meta programming

来源：互联网发布：大医医学数据库编辑：程序博客网时间：2024/05/20 20:58

拜读wuye大神github上的大作cpptemplatetutorial，以C++的模板为基础的元编程入门讲义，收获颇丰，虽然仅是入门，更多的是思维上的拓展

元编程，就是对代码进行操作的编程模式。既然是模版元编程，那么就会有以下优点：

1.使用模板，就可以有代码重用、使用安全、类型安全等好处。

2.利用元编程进行类型识别，STL里也用到这种技术、类型识别完了后就根据不同的类型调用适合的函数，提高了效率

3.运算在编译时完成，不占用运行时间。

模板元编程很好的运用了C++模板特化、偏特化的强大功能，和一些技巧，如模版匹配中的SFINAE、递归模板、整型参数等

另记vczh大神的说法：【类型和值是同一种东西】

小应用：

给类型分配id：

#include <iostream>using std::cout;using std::endl;//--------------------- ID type template-------------//template <typename T> class TypeToID{public:static int const ID = -1;};template <> class TypeToID <uint8_t>{public:static int const ID = 0;};template <> class TypeToID <uint16_t>{public:static int const ID = 1;};template <> class TypeToID <uint32_t>{public:static int const ID = 2;};template <> class TypeToID <uint64_t>{public:static int const ID = 3;};template <> class TypeToID <float>{public:static int const ID = 0xF10A7;};template <> class TypeToID <void* >{public:static int const ID = 0x401d;};template <typename T> class TypeToID<T *>{public:typedef T SameAsT;static int const ID = 0x80000000;};//---------------------  RemovePointer template --------------//template <typename T> class RemovePointer {}; // prototypetemplate <typename T> class RemovePointer<T*>{public:typedef T Result;};int main(){cout << "ID of uint8_t: " << TypeToID<uint8_t>::ID << endl;cout << "ID of uint16_t: " << TypeToID<uint16_t>::ID << endl;cout << "ID of uint32_t: " << TypeToID<uint32_t>::ID << endl;cout << "ID of uint64_t: " << TypeToID<uint64_t>::ID << endl;cout << "ID of float: " << TypeToID<float>::ID << endl;cout << "ID of void*: " << TypeToID<void*>::ID << endl;cout << "ID of float*: " << TypeToID<float*>::ID << endl;// how clever compiler is !cout << TypeToID<TypeToID<float*>::SameAsT>::ID << endl;RemovePointer<float *>::Result x = 0.5f;cout << x << endl;}

2元编程实现斐波那契递归：

// prototypetemplate <int Index>class MetaFibonacci{public:static int const value = MetaFibonacci<Index-1>::value + MetaFibonacci<Index-2>::value;};// special exittemplate <> class MetaFibonacci<1>{public:static int const value = 1;};template <>class MetaFibonacci<2>{public:static int const value = 1;};

3类型判断条件：

// prototype template <typename T, typename N>class Equal{public:static bool const value = false;};// special type template <typename T>class Equal<T, T>{public:static bool const value = true;};bool x = Equal<int, float>::value;bool y = Equal<int, int>::value;

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