What is your first thought when hear "pointer"? You must be thinking about a small arrow which always moving around when you move your mouse, isn't it? :D
No no, what i mean about "pointer" here is, in computer science, a pointer is a programming language object, whose value refers directly to (or "points to") another value stored elsewhere in the computer memory using its address. For high-level programming languages, pointers effectively take the place of general purpose registers in low-level languages such as assembly language or machine code, but may be in available memory. A pointer references a location in memory, and obtaining the value stored at that location is known as dereferencing the pointer. A pointer is a simple, more concrete implementation of the more abstract reference data type. Several languages support some type of pointer, although some have more restrictions on their use than others. As an analogy, a page number in a book's index could be considered a pointer to the corresponding page; dereferencing such a pointer would be done by flipping to the page with the given page number.
Pointers to data significantly improve performance for repetitive operations such as traversing strings, lookup tables, control tables and tree structures. In particular, it is often much cheaper in time and space to copy and dereference pointers than it is to copy and access the data to which the pointers point.
Pointers are also used to hold the addresses of entry points for called subroutines in procedural programming and for run-time linking to dynamic link libraries (DLLs). In object-oriented programming, pointers to functions are used for binding methods, often using what are called virtual method tables.
While "pointer" has been used to refer to references in general, it more properly applies to data structures whose interface explicitly allows the pointer to be manipulated (arithmetically via pointer arithmetic) as a memory address, as opposed to a magic cookie or capability where this is not possible. Because pointers allow both protected and unprotected access to memory addresses, there are risks associated with using them particularly in the latter case. Primitive pointers are often stored in a format similar to an integer; however, attempting to dereference or "look up" a pointer whose value was never a valid memory address would cause a program to crash. To alleviate this potential problem, as a matter of type safety, pointers are considered a separate type parameterized by the type of data they point to, even if the underlying representation is an integer. Other measures may also be taken (such as validation & bounds checking, to verify the contents of the pointer variable contain a value that is both a valid memory address and within the numerical range that the processor is capable of addressing).
Formal Description
A pointer is a kind of reference.
A data primitive (or just primitive) is any datum that can be read from or written to computer memory using one memory access (for instance, both a byte and a word are primitives).
A data aggregate (or just aggregate) is a group of primitives that are logically contiguous in memory and that are viewed collectively as one datum (for instance, an aggregate could be 3 logically contiguous bytes, the values of which represent the 3 coordinates of a point in space). When an aggregate is entirely composed of the same type of primitive, the aggregate may be called an array; in a sense, a multi-byte word primitive is an array of bytes, and some programs use words in this way.
In the context of these definitions, a byte is the smallest primitive; each memory address specifies a different byte. The memory address of the initial byte of a datum is considered the memory address (or base memory address) of the entire datum.
A memory pointer (or just pointer) is a primitive, the value of which is intended to be used as a memory address; it is said that a pointer points to a memory address. It is also said that a pointer points to a datum when the pointer's value is the datum's memory address.
More generally, a pointer is a kind of reference, and it is said that a pointer references a datum stored somewhere in memory; to obtain that datum is to dereference the pointer. The feature that separates pointers from other kinds of reference is that a pointer's value is meant to be interpreted as a memory address, which is a rather low-level concept.
References serve as a level of indirection: A pointer's value determines which memory address (that is, which datum) is to be used in a calculation. Because indirection is a fundamental aspect of algorithms, pointers are often expressed as a fundamental data type in programming languages; in statically (or strongly) typed programming languages, the type of a pointer determines the type of the datum to which the pointer points.
Uses
Pointers are directly supported without restrictions in languages such as PL/I, C, C++, Pascal, and most assembly languages. They are primarily used for constructing references, which in turn are fundamental to constructing nearly all data structures, as well as in passing data between different parts of a program.
In functional programming languages that rely heavily on lists, pointers and references are managed abstractly by the language using internal constructs like cons.
When dealing with arrays, the critical lookup operation typically involves a stage called address calculation which involves constructing a pointer to the desired data element in the array. If the data elements in the array have lengths that are divisible by powers of two, this arithmetic is usually a bit more efficient. Padding is frequently used as a mechanism for ensuring this is the case, despite the increased memory requirement. In other data structures, such as linked lists, pointers are used as references to explicitly tie one piece of the structure to another.
Pointers are used to pass parameters by reference. This is useful if the programmer wants a function's modifications to a parameter to be visible to the function's caller. This is also useful for returning multiple values from a function.
Pointers can also be used to allocate and deallocate dynamic variables and arrays in memory. Since a variable will often become redundant after it has served its purpose, it is a waste of memory to keep it, and therefore it is good practice to deallocate it (using the original pointer reference) when it is no longer needed. Failure to do so may result in a memory leak (where available free memory gradually, or in severe cases rapidly, diminishes because of an accumulation of numerous redundant memory blocks).
Function Pointer
In some languages, a pointer can reference executable code, i.e., it can point to a function, method, or procedure. A function pointer will store the address of a function to be invoked. While this facility can be used to call functions dynamically, it is often a favorite technique of virus and other malicious software writers.
int a, b, x, y;
int sum(int n1, int n2); // Function with two integer parameters returning an integer value
int (*fp)(int, int); // Function pointer which can point to a function like sum
fp = ∑ // fp now points to function sum
x = (*fp)(a, b); // Calls function sum with arguments a and b
y = sum(a, b); // Calls function sum with arguments a and b
Pointers in Java
Unlike C, C++, or Pascal, there is no explicit representation of pointers in Java. Instead, more complex data structures like objects and arrays are implemented using references. The language does not provide any explicit pointer manipulation operators. It is still possible for code to attempt to dereference a null reference (null pointer), however, which results in a run-time exception being thrown. The space occupied by unreferenced memory objects is recovered automatically by garbage collection at run-time.
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