Undefined constructor in inherited templated singleton class c++ - c++

Question

I ran into a problem today, using a tool I have made some times ago : a Singleton templated class.
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;
The problem arise when i try to call TextureManager::getInstance() on the (very basic) inherited class which is the following :
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:
};
During compilation, the linker shows me an error that i understand but I do not really understand the reason :
build/main.o: In function `Singleton<TextureManager>::getInstance()':
main.cpp (.text._ZN9SingletonI14TextureManagerE11getInstanceEv[_ZN9SingletonI14TextureManagerE11getInstanceEv]+0x24): undefined reference to `TextureManager::TextureManager()'
It seems that it can't find the definition of the constructor which is in TextureManager.cpp
TextureManager::TextureManager() = default;
TextureManager::~TextureManager() = default;
This code worked on an older project I compiled using premake so my guess is that my current makefile is rubbish but i can't find why...
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
Now stuck for hours,
all my hopes are on you :p
Edit : full make output
g++ -std=c++11 -Wall -I/usr/X11R6/include/ -o build/main.o -L/usr/X11R6/lib sources/main.cpp -lglut -lGL -lGLU -lGLEW -lm
g++ -std=c++11 -Wall -I/usr/X11R6/include/ -o build/Tools/ThreadManager.o -L/usr/X11R6/lib sources/Tools/ThreadManager.cpp -lglut -lGL -lGLU -lGLEW -lm
g++ -std=c++11 -Wall -I/usr/X11R6/include/ -o build/Graphics/GraphicalEngine.o -L/usr/X11R6/lib sources/Graphics/GraphicalEngine.cpp -lglut -lGL -lGLU -lGLEW -lm
g++ -std=c++11 -Wall -I/usr/X11R6/include/ -o build/Graphics/TextureManager.o -L/usr/X11R6/lib sources/Graphics/TextureManager.cpp -lglut -lGL -lGLU -lGLEW -lm
Linking...
g++ -std=c++11 -Wall -I/usr/X11R6/include/ build/main.o build/Tools/ThreadManager.o build/Graphics/GraphicalEngine.o build/Graphics/TextureManager.o -o bin/game -L/usr/X11R6/lib build/main.o -lglut -lGL -lGLU -lGLEW -lm
build/main.o: In function `Singleton<TextureManager>::getInstance()':
main.cpp:(.text._ZN9SingletonI14TextureManagerE11getInstanceEv[_ZN9SingletonI14TextureManagerE11getInstanceEv]+0x24): undefined reference to `TextureManager::TextureManager()'
collect2: error: ld returned 1 exit status
makefile:18: recipe for target 'bin/game' failed
make: *** [bin/game] Error 1

Answer 1

The problem is that you are only using the first dependency in the $(TARGET) rule:
$(CC) $(CFLAGS) -o $# $(LIBDIR) $< $(LIBS)
# ^^
My comment was actually wrong because you have a different command between the #echo and the actual command:
#echo "$(CC) $(CFLAGS) $^ -o $# $(LIBDIR) $< $(LIBS)"
# ^^ ^^
I do not see a point in adding this echo line here, I would do as follow:
$(TARGET): $(OBJECTS)
#echo "Linking..."
$(CC) $(CFLAGS) $^ -o $# $(LIBDIR) $(LIBS)
# ^^
Had you done so, you would have seen your error sooner I think.