gecko-dev/mfbt/Scoped.h

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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* A number of structures to simplify scope-based RAII management. */
#ifndef mozilla_Scoped_h
#define mozilla_Scoped_h
/*
* Resource Acquisition Is Initialization is a programming idiom used
* to write robust code that is able to deallocate resources properly,
* even in presence of execution errors or exceptions that need to be
* propagated. The Scoped* classes defined in this header perform the
* deallocation of the resource they hold once program execution
* reaches the end of the scope for which they have been defined.
*
* This header provides the following RAII classes:
*
* - |ScopedFreePtr| - a container for a pointer, that automatically calls
* |free()| at the end of the scope;
* - |ScopedDeletePtr| - a container for a pointer, that automatically calls
* |delete| at the end of the scope;
* - |ScopedDeleteArray| - a container for a pointer to an array, that
* automatically calls |delete[]| at the end of the scope.
*
* The general scenario for each of the RAII classes is the following:
*
* ScopedClass foo(create_value());
* // ... In this scope, |foo| is defined. Use |foo.get()| or |foo.rwget()|
* to access the value.
* // ... In case of |return| or |throw|, |foo| is deallocated automatically.
* // ... If |foo| needs to be returned or stored, use |foo.forget()|
*
* Note that the RAII classes defined in this header do _not_ perform any form
* of reference-counting or garbage-collection. These classes have exactly two
* behaviors:
*
* - if |forget()| has not been called, the resource is always deallocated at
* the end of the scope;
* - if |forget()| has been called, any control on the resource is unbound
* and the resource is not deallocated by the class.
*
* Extension:
*
* In addition, this header provides class |Scoped| and macros |SCOPED_TEMPLATE|
* and |MOZ_TYPE_SPECIFIC_SCOPED_POINTER_TEMPLATE| to simplify the definition
* of RAII classes for other scenarios. These macros have been used to
* automatically close file descriptors/file handles when reaching the end of
* the scope, graphics contexts, etc.
*/
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/GuardObjects.h"
#include "mozilla/Move.h"
#include "mozilla/NullPtr.h"
namespace mozilla {
/*
* Scoped is a helper to create RAII wrappers
* Type argument |Traits| is expected to have the following structure:
*
* struct Traits {
* // Define the type of the value stored in the wrapper
* typedef value_type type;
* // Returns the value corresponding to the uninitialized or freed state
* const static type empty();
* // Release resources corresponding to the wrapped value
* // This function is responsible for not releasing an |empty| value
* const static void release(type);
* }
*/
template<typename Traits>
class Scoped
{
public:
typedef typename Traits::type Resource;
explicit Scoped(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM)
: value(Traits::empty())
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
}
explicit Scoped(const Resource& v
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: value(v)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
}
/* Move constructor. */
explicit Scoped(Scoped&& v
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: value(Move(v.value))
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
v.value = Traits::empty();
}
~Scoped() {
Traits::release(value);
}
// Constant getter
operator const Resource&() const { return value; }
const Resource& operator->() const { return value; }
const Resource& get() const { return value; }
// Non-constant getter.
Resource& rwget() { return value; }
/*
* Forget the resource.
*
* Once |forget| has been called, the |Scoped| is neutralized, i.e. it will
* have no effect at destruction (unless it is reset to another resource by
* |operator=|).
*
* @return The original resource.
*/
Resource forget() {
Resource tmp = value;
value = Traits::empty();
return tmp;
}
/*
* Perform immediate clean-up of this |Scoped|.
*
* If this |Scoped| is currently empty, this method has no effect.
*/
void dispose() {
Traits::release(value);
value = Traits::empty();
}
bool operator==(const Resource& other) const {
return value == other;
}
/*
* Replace the resource with another resource.
*
* Calling |operator=| has the side-effect of triggering clean-up. If you do
* not want to trigger clean-up, you should first invoke |forget|.
*
* @return this
*/
Scoped& operator=(const Resource& other) {
return reset(other);
}
Scoped& reset(const Resource& other) {
Traits::release(value);
value = other;
return *this;
}
/* Move assignment operator. */
Scoped& operator=(Scoped&& rhs) {
MOZ_ASSERT(&rhs != this, "self-move-assignment not allowed");
this->~Scoped();
new(this) Scoped(Move(rhs));
return *this;
}
private:
explicit Scoped(const Scoped& value) MOZ_DELETE;
Scoped& operator=(const Scoped& value) MOZ_DELETE;
private:
Resource value;
MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
};
/*
* SCOPED_TEMPLATE defines a templated class derived from Scoped
* This allows to implement templates such as ScopedFreePtr.
*
* @param name The name of the class to define.
* @param Traits A struct implementing clean-up. See the implementations
* for more details.
*/
#define SCOPED_TEMPLATE(name, Traits) \
template<typename Type> \
struct name : public mozilla::Scoped<Traits<Type> > \
{ \
typedef mozilla::Scoped<Traits<Type> > Super; \
typedef typename Super::Resource Resource; \
name& operator=(Resource rhs) { \
Super::operator=(rhs); \
return *this; \
} \
name& operator=(name&& rhs) { \
Super::operator=(Move(rhs)); \
return *this; \
} \
explicit name(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM) \
: Super(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_TO_PARENT) \
{} \
explicit name(Resource rhs \
MOZ_GUARD_OBJECT_NOTIFIER_PARAM) \
: Super(rhs \
MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT) \
{} \
explicit name(name&& rhs \
MOZ_GUARD_OBJECT_NOTIFIER_PARAM) \
: Super(Move(rhs) \
MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT) \
{} \
private: \
explicit name(name&) MOZ_DELETE; \
name& operator=(name&) MOZ_DELETE; \
};
/*
* ScopedFreePtr is a RAII wrapper for pointers that need to be free()d.
*
* struct S { ... };
* ScopedFreePtr<S> foo = malloc(sizeof(S));
* ScopedFreePtr<char> bar = strdup(str);
*/
template<typename T>
struct ScopedFreePtrTraits
{
typedef T* type;
static T* empty() { return nullptr; }
static void release(T* ptr) { free(ptr); }
};
SCOPED_TEMPLATE(ScopedFreePtr, ScopedFreePtrTraits)
/*
* ScopedDeletePtr is a RAII wrapper for pointers that need to be deleted.
*
* struct S { ... };
* ScopedDeletePtr<S> foo = new S();
*/
template<typename T>
struct ScopedDeletePtrTraits : public ScopedFreePtrTraits<T>
{
static void release(T* ptr) { delete ptr; }
};
SCOPED_TEMPLATE(ScopedDeletePtr, ScopedDeletePtrTraits)
/*
* ScopedDeleteArray is a RAII wrapper for pointers that need to be delete[]ed.
*
* struct S { ... };
* ScopedDeleteArray<S> foo = new S[42];
*/
template<typename T>
struct ScopedDeleteArrayTraits : public ScopedFreePtrTraits<T>
{
static void release(T* ptr) { delete [] ptr; }
};
SCOPED_TEMPLATE(ScopedDeleteArray, ScopedDeleteArrayTraits)
/*
* MOZ_TYPE_SPECIFIC_SCOPED_POINTER_TEMPLATE makes it easy to create scoped
* pointers for types with custom deleters; just overload
* TypeSpecificDelete(T*) in the same namespace as T to call the deleter for
* type T.
*
* @param name The name of the class to define.
* @param Type A struct implementing clean-up. See the implementations
* for more details.
* *param Deleter The function that is used to delete/destroy/free a
* non-null value of Type*.
*
* Example:
*
* MOZ_TYPE_SPECIFIC_SCOPED_POINTER_TEMPLATE(ScopedPRFileDesc, PRFileDesc, \
* PR_Close)
* ...
* {
* ScopedPRFileDesc file(PR_OpenFile(...));
* ...
* } // file is closed with PR_Close here
*/
#define MOZ_TYPE_SPECIFIC_SCOPED_POINTER_TEMPLATE(name, Type, Deleter) \
template <> inline void TypeSpecificDelete(Type * value) { Deleter(value); } \
typedef ::mozilla::TypeSpecificScopedPointer<Type> name;
template <typename T> void TypeSpecificDelete(T * value);
template <typename T>
struct TypeSpecificScopedPointerTraits
{
typedef T* type;
const static type empty() { return nullptr; }
const static void release(type value)
{
if (value)
TypeSpecificDelete(value);
}
};
SCOPED_TEMPLATE(TypeSpecificScopedPointer, TypeSpecificScopedPointerTraits)
} /* namespace mozilla */
#endif /* mozilla_Scoped_h */