C++'s pure virtual function implementation and header files

Question

I'm having some trouble implementing pure virtual functions inherited from some abstract class, when the classes in question are divided into *.h and *.cpp files. The compiler (g++) tells me that the derived class cannot be instantiated because of the existence of pure functions.

/** interface.h**/
namespace ns
{
    class Interface {
        public:
            virtual void method()=0;
    }
}

/** interface.cpp**/
namespace ns
{
    //Interface::method()() //not implemented here
}

/** derived.h **/
namespace ns
{
    class Derived : public Interface {
        //note - see below
    }
}

/** derived.cpp **/
namespace ns
{
    void Derived::Interface::method() { /*doSomething*/ }
}

/** main.cpp **/
using namespace ns;
int main()
{
    Interface* instance = new Derived; //compiler error
}

Does this mean that I have to declare the method() twice - in the Interface's *.h and in the derived.h too? Is there no other way around?

First, there's no reason to have a .cpp file in which the only thing is a blank namespace. You don't need an implementation file for a header congaing an abstract class. — Falmarri, Jan 13 '11 at 00:37

score 18 · Answer 1 · edited Feb 21 '23 at 08:08

18

You have to declare your method in the subclass.

// interface.hpp
class Interface {
public:
    virtual void method()=0;
}

// derived.hpp
class Derived : public Interface {
public:
    void method() override;
}

// derived.cpp
void
Derived::method()
{
    // do something
}

edited Feb 21 '23 at 08:08

Moritz Schmidt

2,635
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answered Jan 13 '11 at 00:34

robert

33,242
8
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I think you mean **declare** (not define). – Mike S Sep 10 '15 at 16:49
@MikeSlutsky yes I did. – robert Sep 10 '15 at 17:23
@robert Thank you for the answer. Can you ellaborate on the reasons for that ? Coming from Java it seems very dirty to have to do that. When we have many derived different classes and many methods in the base interface we need to change a lot of files in case of change in the base interface. – Manuel Selva Sep 30 '15 at 15:07
@ManuelSelva you can't define a method in a class unless it's declared in that class. This is independent of whether this class derives from anything. – robert Sep 30 '15 at 15:34

score 16 · Accepted Answer · answered Jan 13 '11 at 00:34

16

You forgot to declare Derived::method().

You tried to define it at least, but wrote Derived::Interface::method() rather than Derived::method(), but you did not even attempt to declare it. Therefore it doesn't exist.

Therefore, Derived has no method(), therefore the pure virtual function method() from Interface was not overridden... and therefore, Derived is also pure virtual and cannot be instantiated.

Also, public void method()=0; is not valid C++; it looks more like Java. Pure virtual member functions have to actually be virtual, but you did not write virtual. And access specifiers are followed by a colon:

public:
    virtual void method() = 0;

answered Jan 13 '11 at 00:34

Lightness Races in Orbit

378,754
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The void bit probably a typo, it's not java either. – time4tea Jan 13 '11 at 01:35
@time4tea: Indeed it's not valid Java either, but with the missing colon and the implication that the access specifier has been seen as something joined to the declaration, it's a damn sight closer than it is to C++! – Lightness Races in Orbit Jan 13 '11 at 01:36
The syntax was my typo, see my comment to the opening post :) sorry about that. Also, the `Derived::Interface::method()` is my attempt to define a function that should be inherited from Interface. My original question was something along the lines "can I implement (define) an inherited pure virtual function without explicitly declaring it in the derived class's `.h` file?". The reason I'd like to do that is to avoid cluttering my header file when the class inherits from several interfaces, containing several pure virtuals each... – Neo Jan 13 '11 at 06:42
@Neo: And the answer is no. :) – Lightness Races in Orbit Jan 13 '11 at 09:39
@Thomalak Geret'kal: Ah. Pity. :( Thanks a lot for your help :) – Neo Jan 13 '11 at 18:43

score 0 · Answer 3 · answered Mar 03 '22 at 04:02

0

You also did not call constructor properly - it's also a method;

 Interface* instance = new Derived;

Should be

 Interface* instance = new Derived();

answered Mar 03 '22 at 04:02

алекс кей

821
9
11

This is C++ and you certainly can create objects with both `new Derived;` and `new Derived();`. There's a certain difference between the two (see [this answer](https://stackoverflow.com/questions/620137/do-the-parentheses-after-the-type-name-make-a-difference-with-new) ), but in this case they're equivalent because there are no fields to initialize. – Neo Mar 03 '22 at 09:16

C++'s pure virtual function implementation and header files

3 Answers3