深度解读Q_D指针与Q_Q指针

首先是Q_D指针:先贴上我们的测试用例.

class TEST;
class TESTPrivate
{
public:
    TESTPrivate(TEST *parent):q_ptr(parent),a(666)
    {cout << "TESTPrivate" << endl;}
    ~TESTPrivate()
    {cout << "~TESTPrivate" << endl;}
private:
    Q_DECLARE_PUBLIC(TEST)

private:
    TEST *q_ptr;
    int a;
};
class TEST
{
public:
    TEST():d_ptr(new TESTPrivate(this)),b(2){}
    ~TEST(){delete d_ptr;}
    int getValue()
    {
        Q_D(TEST);
        return d->a;
    }
private:
    int b;
    TESTPrivate *d_ptr;
    Q_DECLARE_PRIVATE(TEST)
};

int main(int /*argc*/, char */*argv*/[])
{
    TEST t;
    cout << t.getValue() << endl;
}

查看这里面定义的宏:

#define Q_DECLARE_PRIVATE(Class) \
    inline Class##Private* d_func() { return reinterpret_cast(qGetPtrHelper(d_ptr)); } \
    inline const Class##Private* d_func() const { return reinterpret_cast(qGetPtrHelper(d_ptr)); } \
    friend class Class##Private;

#define Q_DECLARE_PUBLIC(Class)                                    \
    inline Class* q_func() { return static_cast(q_ptr); } \
    inline const Class* q_func() const { return static_cast(q_ptr); } \
    friend class Class;

现在,我们手动进行一次宏展开:

//    Q_DECLARE_PUBLIC(TEST)
    inline TEST* q_func() { return static_cast(q_ptr); }
    inline const TEST* q_func() const { return static_cast(q_ptr); }
    friend class TEST;

//    Q_DECLARE_PRIVATE(TEST)
    inline TESTPrivate* d_func() { return reinterpret_cast(qGetPtrHelper(d_ptr)); }
    inline const TESTPrivate* d_func() const { return reinterpret_cast(qGetPtrHelper(d_ptr)); }
    friend class TESTPrivate;

从这里可以看到,实际上宏只是互相声明了友元,并返回了d_ptr指针.

我们查看以下Q_D宏.可以看到这里定义了d指针与q指针.

#define Q_D(Class) Class##Private * const d = d_func()

宏展开,可以看到,所谓的d指针只是获得类Q_DECLARE_PRIVATE(TEST)中函数的返回值:

//        Q_D(TEST);
        TESTPrivate * const d = d_func();

在Q_DECLARE_PRIVATE(TEST)中,查看qGetPtrHelper的宏:

template  static inline T *qGetPtrHelper(T *ptr) { return ptr; }

d指针我们可以不调用d_func(),直接从d_ptr那里获得一份拷贝,并删除多余的func函数,最终结果如下:

class TEST;
class TESTPrivate
{
public:
    TESTPrivate(TEST *parent):q_ptr(parent),a(666)
    {cout << "TESTPrivate" << endl;}
    ~TESTPrivate()
    {cout << "~TESTPrivate" << endl;}
private:
    friend class TEST;
    TEST *q_ptr;
    int a;
};
class TEST
{
public:
    TEST():d_ptr(new TESTPrivate(this)),b(2),c(3){}
    ~TEST(){delete d_ptr;}
    int getValue()
    {
        TESTPrivate *d = d_ptr;
        return d->a;
    }
private:
    int b;
    TESTPrivate *d_ptr;
    friend class TESTPrivate;
};

Q_Q指针与Q_D指针类似,在原理上是一致的.只是在概念中用法不一样.Q_D指针一般用来访问"Private"类中的数据,Q_Q指针则是通过Private类访问"Public"类.private类就是装钱的钱包,public类就是口袋.Qt里面如此封装,应该是尽量降低对OO的破坏吧!毕竟friend本身就违背类OO原则.