Indirection
Encyclopedia
In computer programming
, indirection is the ability to reference something using a name, reference, or container instead of the value itself. The most common form of indirection is the act of manipulating a value through its memory address
. For example, accessing a variable
through the use of a pointer. A stored pointer that exists to provide a reference to an object by double indirection is called an indirection node. In some older computer architectures, indirect words supported a variety of more-or-less complicated addressing mode
s.
A famous aphorism
of David Wheeler goes: "All problems in computer science can be solved by another level of indirection";
this is often deliberately mis-quoted with "abstraction layer
" substituted for "level of indirection". Kevlin Henney
's corollary
to this is, "...except for the problem of too many layers of indirection."
Object-oriented programming
makes use of indirection extensively, a simple example being dynamic dispatch
. Higher-level examples of indirection are the design pattern
s of the proxy
and the proxy server
. Delegation
is another classic example of an indirection pattern. In strongly typed interpreted language
s with dynamic datatypes, most variable references require a level of indirection: first the type of the variable is checked for safety, and then the pointer to the actual value is dereferenced and acted on.
Recursive data types are usually implemented using indirection because otherwise if a value of a datatype can contain the entirety of another value of the same datatype, there is no limit to the size a value of this datatype could need.
When doing symbolic programming from a formal mathematical specification the use of indirection can be quite helpful. To start with a simple example the variables x, y and z in an equation such as z = square root (x squared + y squared) can refer to any number. One could imagine objects for various numbers and then x, y and z could point to the specific numbers being used for a particular problem. The simple example has its limitation as there are infinitely many real numbers. In various other parts of symbolic programming there are only so many symbols. So to move on to a more significant example, in logic the formula α can refer to any formula, so it could be β, γ, δ, ... or η→π, ς v σ,.... When set-builder notation
is employed the statement Δ={α} means the set of all formulae - so although the reference is to α there are two levels of indirection here, the first to the set of all α and then the second to a specific formula for each occurrence of α in the set Δ.
Computer programming
Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
, indirection is the ability to reference something using a name, reference, or container instead of the value itself. The most common form of indirection is the act of manipulating a value through its memory address
Memory address
A digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...
. For example, accessing a variable
Variable (programming)
In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...
through the use of a pointer. A stored pointer that exists to provide a reference to an object by double indirection is called an indirection node. In some older computer architectures, indirect words supported a variety of more-or-less complicated addressing mode
Addressing mode
Addressing modes are an aspect of the instruction set architecture in most central processing unit designs. The various addressing modes that are defined in a given instruction set architecture define how machine language instructions in that architecture identify the operand of each instruction...
s.
A famous aphorism
Aphorism
An aphorism is an original thought, spoken or written in a laconic and memorable form.The term was first used in the Aphorisms of Hippocrates...
of David Wheeler goes: "All problems in computer science can be solved by another level of indirection";
this is often deliberately mis-quoted with "abstraction layer
Abstraction layer
An abstraction layer is a way of hiding the implementation details of a particular set of functionality...
" substituted for "level of indirection". Kevlin Henney
Kevlin Henney
Kevlin Henney is an author who writes on the subject of computer programming in C and C++ for magazines such as the C/C++ Users Journal, Application Development Advisor, JavaSpektrum, C++ Report, Java Report, EXE, and Overload....
's corollary
Corollary
A corollary is a statement that follows readily from a previous statement.In mathematics a corollary typically follows a theorem. The use of the term corollary, rather than proposition or theorem, is intrinsically subjective...
to this is, "...except for the problem of too many layers of indirection."
Object-oriented programming
Object-oriented programming
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
makes use of indirection extensively, a simple example being dynamic dispatch
Dynamic dispatch
In computer science, dynamic dispatch is the process of mapping a message to a specific sequence of code at runtime. This is done to support the cases where the appropriate method can't be determined at compile-time...
. Higher-level examples of indirection are the design pattern
Design pattern (computer science)
In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that...
s of the proxy
Proxy pattern
In computer programming, the proxy pattern is a software design pattern.A proxy, in its most general form, is a class functioning as an interface to something else...
and the proxy server
Proxy server
In computer networks, a proxy server is a server that acts as an intermediary for requests from clients seeking resources from other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource available from a different server...
. Delegation
Delegation (programming)
In object-oriented programming, there are two related notions of delegation.* Most commonly, it refers to a programming language feature making use of the method lookup rules for dispatching so-called self-calls as defined by Lieberman in his 1986 paper "Using Prototypical Objects to Implement...
is another classic example of an indirection pattern. In strongly typed interpreted language
Interpreted language
Interpreted language is a programming language in which programs are 'indirectly' executed by an interpreter program. This can be contrasted with a compiled language which is converted into machine code and then 'directly' executed by the host CPU...
s with dynamic datatypes, most variable references require a level of indirection: first the type of the variable is checked for safety, and then the pointer to the actual value is dereferenced and acted on.
Recursive data types are usually implemented using indirection because otherwise if a value of a datatype can contain the entirety of another value of the same datatype, there is no limit to the size a value of this datatype could need.
When doing symbolic programming from a formal mathematical specification the use of indirection can be quite helpful. To start with a simple example the variables x, y and z in an equation such as z = square root (x squared + y squared) can refer to any number. One could imagine objects for various numbers and then x, y and z could point to the specific numbers being used for a particular problem. The simple example has its limitation as there are infinitely many real numbers. In various other parts of symbolic programming there are only so many symbols. So to move on to a more significant example, in logic the formula α can refer to any formula, so it could be β, γ, δ, ... or η→π, ς v σ,.... When set-builder notation
Set-builder notation
In set theory and its applications to logic, mathematics, and computer science, set-builder notation is a mathematical notation for describing a set by stating the properties that its members must satisfy...
is employed the statement Δ={α} means the set of all formulae - so although the reference is to α there are two levels of indirection here, the first to the set of all α and then the second to a specific formula for each occurrence of α in the set Δ.
See also
- HandleHandle (computing)In computer programming, a handle is a particular kind of smart pointer. Handles are used when application software references blocks of memory or objects managed by another system, such as a database or an operating system....
- Delegation patternDelegation patternIn software engineering, the delegation pattern is a design pattern in object-oriented programming where an object, instead of performing one of its stated tasks, delegates that task to an associated helper object. There is an Inversion of Responsibility in which a helper object, known as a...
- Pointer
- ReferenceReference (computer science)In computer science, a reference is a value that enables a program to indirectly access a particular data item, such as a variable or a record, in the computer's memory or in some other storage device. The reference is said to refer to the data item, and accessing those data is called...
- Fundamental theorem of software engineeringFundamental theorem of software engineeringThe fundamental theorem of software engineering is a term originated by Andrew Koenig to describe a remark by Butler Lampson:The term is facetious, in the sense that it doesn't describe an actual theorem that can be proved, and the problems it solves are not fundamental. Rather, it's a principle...