's permitted by the language, it's not something that I see commonly used (and the fact that it can be done seems to surprise most C++ programmers, even experienced ones).