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Donald Knuth, Structured Programming, with go to Statements[1]

Here, p2 may point to anywhere in memory, so performing the assignment *p2 = 'b'; can corrupt an unknown area of memory or trigger a segmentation fault.

Back pointerIn doubly linked lists or tree structures, a back pointer held on an element 'points back' to the item referring to the current element. These are useful for navigation and manipulation, at the expense of greater memory use.

Wild branch

Where a pointer is used as the address of the entry point to a program or start of a function which doesn't return anything and is also either uninitialized or corrupted, if a call or jump is nevertheless made to this address, a "wild branch" is said to have occurred. The consequences are usually unpredictable and the error may present itself in several different ways depending upon whether or not the pointer is a "valid" address and whether or not there is (coincidentally) a valid instruction (opcode) at that address. The detection of a wild branch can present one of the most difficult and frustrating debugging exercises since much of the evidence may already have been destroyed beforehand or by execution of one or more inappropriate instructions at the branch location. If available, an instruction set simulator can usually not only detect a wild branch before it takes effect, but also provide a complete or partial trace of its history.

Simulation using an array index

Several old versions of BASIC for the Windows platform had support for STRPTR() to return the address of a string, and for VARPTR() to return the address of a variable. Visual Basic 5 also had support for OBJPTR() to return the address of an object interface, and for an ADDRESSOF operator to return the address of a function. The types of all of these are integers, b

Several old versions of BASIC for the Windows platform had support for STRPTR() to return the address of a string, and for VARPTR() to return the address of a variable. Visual Basic 5 also had support for OBJPTR() to return the address of an object interface, and for an ADDRESSOF operator to return the address of a function. The types of all of these are integers, but their values are equivalent to those held by pointer types.

Newer dialects of BASIC, such as FreeBASIC or BlitzMax, have exhaustive pointer implementations, however. In FreeBASIC, arithmetic on ANY point

Newer dialects of BASIC, such as FreeBASIC or BlitzMax, have exhaustive pointer implementations, however. In FreeBASIC, arithmetic on ANY pointers (equivalent to C's void*) are treated as though the ANY pointer was a byte width. ANY pointers cannot be dereferenced, as in C. Also, casting between ANY and any other type's pointers will not generate any warnings.

dim as integer f = 257
dim as any ptr g = @f
dim as integer ptr i = g
assert(*i = 257)
assert( (g + 4) = (@f + 1) )

C and C++

In C and C++ pointers are variables that store addresses and can be null. Each pointer has a type it points to, but one can freely cast between pointer types (but not between a function pointer and an object pointer). A special pointer type called the “void pointer” allows pointing to any (non-function) object, but is limited by the fact that it cannot be dereferenced directly (it shall be cast). The address itself can often be directly manipulated by casting a pointer to and from an integral type of sufficient size, though the results are implementation-defined and may indeed cause undefined behavior; while earlier C standards did not have an integral type that was guaranteed to be large enough, C99 specifies the uintptr_t typedef name defined in <stdint.h>, but an implementation need not provide it.

C++ fully supports C pointers and C

C++ fully supports C pointers and C typecasting. It also supports a new group of typecasting operators to help catch some unintended dangerous casts at compile-time. Since C++11, the C++ standard library also provides smart pointers (unique_ptr, shared_ptr and weak_ptr) which can be used in some situations as a safer alternative to primitive C pointers. C++ also supports another form of reference, quite different from a pointer, called simply a reference or reference type.

Pointer arithmetic, that is, the ability to modify a pointer's target address with arithmetic operations (as well as magnitude comparisons), is restricted by the language standard to remain within the bounds of a single array object (or just after it), and will otherwise invoke undefined behavior. Adding or subtracting from a pointer moves it by a multiple of the size of its datatype. For example, adding 1 to a pointer to 4-byte integer values will increment the pointer's pointed-to byte-address by 4. This has the effect of incrementing the pointer to point at the next element in a contiguous array of integers—which is often the intended result. Pointer arithmetic cannot be performed on void pointers because the void type has no size, and thus the pointed address can not be added to, although gcc and other compilers will perform byte arithmetic on void* as a non-standard extension, treating it as if it were char *.

Pointer arithmetic provides the programmer with a single way of dealing with different types: adding and subtracting the number of elements required instead of the actual offset in bytes. (Pointer arithmetic with char * pointers uses byte offsets, because sizeof(char) is 1 by definition.) In particular, the C definition explicitly declares that the syntax a[n], which is the n-th element of the array a, is equivalent to *(a + n), which is the content of the element pointed by a + n. This implies that n[a] is equivalent to a[n], and one can write, e.g., a[3] or 3[a] equally well to access the fourth element of an array a.

While powerful, pointer arithmetic can be a source of computer bugs. It tends to confuse novice programmers, forcing them into different contexts: an expression can be an ordinary arithmetic one or a pointer arithmetic one, and sometimes it is easy to mistake one for the other. In response to this, many modern high-level computer languages (for example Java) do not permit direct access to memory using addresses. Also, the safe C dialect Cyclone addresses many of the issues with pointers. See C programming language for more discussion.