C++模板元编程(一)

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C++模板元编程(一)

首先看一个例子，利用模板元编程递归将无符号二进制数转换为十进制。

#include<iostream>using namespace std;// 递归，将无符号二进制转换为十进制//template<unsigned long N>class binary{public:    static const unsigned int value = binary<N/10>::value * 2 + N % 10;};// 特化为N=0，终止递归template<>class binary<0>{public:    static const unsigned int value = 0;};int main(){    std::cout << binary<101010>::value << std::endl;    return 0;}

Traits和类型操纵

假设设计一个迭代器的，需要访问迭代器中的类型时。当迭代器是一个类时，可以在其中嵌套指定它的元素类型。但是普通的指针也可以用作合法的迭代器，便不能在其中指定其元素类型。

template<typename ForwardIterator1, typename ForwardIterator2>void iterSwap(ForwardIterator1 i1, ForwardIterator i2){    typename ForwardIterator1::value_type tmp = *i1;    *i1 = *i2;    *i2 = tmp;}

可以在其中引入一个中间层来解决此问题，通过该中间层进行类型计算来获得不同迭代器的元素类型。

/*************************************************************************    > File Name: traits_test.cpp    > Author: corfox    > Mail: corfox@163.com     > Created Time: 2015/12/27 11:39:56 ************************************************************************/#include <iostream>#include <iterator>#include <cstddef>using namespace std;template<typename Tp>class IteratorTraits{    public:        //当使用一个依赖性的名字，且该名字表示一个类型时，C++标准要求使用typename关键字表明        typedef typename Tp::value_type value_type;        typedef typename Tp::reference reference;        typedef typename Tp::pointer pointer;        typedef typename Tp::difference_type difference_type;        typedef typename Tp::iterator_category iterator_category;};// 偏特化Tp为指针类型template<typename Tp>class IteratorTraits<Tp*>{    public:        typedef Tp value_type;        typedef Tp& reference;        typedef Tp* pointer;        typedef std::ptrdiff_t difference_type;        typedef std::random_access_iterator_tag iterator_category;};template<typename ForwardIterator1, typename ForwardIterator2>void iterSwap(ForwardIterator1 i1, ForwardIterator2 i2){    typename IteratorTraits<ForwardIterator1>::value_type tmp = *i1;    *i1 = *i2;    *i2 = tmp;}void f(int *p1, int *p2, size_t length){    for (size_t i = 0; i < length; ++i)     {        iterSwap(p1++, p2++);    }}void showArr(int *p, size_t length){    for (size_t i = 0; i < length; ++i)         cout << *p++ << " ";    cout << endl;}int main(void){    int arr1[5] = { 1, 2, 3, 4, 5 };    int arr2[5] = { 6, 7, 8, 9, 10};    showArr(arr1, 5);    showArr(arr2, 5);    f(arr1, arr2, 5);    showArr(arr1, 5);    showArr(arr2, 5);    return 0;}-----输出：1 2 3 4 56 7 8 9 106 7 8 9 101 2 3 4 5

元函数的定义：一个元函数可以是一个类模板，它的所有参数都是类型；或者一个类，带有一个名为“type”的公有嵌套结构类型(result types)。（摘自：《C++模板元编程》）

练习

/*************************************************************************    > File Name: excercise_210.cpp    > Author: corfox    > Mail: corfox@163.com     > Created Time: 2015/12/28 9:59:50 ************************************************************************/#include <iostream>#include <type_traits>#include <cassert>#include <string>using namespace std;// 2-0struct Test{    virtual ~Test()    {    }};template<typename T>struct add_const_ref{    typedef const T& type;};template<typename T>struct add_const_ref<T&>{    typedef T& type;};// 2-1template<typename T, typename P, typename U, bool same>struct replace_type_indirect;template<typename T, typename P, typename U>struct replace_type{    static bool const value = std::is_same<T, P>::value;    typedef typename replace_type_indirect<T, P, U, value>::type type;};// T与P是相同类型时template<typename T, typename P, typename U>struct replace_type_indirect<T, P, U, true>{    typedef U type;};// T与P不是相同类型时template<typename T, typename P, typename U>struct replace_type_indirect<T*, P, U, false>{    typedef typename replace_type<T, P, U>::type* type;};template<typename T, typename P, typename U>struct replace_type_indirect<T&, P, U, false>{    typedef typename replace_type<T, P, U>::type& type;};template<typename T, typename P, typename U>struct replace_type_indirect<T[], P, U, false>{    typedef typename replace_type<T, P, U>::type type[];};template<typename T, typename P, typename U, int N>struct replace_type_indirect<T[N], P, U, false>{    typedef typename replace_type<T, P, U>::type type[N];};template<typename T, typename P, typename U>struct replace_type_indirect<T(), P, U, false>{    typedef typename replace_type<T, P, U>::type type();};template<typename T, typename P, typename U, typename A>struct replace_type_indirect<T(A), P, U, false>{    typedef typename replace_type<T, P, U>::type type(            typename replace_type<A, P, U>::type);};template<typename T, typename P, typename U, typename A1, typename A2>struct replace_type_indirect<T(A1, A2), P, U, false>{    typedef typename replace_type<T, P, U>::type type(            typename replace_type<A1, P, U>::type,             typename replace_type<A2, P, U>::type);};// 2-2template<typename T, typename S>inline T polymorphic_downcast(S* x){    assert(dynamic_cast<T>(x) == x);    return static_cast<T>(x);}template<typename T, typename S>inline T polymorphic_downcast(S& x){    assert(dynamic_cast<typename std::add_pointer<            typename std::remove_reference<T>::type>::type>(&x) == &x);    return static_cast<T>(x);}struct SubTest : Test {};// 2-3template<typename T> struct type_descriptor{    string value;    type_descriptor()    {        value = "The type T cannot be deduced.";    }    operator const char*()    {        return value.c_str();    }};template<> struct type_descriptor<int>{    string value;    type_descriptor()    {        value = "int";    }    operator const char*()    {        return value.c_str();    }};template<> struct type_descriptor<short int>{    string value;    type_descriptor()    {        value = "short int";    }    operator const char*()    {        return value.c_str();    }};template<> struct type_descriptor<char>{    string value;    type_descriptor()    {        value = "char";    }    operator const char*()    {        return value.c_str();    }};template<> struct type_descriptor<long int>{    string value;    type_descriptor()    {        value = "long int";    }    operator const char*()    {        return value.c_str();    }};template<typename T> struct type_descriptor<T*>{    string value;    operator const char*()    {        value = type_descriptor<T>();        value += "*";        return value.c_str();    }};template<typename T> struct type_descriptor<T&>{    string value;    operator const char*()    {        value = type_descriptor<T>();        value += "&";         return value.c_str();    }};template<typename T> struct type_descriptor<T const>{    string value;    operator const char*()    {        value = type_descriptor<T>();        value += " const";        return value.c_str();    }};int main(void){    cout << "2-0" << endl;    cout << std::boolalpha;    // 测试T是一个引用，返回T    cout << std::is_same<add_const_ref<Test&>::type, Test&>::value << endl;    // 测试T不是一个引用，返回const T&    cout << std::is_same<add_const_ref<Test>::type, Test>::value << endl;    cout << std::is_same<add_const_ref<Test>::type, const Test&>::value << endl;    cout << "2-1" << endl;    cout << std::is_same<replace_type<void*, void, int>::type, int*>::value << endl;    cout << std::is_same<replace_type<int const*[10], int const, long>::type, long*[10]>::value        << endl;    cout << std::is_same<replace_type<char& (*)(char&), char&, long&>::type, long& (*)(long&)>::value        <<endl;    // 2-2    SubTest b;    Test *a_ptr = &b;    SubTest *b_ptr = polymorphic_downcast<SubTest*>(a_ptr);    Test& a_ref = b;    SubTest& b_ref = polymorphic_downcast<SubTest&>(a_ref);    cout << "2-3" << endl;    cout << type_descriptor<int>() << endl;    cout << type_descriptor<char *>() << endl;    cout << type_descriptor<long const*&>() << endl;    return 0;}

参考资料

《C++模板元编程》(David Abrahams, Aleksey Gurtovoy )
c++模板元编程2

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