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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。
#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;
}
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