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ISO C++
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ISO C++
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auto_ptr
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auto_ptr
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auto_ptr
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auto_ptr
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Limitations
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Limitations
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Explaining all of the fun and delicious things that can
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Explaining all of the fun and delicious things that can
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happen with misuse of the auto_ptr class
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happen with misuse of the auto_ptr class
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template (called AP here) would take some
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template (called AP here) would take some
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time. Suffice it to say that the use of AP
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time. Suffice it to say that the use of AP
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safely in the presence of copying has some subtleties.
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safely in the presence of copying has some subtleties.
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The AP class is a really
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The AP class is a really
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nifty idea for a smart pointer, but it is one of the dumbest of
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nifty idea for a smart pointer, but it is one of the dumbest of
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all the smart pointers -- and that's fine.
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all the smart pointers -- and that's fine.
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AP is not meant to be a supersmart solution to all resource
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AP is not meant to be a supersmart solution to all resource
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leaks everywhere. Neither is it meant to be an effective form
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leaks everywhere. Neither is it meant to be an effective form
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of garbage collection (although it can help, a little bit).
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of garbage collection (although it can help, a little bit).
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And it can notbe used for arrays!
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And it can notbe used for arrays!
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AP is meant to prevent nasty leaks in the
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AP is meant to prevent nasty leaks in the
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presence of exceptions. That's all. This
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presence of exceptions. That's all. This
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code is AP-friendly:
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code is AP-friendly:
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// Not a recommend naming scheme, but good for web-based FAQs.
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// Not a recommend naming scheme, but good for web-based FAQs.
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typedef std::auto_ptr<MyClass> APMC;
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typedef std::auto_ptr<MyClass> APMC;
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extern function_taking_MyClass_pointer (MyClass*);
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extern function_taking_MyClass_pointer (MyClass*);
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extern some_throwable_function ();
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extern some_throwable_function ();
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void func (int data)
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void func (int data)
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{
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{
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APMC ap (new MyClass(data));
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APMC ap (new MyClass(data));
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some_throwable_function(); // this will throw an exception
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some_throwable_function(); // this will throw an exception
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function_taking_MyClass_pointer (ap.get());
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function_taking_MyClass_pointer (ap.get());
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}
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}
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When an exception gets thrown, the instance of MyClass that's
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When an exception gets thrown, the instance of MyClass that's
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been created on the heap will be delete'd as the stack is
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been created on the heap will be delete'd as the stack is
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unwound past func().
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unwound past func().
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Changing that code as follows is not AP-friendly:
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Changing that code as follows is not AP-friendly:
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APMC ap (new MyClass[22]);
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APMC ap (new MyClass[22]);
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You will get the same problems as you would without the use
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You will get the same problems as you would without the use
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of AP:
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of AP:
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char* array = new char[10]; // array new...
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char* array = new char[10]; // array new...
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...
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...
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delete array; // ...but single-object delete
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delete array; // ...but single-object delete
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AP cannot tell whether the pointer you've passed at creation points
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AP cannot tell whether the pointer you've passed at creation points
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to one or many things. If it points to many things, you are about
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to one or many things. If it points to many things, you are about
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to die. AP is trivial to write, however, so you could write your
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to die. AP is trivial to write, however, so you could write your
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own auto_array_ptr for that situation (in fact, this has
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own auto_array_ptr for that situation (in fact, this has
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been done many times; check the mailing lists, Usenet, Boost, etc).
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been done many times; check the mailing lists, Usenet, Boost, etc).
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Use in Containers
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Use in Containers
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All of the containers
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All of the containers
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described in the standard library require their contained types
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described in the standard library require their contained types
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to have, among other things, a copy constructor like this:
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to have, among other things, a copy constructor like this:
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struct My_Type
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struct My_Type
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{
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{
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My_Type (My_Type const&);
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My_Type (My_Type const&);
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};
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};
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Note the const keyword; the object being copied shouldn't change.
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Note the const keyword; the object being copied shouldn't change.
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The template class auto_ptr (called AP here) does not
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The template class auto_ptr (called AP here) does not
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meet this requirement. Creating a new AP by copying an existing
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meet this requirement. Creating a new AP by copying an existing
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one transfers ownership of the pointed-to object, which means that
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one transfers ownership of the pointed-to object, which means that
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the AP being copied must change, which in turn means that the
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the AP being copied must change, which in turn means that the
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copy ctors of AP do not take const objects.
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copy ctors of AP do not take const objects.
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The resulting rule is simple: Never ever use a
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The resulting rule is simple: Never ever use a
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container of auto_ptr objects. The standard says that
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container of auto_ptr objects. The standard says that
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undefined behavior is the result, but it is
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undefined behavior is the result, but it is
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guaranteed to be messy.
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guaranteed to be messy.
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To prevent you from doing this to yourself, the
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To prevent you from doing this to yourself, the
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concept checks built
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concept checks built
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in to this implementation will issue an error if you try to
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in to this implementation will issue an error if you try to
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compile code like this:
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compile code like this:
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#include <vector>
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#include <vector>
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#include <memory>
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#include <memory>
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void f()
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void f()
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{
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{
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std::vector< std::auto_ptr<int> > vec_ap_int;
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std::vector< std::auto_ptr<int> > vec_ap_int;
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}
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}
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Should you try this with the checks enabled, you will see an error.
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Should you try this with the checks enabled, you will see an error.
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