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Tub.h
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Tub.h
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/* Copyright (c) 2010-2015 Stanford University
*
* Permission to use, copy, modify, and distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR(S) DISCLAIM ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL AUTHORS BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
#ifndef RAMCLOUD_TUB_H
#define RAMCLOUD_TUB_H
/**
* A Tub holds an object that may be uninitialized; it allows the allocation of
* memory for objects to be separated from its construction and destruction.
* When you initially create a Tub its object is uninitialized (and should not
* be used). You can call #construct and #destroy to invoke the constructor and
* destructor of the embedded object, and #get or #operator-> will return the
* embedded object. The embedded object is automatically destroyed when the Tub
* is destroyed (if it was ever constructed in the first place).
*
* Tubs are useful in situations like the following:
* - You want to create an array of objects, but the objects need complex
* constructors with multiple arguments.
* - You want to create a collection of objects, only some of which will be
* used, and you don't want to pay the cost of constructing objects that will
* never be used.
* - You want automatic destruction of an object but don't want to
* heap-allocate the object (as with std::unique_ptr).
* - You want a way to return failure from a method without using pointers,
* exceptions, or special values (e.g. -1). The Tub gives you a 'maybe'
* object; it may be empty if a failure occurred.
* - You want a singleton, but don't want to deal with heap-allocating an
* object on first use and freeing it later. Instead, just declare your object
* in a tub and do:
* if (!tub) tub.construct();
* - You want optional arguments to a function, but don't want to use pointers
* (i.e. use the Tub's boolean to determine that an argument was passed,
* rather than checking arg != NULL).
*
* Tub is CopyConstructible if and only if ElementType is CopyConstructible,
* and Tub is Assignable if and only if ElementType is Assignable.
*
* \tparam ElementType
* The type of the object to be stored within the Tub.
*/
template<typename ElementType>
class Tub {
public:
/// The type of the object to be stored within the Tub.
typedef ElementType element_type;
/**
* Default constructor: the object starts off uninitialized.
*/
Tub()
: occupied(false)
{}
/**
* Construct an occupied Tub, whose contained object is initialized
* with a copy of the given object.
* \pre
* ElementType is CopyConstructible.
* \param other
* Source of the copy.
*/
Tub(const ElementType& other) // NOLINT
: occupied(false)
{
construct(other);
}
/**
* Copy constructor.
* The object will be initialized if and only if the source of the copy is
* initialized.
* \pre
* ElementType is CopyConstructible.
* \param other
* Source of the copy.
*/
Tub(const Tub<ElementType>& other) // NOLINT
: occupied(false)
{
if (other.occupied)
construct(*other.object); // use ElementType's copy constructor
}
/**
* Destructor: destroy the object if it was initialized.
*/
~Tub() {
destroy();
}
/**
* Assignment: destroy current object if initialized, replace with
* source. Result will be uninitialized if source is uninitialized.
* \pre
* ElementType is Assignable.
*/
Tub<ElementType>&
operator=(const Tub<ElementType>& other) {
if (this != &other) {
if (other.occupied) {
if (occupied) {
#if __GNUC__ && __GNUC__ >= 4 && __GNUC_MINOR__ >= 7
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Warray-bounds"
#endif
*object = *other.object; // use ElementType's assignment
#if __GNUC__ && __GNUC__ >= 4 && __GNUC_MINOR__ >= 7
#pragma GCC diagnostic pop
#endif
} else {
construct(*other.object);
}
} else {
destroy();
}
}
return *this;
}
/**
* Initialize the object.
* If the object was already initialized, it will be destroyed first.
* \param args
* Arguments to ElementType's constructor.
* \return
* A pointer to the newly initialized object.
* \post
* The object is initialized.
*/
template<typename... Args>
ElementType*
construct(Args&&... args) {
destroy();
new(object) ElementType(static_cast<Args&&>(args)...);
occupied = true;
return object;
}
/**
* Destroy the object, leaving the Tub in the same state
* as after the no-argument constructor.
* If the object was not initialized, this will have no effect.
* \post
* The object is uninitialized.
*/
void
destroy() {
if (occupied) {
object->~ElementType();
occupied = false;
}
}
/// See #get().
const ElementType&
operator*() const {
return *get();
}
/// See #get().
ElementType&
operator*() {
return *get();
}
/// See #get().
const ElementType*
operator->() const {
return get();
}
/// See #get().
ElementType*
operator->() {
return get();
}
/**
* Return a pointer to the object.
* \pre
* The object is initialized.
*/
ElementType*
get() {
if (!occupied)
return NULL;
return object;
}
/// See #get().
const ElementType*
get() const {
if (!occupied)
return NULL;
return object;
}
/**
* Return whether the object is initialized.
*/
operator bool() const {
return occupied;
}
private:
/**
* A pointer to where the object is, if it is initialized.
* This must directly precede #raw in the struct.
*/
ElementType object[0];
/**
* A storage area to back the object while it is initialized.
*/
char raw[sizeof(ElementType)];
/**
* Whether the object is initialized.
*/
bool occupied;
};
#endif // RAMCLOUD_TUB_H