libstdc++-v3 HOWTO: Chapter 20: General Utilities
Author: pme@gcc.gnu.org (Phil Edwards)
Description: HOWTO for the libstdc++ chapter 20.
Keywords: HOWTO, libstdc++, GCC, g++, libg++, STL

# Chapter 20: General Utilities

Chapter 20 deals with utility classes and functions, such as the oft-debated `auto_ptr<>`.

# Contents

- `auto_ptr` is not omnipotent
- `auto_ptr` inside container classes
- Functors
- Pairs
- Memory allocators

## `auto_ptr` is not omnipotent

I'm not going to try and explain all of the fun and delicious things that can happen with misuse of the auto_ptr class template (called AP here), nor am I going to try and teach you how to use AP safely in the presence of copying. The AP class is a really nifty idea for a smart pointer, but it is one of the dumbest of all the smart pointers -- and that's fine.

AP is not meant to be a supersmart solution to all resource leaks everywhere. Neither is it meant to be an effective form of garbage collection (although it can help, a little bit). And it can not be used for arrays!

AP is meant to prevent nasty leaks in the presence of exceptions. That's all. This code is AP-friendly:

        // not a recommend naming scheme, but good for web-based FAQs
        typedef std::auto_ptr<MyClass>  APMC;

        extern function_taking_MyClass_pointer (MyClass*);
        extern some_throwable_function ();

        void func (int data)
        {
            APMC  ap (new MyClass(data));

            some_throwable_function();   // this will throw an exception

            function_taking_MyClass_pointer (ap.get());
        }

When an exception gets thrown, the instance of MyClass that's been created on the heap will be `delete`'d as the stack is unwound past `func()`.

Changing that code as follows is not AP-friendly:

            APMC  ap (new MyClass[22]);

You will get the same problems as you would without the use of AP:

            char*  array = new char[10];       // array new...
            ...
            delete array;                      // ...but single-object delete

AP cannot tell whether the pointer you've passed at creation points to one or many things. If it points to many things, you are about to die. AP is trivial to write, however, so you could write your own `auto_array_ptr` for that situation (in fact, this has been done many times; check the mailing lists, Usenet, Boost, etc).

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## `auto_ptr` inside container classes

All of the containers described in the standard library require their contained types to have, among other things, a copy constructor like this:

        struct My_Type
        {
            My_Type (My_Type const&);
        };

Note the const keyword; the object being copied shouldn't change. The template class `auto_ptr` (called AP here) does not meet this requirement. Creating a new AP by copying an existing one transfers ownership of the pointed-to object, which means that the AP being copied must change, which in turn means that the copy ctors of AP do not take const objects.

The resulting rule is simple: Never ever use a container of auto_ptr objects. The standard says that "undefined" behavior is the result, but it is guaranteed to be messy.

To prevent you from doing this to yourself, the concept checks built in to this implementation will issue an error if you try to compile code like this:

        #include <vector>
        #include <memory>

        void f()
        {
            std::vector< std::auto_ptr<int> >   vec_ap_int;
        }

Should you try this with the checks enabled, you will see an error.

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## Functors

If you don't know what functors are, you're not alone. Many people get slightly the wrong idea. In the interest of not reinventing the wheel, we will refer you to the introduction to the functor concept written by SGI as part of their STL, in their http://www.sgi.com/tech/stl/functors.html.

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## Pairs

The `pair<T1,T2>` is a simple and handy way to carry around a pair of objects. One is of type T1, and another of type T2; they may be the same type, but you don't get anything extra if they are. The two members can be accessed directly, as `.first` and `.second`.

Construction is simple. The default ctor initializes each member with its respective default ctor. The other simple ctor,

        pair (const T1& x, const T2& y);

does what you think it does, `first` getting `x` and `second` getting `y`.

There is a copy constructor, but it requires that your compiler handle member function templates:

        template <class U, class V> pair (const pair<U,V>& p);

The compiler will convert as necessary from U to T1 and from V to T2 in order to perform the respective initializations.

The comparison operators are done for you. Equality of two `pair<T1,T2>`s is defined as both `first` members comparing equal and both `second` members comparing equal; this simply delegates responsibility to the respective `operator==` functions (for types like MyClass) or builtin comparisons (for types like int, char, etc).

The less-than operator is a bit odd the first time you see it. It is defined as evaluating to:

        x.first  <  y.first  ||
            ( !(y.first  <  x.first)  &&  x.second  <  y.second )

The other operators are not defined using the `rel_ops` functions above, but their semantics are the same.

Finally, there is a template function called `make_pair` that takes two references-to-const objects and returns an instance of a pair instantiated on their respective types:

        pair<int,MyClass> p = make_pair(4,myobject);

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## Memory allocators

The available free store ("heap") management classes are described here.

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