最近学习C++11的variadic template argument，终于可以摆脱用fpmacro模板来复制一大堆代码的做法了，好开心。这个例子的main函数用lambda写了一个斐波那契数列的递归计算函数。跟以往不同的是，在Y函数的帮助下，这个lambda表达是可以成功看到自己，然后递归调用。当然这仍然需要用普通的C++递归来实现，并不是λ-calculus那个高大上的Y Combinator。
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#include <functional>#include <memory>#include <iostream>#include <string>using namespace std;template<typename TResult, typename ...TArgs>class YBuilder{private: function<TResult(function<TResult(TArgs...)>, TArgs...)> partialLambda;public: YBuilder(function<TResult(function<TResult(TArgs...)>, TArgs...)> _partialLambda) :partialLambda(_partialLambda) { } TResult operator()(TArgs ...args)const { return partialLambda( [this](TArgs ...args) { return this->operator()(args...); }, args...); }};template<typename TMethod>struct PartialLambdaTypeRetriver{ typedef void FunctionType; typedef void LambdaType; typedef void YBuilderType;};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;};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;}; template<typename TLambda>function<typename PartialLambdaTypeRetriver<decltype(&TLambda::operator())>::FunctionType> Y(TLambda partialLambda){ return typename PartialLambdaTypeRetriver<decltype(&TLambda::operator())>::YBuilderType(partialLambda);} 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); }); for (int i = 0; i < 10; i++) { cout << fib(i) << " "; } cout << endl;} |
