最近學習C++11的variadic template argument，終于可以擺脫用fpmacro模板來復制一大堆代碼的做法了，好開心。這個例子的main函數用lambda寫了一個斐波那契數列的遞歸計算函數。跟以往不同的是，在Y函數的幫助下，這個lambda表達是可以成功看到自己，然后遞歸調用。當然這仍然需要用普通的C++遞歸來實現，并不是λ-calculus那個高大上的Y Combinator。

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#include <functional>

#include <memory>

#include <iostream>

#include <string>

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using namespace std;

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template<typename TResult, typename ...TArgs>

class YBuilder

{

private:

????function<TResult(function<TResult(TArgs...)>, TArgs...)> partialLambda;

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public:

????YBuilder(function<TResult(function<TResult(TArgs...)>, TArgs...)> _partialLambda)

????????:partialLambda(_partialLambda)

????{

????}

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????TResult operator()(TArgs ...args)const

????{

????????return partialLambda(

????????????[this](TArgs ...args)

????????????{

????????????????return this->operator()(args...);

????????????}, args...);

????}

};

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template<typename TMethod>

struct PartialLambdaTypeRetriver

{

????typedef void FunctionType;

????typedef void LambdaType;

????typedef void YBuilderType;

};

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template<typename TClass, typename TResult, typename ...TArgs>

struct PartialLambdaTypeRetriver<TResult(__thiscall TClass::*)(function<TResult(TArgs...)>, TArgs...)>

{

????typedef TResult FunctionType(TArgs...);

????typedef TResult LambdaType(function<TResult(TArgs...)>, TArgs...);

????typedef YBuilder<TResult, TArgs...> YBuilderType;

};

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template<typename TClass, typename TResult, typename ...TArgs>

struct PartialLambdaTypeRetriver<TResult(__thiscall TClass::*)(function<TResult(TArgs...)>, TArgs...)const>

{

????typedef TResult FunctionType(TArgs...);

????typedef TResult LambdaType(function<TResult(TArgs...)>, TArgs...);

????typedef YBuilder<TResult, TArgs...> YBuilderType;

};

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template<typename TLambda>

function<typename PartialLambdaTypeRetriver<decltype(&TLambda::operator())>::FunctionType> Y(TLambda partialLambda)

{

????return typename PartialLambdaTypeRetriver<decltype(&TLambda::operator())>::YBuilderType(partialLambda);

}

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int _tmain(int argc, _TCHAR* argv[])

{

????auto fib = Y([](function<int(int)> self, int index)

????{

????????return index<2

?????????????1

????????????:self(index-1)+self(index-2);

????});

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????for (int i = 0; i < 10; i++)

????{

????????cout << fib(i) << " ";

????}

????cout << endl;

}