纯虚函数和抽象类 [英] pure virtual function and abstract class

问题描述

我有以下类，Base 和 Derived，当我编译时，编译器抱怨它无法创建 DLog 的实例，因为它是抽象的.

I have the following classes, Base and Derived and when I compile the compiler complains that it cannot create an instance of DLog because it is abstract.

谁能告诉我如何解决这个错误?

Can someone tell me how I can fix this error?

我猜这是因为不是两个纯虚函数都没有在 Derived 中实现.

I'm guessing it's because not both pure virtual functions are not implemented in the Derived.

class Logger
{
public:

    virtual void log(int debugLevel, char* fmt, ...) = 0;
    virtual void log(string logText, int debugLevel, string threadName = "") = 0;

    static Logger* defaultLogger() {return m_defaultLogger;}
    static void setDefaultLogger(Logger& logger) {m_defaultLogger = &logger;}

protected:

    static Logger* m_defaultLogger;
};

class DLog : public Logger
{
public:
    DLog();
    ~DLog();

    static DLog *Instance();
    static void Destroy();

    void SetLogFilename(std::string filename);
    void SetOutputDebug(bool enable);
    std::string getKeyTypeName(long lKeyType);
    std::string getScopeTypeName(long lScopeType);
    std::string getMethodName(long lMethod);

    virtual void log(string logText, int debugLevel)
    {
        Log(const_cast<char*>(logText.c_str()));
    }

    void Log(char* fmt, ...);

private:

    static DLog *m_instance;

    std::string m_filename;
    bool m_bOutputDebug;
};

//DLog 实例化为单例

// DLog instantion as a singleton

DLog *DLog::Instance()
{
    if (!m_instance)
        m_instance = new DLog();
    return m_instance;
}

推荐答案

virtual void log(string logText, int debugLevel, string threadName = "") = 0;

尚未在 DLog 类中实现.您必须实现它，因为它在基类中是纯虚拟的.

has not been implemented in class DLog. You have to implement it because it's pure virtual in the base class.

您可能是在 DLog 中第一次重载 log 时的意思:

You probably meant this in your first overload of log in DLog:

virtual void log(string logText, int debugLevel, string /*threadname*/)
{
    Log(const_cast<char*>(logText.c_str()));
}

您还没有实现

virtual void log(int debugLevel, char* fmt, ...) = 0;

请注意，尽管使用 const_cast 是一个非常糟糕的主意并且是未定义的行为.您可以通过执行以下操作来获得明确定义的行为:

Note here though that using the const_cast is a very bad idea and is undefined behavior. You can get well defined behavior by doing something like this instead:

virtual void log(string logText, int debugLevel, string /*threadname*/)
{
    logText.push_back(''); // Add null terminator
    Log(&logText[0]); // Send non-const string to function
    logText.pop_back(); // Remove null terminator
}

不过更好的是，首先将Log"设置为常量.

Better yet though, just make "Log" const-correct in the first place.

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