Opaque Pointer in C++

Opaque Pointer in C++

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Updated on Feb 23, 2023 15:51 IST

In C++, an opaque pointer is a type of pointer that points to a data structure whose implementation details are hidden or not exposed to the user. This article will discuss what an opaque operator is and why to use it.

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Opaque refers to something that is not transparent and cannot be seen through. In C++, an opaque pointer is a type of pointer that directs to a data structure whose content is hidden or unknown at the point of declaration.

In this tutorial, we will discuss the use of opaque pointers in C++ with the aid of suitable examples.

We will be covering the following sections:

What is an Opaque Pointer in C++?

In C++, an opaque pointer is a type of pointer that points to a data structure whose implementation details are hidden or not exposed to the user. The term “opaque” is used to describe this type of pointer because the contents of the data structure are not visible to the user and are therefore treated as an unknown, or opaque, type.

The purpose of an opaque pointer is to provide encapsulation and abstraction, which are important principles in software engineering:

  • Encapsulation refers to the practice of hiding implementation details of an object or data structure from the user so that changes to the implementation do not affect the user’s code. 
  • Abstraction refers to the practice of presenting an interface to the user that abstracts away implementation details, making the interface easier to use and less error-prone.

In the case of an opaque pointer, the implementation details of the data structure are hidden from the user, so the user cannot directly access or modify the data. Instead, the user must use a set of functions or methods provided by the library or framework to interact with the data. This allows the library or framework to maintain control over the data and to ensure that the data is used in a consistent and safe manner.

Also Read: Difference between Abstraction and Encapsulation

Also Read: Introduction to Pointers in C++

Why Use Opaque Pointer in C++?

By using an opaque pointer, you can hide the implementation details of a data structure and provide a clean, well-defined interface to the user of the data structure. This allows you to change the implementation details of the data structure without affecting the user’s code, as long as you maintain the same interface.

For example, imagine you are writing a library that provides a set of functions for working with a complex data structure. You want to hide the details of the data structure from the user of the library to prevent them from accessing the data directly and potentially corrupting it. By using an opaque pointer to represent the data structure, you can provide a set of functions that operate on the data without exposing the details of the data structure to the user. This allows you to change the implementation details of the data structure without affecting the user’s code as long as you maintain the same set of functions.

Overall, an opaque pointer is a powerful tool for achieving encapsulation and information hiding in C++, which can lead to more robust, maintainable, and secure code.

Must Read: What is C++ Programming?

Examples of Using Opaque Pointer

Object-Oriented Programming

As discussed above, in C++, objects are often implemented as opaque pointers to hide implementation details and provide encapsulation and abstraction.

For example, a class might have a private data member that is a pointer to an opaque data structure, and public member functions that operate on that data structure.

 
class MyClass {
private:
struct MyDataStructure;
MyDataStructure* data;
public:
MyClass();
~MyClass();
void doSomething();
};
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Here, MyDataStructure is an opaque data structure that is not exposed to the user of the class. The user can only interact with the data structure through the public member functions of MyClass.

Libraries and Frameworks

Another common use of opaque pointers is in libraries and frameworks that provide complex data structures and algorithms.

For example, the GTK+ GUI toolkit uses opaque pointers extensively to encapsulate data structures such as widgets, windows, and events.

 
GtkWidget* gtk_button_new_with_label(const gchar *label);
void gtk_container_add(GtkContainer *container, GtkWidget *widget);
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In this example, GtkWidget is an opaque data structure that represents a graphical user interface (GUI) widget. The user of the library can create a new button widget using gtk_button_new_with_label, and add it to a container using gtk_container_add, but they cannot access the details of the GtkWidget structure directly.

Also Read: Difference Between Libraries and Framework

Database Access

Opaque pointers can also be used in database access libraries to represent database connections, transactions, and queries. For example, the SQLite library uses opaque pointers to encapsulate database connections and prepared statements.

 
sqlite3* sqlite3_open(const char *filename, sqlite3 **ppDb);
int sqlite3_prepare_v2(sqlite3 *db, const char *zSql, int nByte, sqlite3_stmt **ppStmt, const char **pzTail);
int sqlite3_bind_int(sqlite3_stmt*, int, int);
int sqlite3_step(sqlite3_stmt*);
int sqlite3_finalize(sqlite3_stmt *pStmt);
int sqlite3_close(sqlite3*);
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In this example, sqlite3 is an opaque data type that represents a database connection. The user can open a new database connection using sqlite3_open, prepare a SQL statement using sqlite3_prepare_v2, bind parameters to the statement using sqlite3_bind_int, execute the statement using sqlite3_step, finalize the statement using sqlite3_finalize, and close the database connection using sqlite3_close, but they cannot access the details of the sqlite3 structure directly.

Must Check: C++ Online Courses and Certifications

Endnotes

Opaque pointers are useful in a variety of programming contexts, including object-oriented programming, libraries and frameworks, and database access. They provide a powerful mechanism for achieving encapsulation, abstraction, and information hiding and can help to reduce code complexity and improve security. By using opaque pointers, C++ developers can control the level of access that the user has to a data structure and modify the implementation of the data structure without affecting the user code.
Explore our C++ articles to find out more about the language and consolidate your knowledge of the fundamentals.

Contributed By: Prerna Singh

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