template <class C>
class Singleton {
    /*! class Singleton
    * brief Singleton Template
    */
public:
    /*!
    * brief Public access interface
    * return Singleton instance
    */
    static C *getInstance (){
        // Create new instance if not initialised
        if (_singleton == NULL){
            _singleton = new C;
        }
        return (static_cast<C*> (_singleton));
    }

    /*!
    * brief Public access destructor
    */
    static void kill(){
        // Clean Singleton object and free memory
        if (_singleton != NULL){
            delete _singleton;
            _singleton = NULL;
        }
    }
protected:
    /*!
     * brief Empty constructor
     */
    Singleton() = default;
    /*!
     * brief Destructor
     */
    virtual ~Singleton() = default;

private:
    static C *_singleton; /*!< Unique instance */
};

template <class C>
C *Singleton<C>::_singleton = NULL;

class TextureManager : public Singleton<TextureManager> {
    friend class Singleton<TextureManager>;
    /*! class TextureManager
    * brief Textures Container
    */
public:
protected:
    /*!
     * brief Empty constructor
     */
    TextureManager();
    /*!
     * brief Destructor
     */
    ~TextureManager() override;
private:
};

build/main.o: In function `Singleton<TextureManager>::getInstance()':
main.cpp (.text._ZN9SingletonI14TextureManagerE11getInstanceEv[_ZN9SingletonI14TextureManagerE11getInstanceEv]+0x24): undefined reference to `TextureManager::TextureManager()'

TextureManager::TextureManager() = default;
TextureManager::~TextureManager() = default;

INCLUDE = -I/usr/X11R6/include/
LIBDIR  = -L/usr/X11R6/lib

SRCDIR := sources
BUILDDIR := build
TARGET := bin/game

SRCEXT := cpp
SOURCES := $(shell find $(SRCDIR) -type f -name *.$(SRCEXT))
OBJECTS := $(patsubst $(SRCDIR)/%,$(BUILDDIR)/%,$(SOURCES:.$(SRCEXT)=.o))

FLAGS = -std=c++11 -Wall
CC = g++
CFLAGS = $(FLAGS) $(INCLUDE)
LIBS =  -lglut -lGL -lGLU -lGLEW -lm

$(TARGET): $(OBJECTS)
    @echo "Linking..."
    @echo "$(CC) $(CFLAGS) $^ -o $@ $(LIBDIR) $< $(LIBS)"; $(CC) $(CFLAGS) -o $@ $(LIBDIR) $< $(LIBS)

$(BUILDDIR)/%.o: $(SRCDIR)/%.$(SRCEXT)
    @mkdir -p $(@D)
    @echo "$(CC) $(CFLAGS) -o $@ $(LIBDIR) $< $(LIBS)"; $(CC) $(CFLAGS) -c -o $@ $(LIBDIR) $< $(LIBS)

clean:
    @echo "Cleaning..."
    @echo "$(RM) -r $(BUILDDIR) $(TARGET)"; $(RM) -r $(BUILDDIR) $(TARGET)

.PHONY: clean