Editing Wrapper library

{{Short description|Software library for programming}}
'''Wrapper libraries''' (or '''library wrappers''') consist of a thin layer of code (a "[[shim (computing)|shim]]") which translates a [[Library (computing)|library]]'s existing interface into a compatible interface. This is done for several reasons:

* To refine a poorly designed or complicated interface
* Allow code to work together which otherwise cannot (e.g. incompatible data formats)
* Enable cross language and/or [[Run-time system|runtime]] interoperability

Wrapper libraries can be implemented using the [[Adapter pattern|adapter]], [[Facade pattern|façade]], and to a lesser extent, [[Proxy pattern|proxy]] [[Design pattern (computer science)|design patterns]].

== Structure and implementation ==
The specific way in which a wrapper library is implemented is highly specific to the environment it is being written in and the scenarios which it intends to address. This is especially true in the case when [[#Cross-language.2Fruntime_interoperability|cross-language/runtime interoperability]] is a consideration.

=== Example ===
The following provides a general illustration of a common wrapper library implementation. In this example, a [[C++]] interface acts as a "wrapper" around a [[C (programming language)|C]] interface.

==== C interface ====
<syntaxhighlight lang="C">
int pthread_mutex_init(pthread_mutex_t * mutex , pthread_mutexattr_t * attr);
int pthread_mutex_destroy (pthread_mutex_t * mutex);
int pthread_mutex_lock (pthread_mutex_t * mutex );
int pthread_mutex_unlock (pthread_mutex_t * mutex );
</syntaxhighlight>

==== C++ wrapper ====
<syntaxhighlight lang="cpp">
class Mutex
{
     pthread_mutex_t mutex;

public:
     Mutex() 
     {
          pthread_mutex_init(&mutex, 0);
     }

     ~Mutex()
     {
          pthread_mutex_destroy(&mutex);
     }

private:
     friend class Lock;

     void lock()
     {
          pthread_mutex_lock(&mutex);
     }

     void unlock()
     {
          pthread_mutex_unlock(&mutex);
     }
};

class Lock
{
private:
      Mutex &mutex;

public:
      Lock(Mutex &mutex): mutex{mutex}
      {
            mutex.lock();
      }

      ~Lock()
      {
            mutex.unlock();
      }
};
</syntaxhighlight>

The original C interface can be regarded as error prone, particularly in the case where users of the library forget to unlock an already locked mutex. The new interface effectively utilizes [[resource acquisition is initialization]] (RAII) in the new {{mono|Mutex}} and {{mono|Lock}} classes to ensure {{mono|Mutex}}s are eventually unlocked and {{mono|pthread_mutex_t}} objects are automatically released.

The above code closely mimics the implementation of {{mono|boost::scoped_lock}} and {{mono|boost::mutex}} which are part of the [http://www.boost.org/doc/libs/1_38_0/doc/html/thread.html boost::thread] library.

=== Driver wrappers ===
{{Further|Driver wrapper}}

== Cross-language/runtime interoperability ==
Some wrapper libraries exist to act as a bridge between a client application and a library written using an incompatible technology. For instance, a [[Java (programming language)|Java]] application may need to execute a [[system call]]. However system calls are typically exposed as C library functions. To resolve this issue Java implements wrapper libraries which make these system calls callable from a Java application.

In order to achieve this, languages like Java provide mechanisms that make this possible. Some examples of these mechanisms include:

* [[Java Native Interface|Java Native Interface (JNI)]]
* [[Java Native Access|Java Native Access (JNA)]]
* [https://docs.python.org/library/ctypes.html A foreign function library for Python]
* [[Managed Extensions for C++|Managed Extensions]]
* [[SWIG|SWIG (Simplified Wrapper and Interface Generator)]]

== Existing wrapper libraries ==
Some examples of existing wrapper libraries:

* [http://sourceware.org/pthreads-win32/ Pthreads for WIN32]
* [http://pyopengl.sourceforge.net/ OpenGL Bindings for Python]
* [http://www.tangentsoft.net/mysql++/ MySQL++]
* [http://code.google.com/p/javacv/ JavaCV]
* [[WineD3D]]

==See also==
* [[Wrapper function]]
* [[Wrapper pattern]]
* [[Glue code]]

[[Category:Computer libraries]]