More C++ Idioms/Scope Guard

Scope Guard

Intent

• To ensure that resources are always released in face of an exception but not while returning normally
• To provide basic exception safety guarantee

Motivation

Resource Acquisition is Initialization (RAII) idiom allows us to acquire resources in the constructor and release them in the destructor when scope ends successfully or due to an exception. It will always release resources. This is not very flexible. Sometime we don't want to release resources if no exception is thrown but we do want to release them if an exception is thrown.

Solution and Sample Code

Enhance the typical implementation of the Resource Acquisition is Initialization (RAII) idiom with a conditional check.

class ScopeGuard
{
public:
  ScopeGuard () 
   : engaged_ (true) 
  { /* Acquire resources here. */ }
  
  ~ScopeGuard ()  
  { 
    if (engaged_) 
     { /* Release resources here. */} 
  }
  void release () 
  { 
     engaged_ = false; 
     /* Resources will no longer be released */ 
  }
private:
  bool engaged_;
};
void some_init_function ()
{
  ScopeGuard guard;
  // ...... Something may throw here. If it does we release resources.
  guard.release (); // Resources will not be released in normal execution.
}

Known Uses

• boost::mutex::scoped_lock
• boost::scoped_ptr
• std::auto_ptr
• ACE_Guard
• ACE_Read_Guard
• ACE_Write_Guard
• ACE_Auto_Ptr
• ACE_Auto_Array_Ptr

Related Idioms

• Resource Acquisition Is Initialization (RAII)

References

• Generic: Change the Way You Write Exception-Safe Code — Forever
• SCOPE_FAILURE and SCOPE_SUCCESS in C++: ScopeGuards without need to release/commit by hands