（原创）一个和c#中LazyT类似的c++ LazyT类的实现

　　在.net 4.0中增加一个延迟加载类Lazy<T>，它的作用是实现按需延迟加载，也许很多人用过。一个典型的应用场景是这样的：当初始化某个对象时，该对象引用了一个大对象，需要创建，这个对象的创建时需要较长的时间，同时也需要在托管堆上分配较多的空间，这样可能会在初始化时变得很慢，尤其是UI应用时，会导致用户体验很差。其实狠多时候并不需要马上就获取大数据，只是在需要时获取，这种场景就很适合延迟加载了。先看看c#中Lazy<T>如何使用的吧：

class LargeObject
{public int InitializedBy { get { return initBy; } }int initBy = 0;public LargeObject(int initializedBy){initBy = initializedBy;Console.WriteLine("LargeObject was created on thread id {0}.", initBy);}public long[] Data = new long[100000000];
}

class TestLazy
{Lazy<LargeObject> lazyLargeObject = null;public TestLazy(){//创建一个延迟加载对象lazyLargeObject = new Lazy<LargeObject>(InitLargeObject);}public void ReallyLoad(){//此时真正加载
        lazyLargeObject.Value;Console.WriteLine("lazy load big object");//do something
    }
}void Test()
{TestLazy t = new TestLazy();t.ReallyLoad(); //这时，真正延迟加载时才会打印"lazy load big object"
}

　　c++中目前还没有类似的Lazy<T>延迟加载类，其实延迟加载类内部用到了lamda表达式，将函数封装到lamda表达式中去，而不是马上求值，而是在需要的时候再调用lamda表达式去求值。c++11 中有lamda表达式和function，正好做这个事情，看看c++11如何实现类似c#的Lazy<T>延迟加载类吧。

#include <boost/optional.hpp>
template<typename T>
struct Lazy
{Lazy(){}template <typename Func, typename... Args>Lazy(Func& f, Args && ... args){m_func = [&f, &args...]{return f(args...); };}T& Value(){if (!m_value.is_initialized()){m_value = m_func();}return *m_value;}bool IsValueCreated() const{return m_value.is_initialized();}private:std::function<T()> m_func;boost::optional<T> m_value;
};template<class Func, typename... Args>
Lazy<typename std::result_of<Func(Args...)>::type>lazy(Func && fun, Args && ... args)
{return Lazy<typename std::result_of<Func(Args...)>::type>(std::forward<Func>(fun), std::forward<Args>(args)...);
}

再看看测试代码：

struct BigObject
{BigObject(){cout << "lazy load big object" << endl;}
};struct MyStruct
{MyStruct(){m_obj = lazy([]{return std::make_shared<BigObject>(); });}void Load(){m_obj.Value();}Lazy< std::shared_ptr<BigObject>> m_obj;
};int Foo(int x)
{return x * 2;
}void TestLazy()
{//带参数的普通函数int y = 4;auto lazyer1 = lazy(Foo, y);cout << lazyer1.Value() << endl;//不带参数的lamdaLazy<int> lazyer2 = lazy([]{return 12; });cout << lazyer2.Value() << endl;//带参数的fucntionstd::function < int(int) > f = [](int x){return x + 3; };auto lazyer3 = lazy(f, 3);cout << lazyer3.Value() << endl;//延迟加载大对象
    MyStruct t;t.Load();
}