c++ - 继承的模板化单例类 C++ 中的未定义构造函数

Question

我今天遇到了一个问题，使用了我之前制作的工具:单例模板类。

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;

当我尝试在(非常基本的)继承类上调用 TextureManager::getInstance() 时，问题出现了:

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:
};

在编译过程中，链接器向我显示了一个我理解但我并不真正理解原因的错误:

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

似乎找不到TextureManager.cpp中构造函数的定义

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

此代码适用于我使用 premake 编译的旧项目，所以我猜测我当前的 makefile 是垃圾，但我找不到原因...

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

现在卡了几个小时，我所有的希望都寄托在你身上:p

编辑:完整的输出

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

问题是您只使用了 $(TARGET) 规则中的第一个依赖项:

$(CC) $(CFLAGS) -o $@ $(LIBDIR) $< $(LIBS)
#                               ^^

我的评论实际上是错误的，因为你在 @echo 和实际命令之间有不同的命令:

@echo "$(CC) $(CFLAGS) $^ -o $@ $(LIBDIR) $< $(LIBS)"
#                      ^^                 ^^

我看不出在此处添加此 echo 行有什么意义，我会按以下步骤操作:

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

如果你这样做了，我想你会更快地发现你的错误。