Error building with make: Undefined reference to `CEGUI::NullRenderer::create()'

Question

Lately someone reminded me of a game I used to play back when I was growing up (not so long ago) and I wanted to try getting it to run again. It hasn't been developed for almost ten years so no of the prebuilt packages work. I grabbed an the source code for the game and for an old version of the CEGUI library and got to work.

I fixed a few issues in the make process but now I'm stuck with this error message:

/usr/bin/ld: smc-video.o: in function `SMC::cVideo::Init_CEGUI_Fake() const':
video.cpp:107: undefined reference to `CEGUI::NullRenderer::create()'

I have little experience with C++, but I managed to understand that it's a linker problem that cannot find the function declaration. So I took a look in the file that should have the declaration and indeed there is a create function:

/*!
\brief
    Create an NullRenderer object 
*/
static NullRenderer& create();

But doesn't it miss a declaration? I've checked that the linker looks for the function in the correct file as adding {} to the code does prevent the error (it does of course lead to different errors) which makes me believe that there is nothing wrong with make. I've also looked for the declaration in the includes from the CEGUINullRenderer.h but didn't find them. (There is no CEGUINullRenderer.cpp)

I tried just returning a NullRenderer object from the create function but that doesn't really work. What am I missing here?

video.cpp (up till the part that's throwing the error):

/***************************************************************************
 * video.cpp  -  General video functions
 *
 * Copyright (C) 2005 - 2011 Florian Richter
 ***************************************************************************/
/*
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.
   
   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include "../video/video.h"
#include "../gui/hud.h"
#include "../user/preferences.h"
#include "../core/framerate.h"
#include "../video/font.h"
#include "../core/game_core.h"
#include "../video/img_settings.h"
#include "../input/mouse.h"
#include "../video/renderer.h"
#include "../core/main.h"
#include "../core/math/utilities.h"
#include "../core/i18n.h"
#include "../core/math/size.h"
#include "../core/filesystem/filesystem.h"
#include "../core/filesystem/resource_manager.h"
#include "../gui/spinner.h"
// SDL
#include "SDL_opengl.h"
// CEGUI
#include "CEGUIDefaultResourceProvider.h"
#include "CEGUIDefaultLogger.h"
#include "CEGUIExceptions.h"
#include "CEGUIWindowFactoryManager.h"
#include "CEGUIImagesetManager.h"
#include "CEGUIFontManager.h"
#include "CEGUIWindowManager.h"
#include "CEGUISchemeManager.h"
#include "falagard/CEGUIFalWidgetLookManager.h"
#include "elements/CEGUIProgressBar.h"
#include "RendererModules/Null/CEGUINullRenderer.h"
// png
#include <png.h>
#ifndef PNG_COLOR_TYPE_RGBA
    #define PNG_COLOR_TYPE_RGBA PNG_COLOR_TYPE_RGB_ALPHA
#endif

namespace SMC
{

/* *** *** *** *** *** *** *** Video class *** *** *** *** *** *** *** *** *** *** */

cVideo :: cVideo( void )
{
    m_opengl_version = 0;

    m_double_buffer = 0;

    m_rgb_size[0] = 0;
    m_rgb_size[1] = 0;
    m_rgb_size[2] = 0;

    m_default_buffer = GL_BACK;
    m_max_texture_size = 512;
    
    m_audio_init_failed = 0;
    m_joy_init_failed = 0;
    m_geometry_quality = cPreferences::m_geometry_quality_default;
    m_texture_quality = cPreferences::m_texture_quality_default;

    SDL_VERSION( &wm_info.version );
#ifdef __unix__
    glx_context = NULL;
#endif
    m_render_thread = boost::thread();

    m_initialised = 0;
}

cVideo :: ~cVideo( void )
{

}

void cVideo :: Init_CEGUI_Fake( void ) const
{
    // create fake Resource Provider
    CEGUI::DefaultResourceProvider *rp = new CEGUI::DefaultResourceProvider();
    // set Resource Provider directories
    if( CEGUI::System::getDefaultXMLParserName().compare( "XercesParser" ) == 0 )
    {
        // This is needed for Xerces to specify the schemas location
        rp->setResourceGroupDirectory( "schemas", DATA_DIR "/" GAME_SCHEMA_DIR "/" );
    }
    // get a directory to dump the CEGUI log
#ifdef _WIN32
    // fixme : Workaround for std::string to CEGUI::String utf8 conversion. Check again if CEGUI 0.8 works with std::string utf8
    CEGUI::String log_dump_dir = (const CEGUI::utf8*)((Get_Temp_Directory() + "cegui.log").c_str());
#else
    CEGUI::String log_dump_dir = "/dev/null";
#endif
    // create fake system and renderer
    pGuiSystem = &CEGUI::System::create( CEGUI::NullRenderer::create(), rp, NULL, NULL, NULL, "", log_dump_dir );
}

CEGUINullRenderer.h:

/***********************************************************************
    filename:   CEGUINullRenderer.h
    created:    Fri Jan 15 2010
    author:     Eugene Marcotte
*************************************************************************/
/***************************************************************************
 *   Copyright (C) 2004 - 2010 Paul D Turner & The CEGUI Development Team
 *
 *   Permission is hereby granted, free of charge, to any person obtaining
 *   a copy of this software and associated documentation files (the
 *   "Software"), to deal in the Software without restriction, including
 *   without limitation the rights to use, copy, modify, merge, publish,
 *   distribute, sublicense, and/or sell copies of the Software, and to
 *   permit persons to whom the Software is furnished to do so, subject to
 *   the following conditions:
 *
 *   The above copyright notice and this permission notice shall be
 *   included in all copies or substantial portions of the Software.
 *
 *   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 *   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 *   MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 *   IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
 *   OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
 *   ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 *   OTHER DEALINGS IN THE SOFTWARE.
 ***************************************************************************/
#ifndef _CEGUINullRenderer_h_
#define _CEGUINullRenderer_h_

#include "../../CEGUIRenderer.h"
#include "../../CEGUISize.h"
#include "../../CEGUIVector.h"

#include <vector>
#include <map>

#if (defined( __WIN32__ ) || defined( _WIN32 )) && !defined(CEGUI_STATIC)
#   ifdef NULL_GUIRENDERER_EXPORTS
#       define NULL_GUIRENDERER_API __declspec(dllexport)
#   else
#       define NULL_GUIRENDERER_API __declspec(dllimport)
#   endif
#else
#   define NULL_GUIRENDERER_API
#endif

#if defined(_MSC_VER)
#   pragma warning(push)
#   pragma warning(disable : 4251)
#endif


// Start of CEGUI namespace section
namespace CEGUI
{
class NullGeometryBuffer;
class NullTexture;
class NullRenderTarget;

//! CEGUI::Renderer implementation for no particular engine
class NULL_GUIRENDERER_API NullRenderer : public Renderer
{
public:
    /*!
    \brief
        Convenience function that creates all the necessary objects
        then initialises the CEGUI system with them.

        This will create and initialise the following objects for you:
        - CEGUI::NullRenderer
        - CEGUI::DefaultResourceProvider
        - CEGUI::System

    \return
        Reference to the CEGUI::NullRenderer object that was created.

    */
    static NullRenderer& bootstrapSystem();
    
    /*!
    \brief
        Convenience function to cleanup the CEGUI system and related objects
        that were created by calling the bootstrapSystem function.

        This function will destroy the following objects for you:
        - CEGUI::System
        - CEGUI::DefaultResourceProvider
        - CEGUI::NullRenderer

    \note
        If you did not initialise CEGUI by calling the bootstrapSystem function,
        you should \e not call this, but rather delete any objects you created
        manually.
    */
    static void destroySystem();

    /*!
    \brief
        Create an NullRenderer object 
    */
    static NullRenderer& create();

    //! destory an NullRenderer object.
    static void destroy(NullRenderer& renderer);

    // implement CEGUI::Renderer interface
    RenderingRoot& getDefaultRenderingRoot();
    GeometryBuffer& createGeometryBuffer();
    void destroyGeometryBuffer(const GeometryBuffer& buffer);
    void destroyAllGeometryBuffers();
    TextureTarget* createTextureTarget();
    void destroyTextureTarget(TextureTarget* target);
    void destroyAllTextureTargets();
    Texture& createTexture();
    Texture& createTexture(const String& filename, const String& resourceGroup);
    Texture& createTexture(const Size& size);
    void destroyTexture(Texture& texture);
    void destroyAllTextures();
    void beginRendering();
    void endRendering();
    void setDisplaySize(const Size& sz);
    const Size& getDisplaySize() const;
    const Vector2& getDisplayDPI() const;
    uint getMaxTextureSize() const;
    const String& getIdentifierString() const;

protected:
    //! default constructor.
    NullRenderer();
    //! common construction things.
    void constructor_impl();
    //! destructor.
    virtual ~NullRenderer();

    //! String holding the renderer identification text.
    static String d_rendererID;
    //! What the renderer considers to be the current display size.
    Size d_displaySize;
    //! What the renderer considers to be the current display DPI resolution.
    Vector2 d_displayDPI;
    //! The default rendering root object
    RenderingRoot* d_defaultRoot;
    //! The default RenderTarget (used by d_defaultRoot)
    NullRenderTarget* d_defaultTarget;
    //! container type used to hold TextureTargets we create.
    typedef std::vector<TextureTarget*> TextureTargetList;
    //! Container used to track texture targets.
    TextureTargetList d_textureTargets;
    //! container type used to hold GeometryBuffers we create.
    typedef std::vector<NullGeometryBuffer*> GeometryBufferList;
    //! Container used to track geometry buffers.
    GeometryBufferList d_geometryBuffers;
    //! container type used to hold Textures we create.
    typedef std::vector<NullTexture*> TextureList;
    //! Container used to track textures.
    TextureList d_textures;
    //! What the renderer thinks the max texture size is.
    uint d_maxTextureSize;
};


} // End of  CEGUI namespace section

#if defined(_MSC_VER)
#   pragma warning(pop)
#endif

#endif  // end of guard _CEGUINullRenderer_h_

Answer 1

You've declared it, which means the compiler is happy to compile it. The declaration is like the name of the function: the compiler knows that the function exists and can compile code that calls it.

But in order to link it, you have to link in the definition (the implementation) of that function. The linker is telling you that it doesn't have the actual implementation of that function that you declared.

Most likely your link line is wrong, almost surely you either forgot to put the library on the link line at all or else you put it in the wrong place, but since you didn't provide us with any information about the link command that was run to generate that error message or the makefile rule you used to link, there's no way we can help.