ScopeGuard 取代 std::auto_ptr 实现异常安全
Posted: October 27, 2006 at 6:10 pm | Tags: blog, class, 类转至：http://www.cppblog.com/eXile
为了实现异常安全，经常见到下列代码，而且这也是被标准推荐的方式：
void f()
{
std::auto_ptr<SomeType> ptr(new SomeType);
//…and other operation
}
boost替代scoped_ptr 来强化这个概念。这种方案的缺陷是只能用于删除指针，它实际上表达的是以下概念：
void f()
{
SomeType * ptr = new SomeType;
ON_BLOCK_EXIT(delete ptr);
//…and other operation
}
如何在退出作用域时自动执行所指定的函数，实现更广泛意义上的异常安全，而不是scoped_ptr, scoped_array, scoped_function 等一系列替代品。LOKI的那个变态大师又提出了一种更好的办法，类似于下列代码：
template <class T> inline void Delete(T* p)
{ delete p; }
void f()
{
SomeType * ptr = new SomeType;
ON_BLOCK_EXIT(&Delete, ptr);
//…and other operation
}
实现 ON_BLOCK_EXIT 的自动调用，关键是实现一个高效的 ScopeGuard，LOKI中实现大致如下：
class ScopeGuardImplBase
{
public:
void Dismiss() const throw()
{ dismissed_ = true; }
protected:
ScopeGuardImplBase() : dismissed_(false)
{}
ScopeGuardImplBase(const ScopeGuardImplBase& other)
: dismissed_(other.dismissed_)
{ other.Dismiss(); }
~ScopeGuardImplBase() {} // nonvirtual (see below why)
mutable bool dismissed_;
private:
// Disable assignment
ScopeGuardImplBase& operator=(
const ScopeGuardImplBase&);
};
template <typename Fun, typename Parm>class ScopeGuardImpl1 : public ScopeGuardImplBase{public: ScopeGuardImpl1(const Fun& fun, const Parm& parm) : fun_(fun), parm_(parm) {} ~ScopeGuardImpl1() { if (!dismissed_) fun_(parm_); }private: Fun fun_; const Parm parm_;};
template <typename Fun, typename Parm>ScopeGuardImpl1<Fun, Parm>MakeGuard(const Fun& fun, const Parm& parm){ return ScopeGuardImpl1<Fun, Parm>(fun, parm);}
typedef const ScopeGuardImplBase& ScopeGuard;
#define ON_BLOCK_EXIT ScopeGuard g = MakeGuard
这只是实现了一个参数的情况，并作了简化处理，更具体的实现，请参看Loki库。
