虚函数表

让我们从虚函数表及其工作原理的一些背景知识开始(来源):

[20.3] What's the difference between how virtual and non-virtual member functions are called? Non-virtual member functions are resolved statically. That is, the member function is selected statically (at compile-time) based on the type of the pointer (or reference) to the object. In contrast, virtual member functions are resolved dynamically (at run-time). That is, the member function is selected dynamically (at run-time) based on the type of the object, not the type of the pointer/reference to that object. This is called "dynamic binding." Most compilers use some variant of the following technique: if the object has one or more virtual functions, the compiler puts a hidden pointer in the object called a "virtual-pointer" or "v-pointer." This v-pointer points to a global table called the "virtual-table" or "v-table." The compiler creates a v-table for each class that has at least one virtual function. For example, if class Circle has virtual functions for draw() and move() and resize(), there would be exactly one v-table associated with class Circle, even if there were a gazillion Circle objects, and the v-pointer of each of those Circle objects would point to the Circle v-table. The v-table itself has pointers to each of the virtual functions in the class. For example, the Circle v-table would have three pointers: a pointer to Circle::draw(), a pointer to Circle::move(), and a pointer to Circle::resize(). During a dispatch of a virtual function, the run-time system follows the object's v-pointer to the class's v-table, then follows the appropriate slot in the v-table to the method code. The space-cost overhead of the above technique is nominal: an extra pointer per object (but only for objects that will need to do dynamic binding), plus an extra pointer per method (but only for virtual methods). The time-cost overhead is also fairly nominal: compared to a normal function call, a virtual function call requires two extra fetches (one to get the value of the v-pointer, a second to get the address of the method). None of this runtime activity happens with non-virtual functions, since the compiler resolves non-virtual functions exclusively at compile-time based on the type of the pointer.

我的问题，或者我是怎么来的

我尝试使用类似这样的东西，现在cubefile基类与模板优化加载函数，这将实现不同类型的立方体(一些存储像素，一些通过图像等)。

一些代码:

virtual void  LoadCube(UtpBipCube<float> &Cube,long LowerLeftRow=0,long LowerLeftColumn=0,
        long UpperRightRow=-1,long UpperRightColumn=-1,long LowerBand=0,long UpperBand=-1) = 0;
virtual void  LoadCube(UtpBipCube<short> &Cube, long LowerLeftRow=0,long LowerLeftColumn=0,
        long UpperRightRow=-1,long UpperRightColumn=-1,long LowerBand=0,long UpperBand=-1) = 0;
virtual void  LoadCube(UtpBipCube<unsigned short> &Cube, long LowerLeftRow=0,long LowerLeftColumn=0,
        long UpperRightRow=-1,long UpperRightColumn=-1,long LowerBand=0,long UpperBand=-1) = 0;

我想要它是什么，但它不会编译由于虚拟模板组合:

template<class T>
    virtual void  LoadCube(UtpBipCube<T> &Cube,long LowerLeftRow=0,long LowerLeftColumn=0,
            long UpperRightRow=-1,long UpperRightColumn=-1,long LowerBand=0,long UpperBand=-1) = 0;

我最终将模板声明移到了类级别。这种解决方案将迫使程序在读取数据之前了解它们将要读取的特定类型的数据，这是不可接受的。

解决方案

警告，这不是很漂亮，但它允许我删除重复的执行代码

1)在基类中

virtual void  LoadCube(UtpBipCube<float> &Cube,long LowerLeftRow=0,long LowerLeftColumn=0,
            long UpperRightRow=-1,long UpperRightColumn=-1,long LowerBand=0,long UpperBand=-1) = 0;
virtual void  LoadCube(UtpBipCube<short> &Cube, long LowerLeftRow=0,long LowerLeftColumn=0,
            long UpperRightRow=-1,long UpperRightColumn=-1,long LowerBand=0,long UpperBand=-1) = 0;
virtual void  LoadCube(UtpBipCube<unsigned short> &Cube, long LowerLeftRow=0,long LowerLeftColumn=0,
            long UpperRightRow=-1,long UpperRightColumn=-1,long LowerBand=0,long UpperBand=-1) = 0;

2)和在儿童班

void  LoadCube(UtpBipCube<float> &Cube, long LowerLeftRow=0,long LowerLeftColumn=0,
        long UpperRightRow=-1,long UpperRightColumn=-1,long LowerBand=0,long UpperBand=-1)
{ LoadAnyCube(Cube,LowerLeftRow,LowerLeftColumn,UpperRightRow,UpperRightColumn,LowerBand,UpperBand); }

void  LoadCube(UtpBipCube<short> &Cube, long LowerLeftRow=0,long LowerLeftColumn=0,
        long UpperRightRow=-1,long UpperRightColumn=-1,long LowerBand=0,long UpperBand=-1)
{ LoadAnyCube(Cube,LowerLeftRow,LowerLeftColumn,UpperRightRow,UpperRightColumn,LowerBand,UpperBand); }

void  LoadCube(UtpBipCube<unsigned short> &Cube, long LowerLeftRow=0,long LowerLeftColumn=0,
        long UpperRightRow=-1,long UpperRightColumn=-1,long LowerBand=0,long UpperBand=-1)
{ LoadAnyCube(Cube,LowerLeftRow,LowerLeftColumn,UpperRightRow,UpperRightColumn,LowerBand,UpperBand); }

template<class T>
void  LoadAnyCube(UtpBipCube<T> &Cube, long LowerLeftRow=0,long LowerLeftColumn=0,
        long UpperRightRow=-1,long UpperRightColumn=-1,long LowerBand=0,long UpperBand=-1);

注意，LoadAnyCube没有在基类中声明。

下面是另一个堆栈溢出的答案: 需要一个虚拟模板成员解决方案。

如果预先知道模板方法的类型集，则'虚拟模板方法'有一个变通方法。

为了说明这个想法，在下面的例子中只使用了两种类型(int和double)。

在那里，一个“虚拟”模板方法(Base:: method)调用相应的虚拟方法(Base:: VMethod之一)，后者反过来调用模板方法实现(Impl::TMethod)。

只需要在派生实现(AImpl, BImpl)中实现模板方法TMethod，并使用derived <*Impl>。

class Base
{
public:
    virtual ~Base()
    {
    }

    template <typename T>
    T Method(T t)
    {
        return VMethod(t);
    }

private:
    virtual int VMethod(int t) = 0;
    virtual double VMethod(double t) = 0;
};

template <class Impl>
class Derived : public Impl
{
public:
    template <class... TArgs>
    Derived(TArgs&&... args)
        : Impl(std::forward<TArgs>(args)...)
    {
    }

private:
    int VMethod(int t) final
    {
        return Impl::TMethod(t);
    }

    double VMethod(double t) final
    {
        return Impl::TMethod(t);
    }
};

class AImpl : public Base
{
protected:
    AImpl(int p)
        : i(p)
    {
    }

    template <typename T>
    T TMethod(T t)
    {
        return t - i;
    }

private:
    int i;
};

using A = Derived<AImpl>;

class BImpl : public Base
{
protected:
    BImpl(int p)
        : i(p)
    {
    }

    template <typename T>
    T TMethod(T t)
    {
        return t + i;
    }

private:
    int i;
};

using B = Derived<BImpl>;

int main(int argc, const char* argv[])
{
    A a(1);
    B b(1);
    Base* base = nullptr;

    base = &a;
    std::cout << base->Method(1) << std::endl;
    std::cout << base->Method(2.0) << std::endl;

    base = &b;
    std::cout << base->Method(1) << std::endl;
    std::cout << base->Method(2.0) << std::endl;
}

输出:

0
1
2
3

注: Base::Method对于实际代码来说实际上是多余的(VMethod可以被设为public并直接使用)。 我添加它，使它看起来像一个实际的“虚拟”模板方法。

从c++模板的完整指南:

Member function templates cannot be declared virtual. This constraint is imposed because the usual implementation of the virtual function call mechanism uses a fixed-size table with one entry per virtual function. However, the number of instantiations of a member function template is not fixed until the entire program has been translated. Hence, supporting virtual member function templates would require support for a whole new kind of mechanism in C++ compilers and linkers. In contrast, the ordinary members of class templates can be virtual because their number is fixed when a class is instantiated

在其他答案中，建议的模板函数是一个门面，并不能提供任何实际的好处。

模板函数对于只编写一次代码很有用 不同的类型。 虚函数对于为不同的类提供公共接口非常有用。

该语言不允许虚拟模板函数，但通过一个变通方法，可以同时拥有两者，例如，每个类都有一个模板实现和一个虚拟公共接口。

但是，有必要为每个模板类型组合定义一个虚拟包装器函数:

#include <memory>
#include <iostream>
#include <iomanip>

//---------------------------------------------
// Abstract class with virtual functions
class Geometry {
public:
    virtual void getArea(float &area) = 0;
    virtual void getArea(long double &area) = 0;
};

//---------------------------------------------
// Square
class Square : public Geometry {
public:
    float size {1};

    // virtual wrapper functions call template function for square
    virtual void getArea(float &area) { getAreaT(area); }
    virtual void getArea(long double &area) { getAreaT(area); }

private:
    // Template function for squares
    template <typename T>
    void getAreaT(T &area) {
        area = static_cast<T>(size * size);
    }
};

//---------------------------------------------
// Circle
class Circle : public Geometry  {
public:
    float radius {1};

    // virtual wrapper functions call template function for circle
    virtual void getArea(float &area) { getAreaT(area); }
    virtual void getArea(long double &area) { getAreaT(area); }

private:
    // Template function for Circles
    template <typename T>
    void getAreaT(T &area) {
        area = static_cast<T>(radius * radius * 3.1415926535897932385L);
    }
};


//---------------------------------------------
// Main
int main()
{
    // get area of square using template based function T=float
    std::unique_ptr<Geometry> geometry = std::make_unique<Square>();
    float areaSquare;
    geometry->getArea(areaSquare);

    // get area of circle using template based function T=long double
    geometry = std::make_unique<Circle>();
    long double areaCircle;
    geometry->getArea(areaCircle);

    std::cout << std::setprecision(20) << "Square area is " << areaSquare << ", Circle area is " << areaCircle << std::endl;
    return 0;
}

输出:

方形面积为1，圆形面积为3.1415926535897932385

在这里试试

至少在gcc 5.4中，虚函数可以是模板成员，但必须是模板本身。

#include <iostream>
#include <string>
class first {
protected:
    virtual std::string  a1() { return "a1"; }
    virtual std::string  mixt() { return a1(); }
};

class last {
protected:
    virtual std::string a2() { return "a2"; }
};

template<class T>  class mix: first , T {
    public:
    virtual std::string mixt() override;
};

template<class T> std::string mix<T>::mixt() {
   return a1()+" before "+T::a2();
}

class mix2: public mix<last>  {
    virtual std::string a1() override { return "mix"; }
};

int main() {
    std::cout << mix2().mixt();
    return 0;
}

输出

mix before a2
Process finished with exit code 0

我目前的解决方案如下(禁用RTTI -你也可以使用std::type_index):

#include <type_traits>
#include <iostream>
#include <tuple>

class Type
{
};

template<typename T>
class TypeImpl : public Type
{

};

template<typename T>
inline Type* typeOf() {
    static Type* typePtr = new TypeImpl<T>();
    return typePtr;
}

/* ------------- */

template<
    typename Calling
    , typename Result = void
    , typename From
    , typename Action
>
inline Result DoComplexDispatch(From* from, Action&& action);

template<typename Cls>
class ChildClasses
{
public:
    using type = std::tuple<>;
};

template<typename... Childs>
class ChildClassesHelper
{
public:
    using type = std::tuple<Childs...>;
};

//--------------------------

class A;
class B;
class C;
class D;

template<>
class ChildClasses<A> : public ChildClassesHelper<B, C, D> {};

template<>
class ChildClasses<B> : public ChildClassesHelper<C, D> {};

template<>
class ChildClasses<C> : public ChildClassesHelper<D> {};

//-------------------------------------------

class A
{
public:
    virtual Type* GetType()
    {
        return typeOf<A>();
    }

    template<
        typename T,
        bool checkType = true
    >
        /*virtual*/void DoVirtualGeneric()
    {
        if constexpr (checkType)
        {
            return DoComplexDispatch<A>(this, [&](auto* other) -> decltype(auto)
                {
                    return other->template DoVirtualGeneric<T, false>();
                });
        }
        std::cout << "A";
    }
};

class B : public A
{
public:
    virtual Type* GetType()
    {
        return typeOf<B>();
    }
    template<
        typename T,
        bool checkType = true
    >
    /*virtual*/void DoVirtualGeneric() /*override*/
    {
        if constexpr (checkType)
        {
            return DoComplexDispatch<B>(this, [&](auto* other) -> decltype(auto)
                {
                    other->template DoVirtualGeneric<T, false>();
                });
        }
        std::cout << "B";
    }
};

class C : public B
{
public:
    virtual Type* GetType() {
        return typeOf<C>();
    }

    template<
        typename T,
        bool checkType = true
    >
    /*virtual*/void DoVirtualGeneric() /*override*/
    {
        if constexpr (checkType)
        {
            return DoComplexDispatch<C>(this, [&](auto* other) -> decltype(auto)
                {
                    other->template DoVirtualGeneric<T, false>();
                });
        }
        std::cout << "C";
    }
};

class D : public C
{
public:
    virtual Type* GetType() {
        return typeOf<D>();
    }
};

int main()
{
    A* a = new A();
    a->DoVirtualGeneric<int>();
}

// --------------------------

template<typename Tuple>
class RestTuple {};

template<
    template<typename...> typename Tuple,
    typename First,
    typename... Rest
>
class RestTuple<Tuple<First, Rest...>> {
public:
    using type = Tuple<Rest...>;
};

// -------------
template<
    typename CandidatesTuple
    , typename Result
    , typename From
    , typename Action
>
inline constexpr Result DoComplexDispatchInternal(From* from, Action&& action, Type* fromType)
{
    using FirstCandidate = std::tuple_element_t<0, CandidatesTuple>;

    if constexpr (std::tuple_size_v<CandidatesTuple> == 1)
    {
        return action(static_cast<FirstCandidate*>(from));
    }
    else {
        if (fromType == typeOf<FirstCandidate>())
        {
            return action(static_cast<FirstCandidate*>(from));
        }
        else {
            return DoComplexDispatchInternal<typename RestTuple<CandidatesTuple>::type, Result>(
                from, action, fromType
            );
        }
    }
}

template<
    typename Calling
    , typename Result
    , typename From
    , typename Action
>
inline Result DoComplexDispatch(From* from, Action&& action)
{
    using ChildsOfCalling = typename ChildClasses<Calling>::type;
    if constexpr (std::tuple_size_v<ChildsOfCalling> == 0)
    {
        return action(static_cast<Calling*>(from));
    }
    else {
        auto fromType = from->GetType();
        using Candidates = decltype(std::tuple_cat(std::declval<std::tuple<Calling>>(), std::declval<ChildsOfCalling>()));
        return DoComplexDispatchInternal<Candidates, Result>(
            from, std::forward<Action>(action), fromType
        );
    }
}

我唯一不喜欢的是你必须定义/注册所有的子类。