下面将介绍泛型编程的最重要的组件,他们是:
1。编译期断言
2。编译期转换检测
3。编译期选择
4。编译期多态
5。代码容器
再一次说,这是泛型里最重要的组件,值得你去理解和使用,他们的实现有很多版本,Boost和Loki都有实现,在认真比较以后,我认为Loki确实更加简单和强大,所以我选择Loki的代码进行示例,如果你有兴趣深入了解,请读《C++设计新思维》。
值得注意的是,为了便于理解,我把Loki的代码进行了适当修改。
1。编译期断言
实现:
template<int> struct CompileTimeError;
template<> struct CompileTimeError<true> {};
#define STATIC_CHECK(expr, msg) \
{ CompileTimeError<((expr) != 0)> ERROR_##msg; (void)ERROR_##msg; }
原理:
利用template<> struct CompileTimeError<false>并没有特化体,所以产生他的实例ERROR_##msg编译器就会报错。
使用:
const int i = 0;
STATIC_CHECK(i==0, "i因该等于0");
2。编译期转换检测
实现:
template <class T, class U>
struct ConversionHelper
{
typedef char Small;
struct Big { char dummy[2]; };
static Big Test(...);
static Small Test(U);
static T MakeT();
};
template <class T, class U>
struct Conversion
{
typedef ConversionHelper<T, U> H;
enum { exists = sizeof(typename H::Small) == sizeof(H::Test(H::MakeT())) };
enum { exists2Way = exists && Conversion<U, T>::exists };
enum { sameType = false };
};
template <class T>
struct Conversion<T, T>
{
enum { exists = 1, exists2Way = 1,sameType = 1 };
};
template <class T>
struct Conversion<void, T>
{
enum { exists = 1, exists2Way = 0,sameType = 0 };
};
template <class T>
struct Conversion<T, void>
{
enum { exists = 1, exists2Way = 0,sameType = 0 };
};
template <>
class Conversion<void, void>
{
public:
enum { exists = 1, exists2Way = 1,sameType = 1 };
};
#define SUPERSUBCLASS(T, U) \
(Conversion<const U*, const T*>::exists && \
!Conversion<const T*, const void*>::sameType)
#define SUPERSUBCLASS_STRICT(T, U) \
(SUPERSUBCLASS(T, U) && \
!Conversion<const T, const U>::sameType)
原理:
利用函数的参数列表的型别推测机制得到编译器使用哪个函数,而函数返回型别,接着利用 sizeof()一定会在编译期求出值来获得返回型别的大小,依此来判断调用了哪个函数,然后在利用enum是编译期确定的常数,把sizeof()返回的值具体化出来。
使用:
class A { };
class B : public A { };
STATIC_CHECK( SUPERSUBCLASS(A, B), "A因该是B的基类");
3。编译期选择
实现:
template <bool flag, typename T, typename U>
struct Select
{
typedef T Result;
};
template <typename T, typename U>
struct Select<false, T, U>
{
typedef U Result;
};
原理:
模板偏特化
使用:
class A { };
class B { };
template<bool b>
class C : public Select<b, A, B> { };
4。编译期多态
实现:
template<class T>
class Relex
{
private:
Relex(const Relex&);
Relex& operator = (const Relex&);
public:
typedef T _HostType;
typedef T& _HostReference;
typedef T* _HostPointer;
public:
Relex() { }
T* GetHostPtr() { return reinterpret_cast<T*>(this); }
};
原理:
模板的迭代编译
使用:
template<class T>
struct A : Relex<T>
{
void func1() { GetHostPtr()->func2(); }
};
template<class T>
struct B : Relex<T>
{
void func2() { GetHostPtr()->func1(); }
};
template<template<class>class T1, template<class>class T2>
class C_Impl : public T1<C_Impl>, public public T2<C_Impl> { };
typedef C_Impl<A, B> C;
5。代码容器
这里的实现非常巨大,就不写了,你可以自己去看代码
实现:
template <class T, class U>
struct Typelist //请想想STL里的list,非常象。这里的Tail又是一个Typelist,这样形成一个链
{
typedef T Head;
typedef U Tail;
};
//Typelist的头是Typelist<T, NullType>,我们把NullType作为链的尾。
struct NullType { };
struct EmptyType { };
//下面是操作Typelist的类
template <class TList> struct Length;
template <class TList, unsigned int index> struct TypeAt;
template <class TList, unsigned int index,typename DefaultType = NullType> struct TypeAtNonStrict;
template <class TList, class T> struct IndexOf;
template <class TList, class T> struct Append;
template <class TList, class T> struct Erase;
template <class TList, class T> struct EraseAll;
template <class TList> struct NoDuplicates;
template <class TList, class T, class U> struct Replace;
template <class TList, class T, class U> struct ReplaceAll;
template <class TList> struct Reverse;
template <class TList, class T> struct MostDerived;
template <class TList> struct DerivedToFront;
//下面就是代码容器,典型的模板递归和模板偏特化的运用,请仔细看,其实很简单
template <class TList, template <class> class Unit>
class GenScatterHierarchy;
template <class T1, class T2, template <class> class Unit>
class GenScatterHierarchy<Typelist<T1, T2>, Unit>
: public GenScatterHierarchy<T1, Unit>
, public GenScatterHierarchy<T2, Unit>
{
public:
typedef Typelist<T1, T2> TList;
typedef GenScatterHierarchy<T1, Unit> LeftBase;
typedef GenScatterHierarchy<T2, Unit> RightBase;
template <typename T> struct Rebind
{
typedef Unit<T> Result;
};
};
template <class AtomicType, template <class> class Unit>
class GenScatterHierarchy : public Unit<AtomicType>
{
typedef Unit<AtomicType> LeftBase;
template <typename T> struct Rebind
{
typedef Unit<T> Result;
};
};
template <template <class> class Unit>
class GenScatterHierarchy<NullType, Unit>
{
template <typename T> struct Rebind
{
typedef Unit<T> Result;
};
};
template
<
class TList,
template <class AtomicType, class Base> class Unit,
class Root = EmptyType
>
class GenLinearHierarchy;
template
<
class T1,
class T2,
template <class, class> class Unit,
class Root
>
class GenLinearHierarchy<Typelist<T1, T2>, Unit, Root>
: public Unit< T1, GenLinearHierarchy<T2, Unit, Root> > { };
template
<
class T,
template <class, class> class Unit,
class Root
>
class GenLinearHierarchy<Typelist<T, NullType>, Unit, Root>
: public Unit<T, Root> { };
template <int i, class H>
typename FieldHelper<H, i>::ResultType& Field(H& obj);
原理:
metaprogram,也就是模板递归,模板偏特化。
使用:
使用Loki里提供的宏,Loki提供了50个宏
运用1,实现Tuple:
template <class T>
struct TupleUnit
{
T value_;
operator T&() { return value_; }
operator const T&() const { return value_; }
};
template <class TList>
struct Tuple : public GenScatterHierarchy<TList, TupleUnit> { };
class A { };
class B { };
class C { };
class D { };
typedef Tuple<TYPELIST_4(A, B, C, D)> TupleClass;
TupleClass obj;
B& b = Field<1>(obj);
运用2,实现一个抽象工厂:
namespace Noir_Impl
{
template<class P>
struct CAF_Product { P* Create_Impl() { return new P; } };
template<class CProductList>
class CAF_AbstractFactory : public Loki::GenScatterHierarchy< CProductList, CAF_Product > { };
};
template<class IProductList, class CProductList>
struct Simple_AbstractFactory : public IProductList, private Noir_Impl::CAF_AbstractFactory< CProductList >
{
template<class IP> IP* Create()
{
typedef TypeAt< CProductList, IndexOf< IProductList, IP >::value > CP;
return CP::Create_Impl();
}
};
class IA { };
class IB { };
class IC { };
class ID { };
class A : public IA { };
class B : public IB { };
class C : public IC { };
class D : public ID { };
typedef Simple_AbstractFactory< TYPELIST_4(IA, IB, IC, ID), TYPELIST_4(A, B, C, D) > MyAF;
MyAf factory;
IA* pA = factory.Create<A>();
IB* pB = factory.Create<B>();
IC* pC = factory.Create<C>();
ID* pD = factory.Create<D>();