C++ - LNK 2019 Error, Trying to Define a Subclass Static Function

Question

The call that generates the unresolved external symbol:

#include <string.h>
#include "GContext.h"
#include "GBitmap.h"
#include "GColor.h"

int main(int argc, char** argv) {
    const int W = 100;
    const int H = 100;
    GContext* ctx = GContext::Create(W, H);

The abstract class method signature:

#ifndef GContext_DEFINED
#define GContext_DEFINED

#include "GTypes.h"

class GBitmap;
class GColor;

class GContext {
public:
    GContext() {}
    virtual ~GContext() {}


    virtual void getBitmap(GBitmap*) const = 0;

    virtual void clear(const GColor&) = 0;


    static GContext* Create(const GBitmap&);

    static GContext* Create(int width, int height);
};

#endif

And the Current Derived Class Implementation and Method Signature:

#include "GColor.h"
#include "GPixel.h"
#include "GBitmap.h"
#include "GContext.h"
#include "GTypes.h"
class myGContext : public GContext
{
public:
        myGContext() : GContext(){}
        static const GBitmap* bitmap;

        void getBitmap(GBitmap* bitmap) const
        {

        }

        void clear(const GColor& gcolor)
        {
        int length = sizeof( (GPixel)(bitmap->fPixels) ) / sizeof(GPixel);
        for (int i = 0; i < length; i++)
        {
            (bitmap->fPixels)[i]
        }

        }

        static GContext* Create(const GBitmap& gbitmap)
        { 
        GContext::Create(gbitmap);
        bitmap = &gbitmap;
        GContext* g = new myGContext();
        return g;
        }


        static GContext* Create(int width, int height)
        {
        GContext::Create(width,height);
        GContext* g = new myGContext();
        return g;

    }
};

So I understand that I need to define both types of the function GContext::Create() to resolve the external symbol error, but I need to define them in my derived Class. Which I thought I was doing right, any ideas?

Did you defined you static methods inside your `GContext` class too ? — Pierre Fourgeaud, Sep 04 '13 at 20:54

score 0 · Answer 1 · answered Sep 04 '13 at 20:27

0

no Inheritance does not work, this is not like a virtual function.

answered Sep 04 '13 at 20:27

dzada

5,344
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score 0 · Answer 2 · answered Sep 04 '13 at 20:40

I'm not exactly sure what you're trying to do but if you

Need to have static functions
Need both base and derived classes to have their own implementation
derived needs access to the base class' functions

this is all achievable:

#include <iostream>

class A {
public:
    A() {}
    static void f() { std::cout << "A f" << std::endl; }
};

class B : public A {
public:
    B() {}
    static void f() { std::cout << "B f" << std::endl; }
};




int main(int argc, char* argv[]) {

    A a;
    B b;

    a.f();
    b.f();
    b.A::f();
    return 0;
}

Output is

A f
B f
A f

score 0 · Answer 3 · edited Jun 20 '20 at 09:12

I think it is just because you static method is not defined in your base class. From here it is said that LNK2019 can also occur when a static data member is declared but not defined.

Also, be careful when you try to redefine static methods inside subclasses:

You cannot override a static method in a subclass, you can only hide it.

And from the C++ standard:

9.4.1 Static member functions [class.static.mfct]

2/ A static member function shall not be virtual. There shall not be a static and a non-static member function with the same name and the same parameter types (13.1). A static member function shall not be declared const, volatile, or const volatile.

Example:

#include <iostream>

class Foo
{
public:
  static void func() { std::cout << "Foo::func" << std::endl; }
};

class Bar : public Foo
{
public:
  static void func() { std::cout << "Bar::func" << std::endl; }
};

int main(void)
{
  Foo::func();     // Works
  Bar::func();     // Works

  Foo foo;
  Bar bar;

  foo.func();        // Works
  bar.func();        // Works
  bar.Foo::func();   // Works

  Foo* foobar = new Bar;

  foobar->func();      // Not the result expected
                       // Because no override.
  return 0;
}

C++ - LNK 2019 Error, Trying to Define a Subclass Static Function

3 Answers3