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...
, a postcondition is a condition or predicate that must always be true just after the execution of some section of code or after an operation in a formal specification
Formal specification
In computer science, a formal specification is a mathematical description of software or hardware that may be used to develop an implementation. It describes what the system should do, not how the system should do it...
. Postconditions are sometimes tested using assertions
Assertion (computing)
In computer programming, an assertion is a predicate placed in a program to indicate that the developer thinks that the predicate is always true at that place.For example, the following code contains two assertions:...
within the code itself. Often, postconditions are simply included in the documentation of the affected section of code.
In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n...
is always an integer and greater than or equal to 1. So a program that calculates the factorial of an input number would have postconditions that the result after the calculation be an integer and that it be greater than or equal to 1. Another example: a program that calculates the square root
Square root
In mathematics, a square root of a number x is a number r such that r2 = x, or, in other words, a number r whose square is x...
of an input number might have the postconditions that the result be a number and that its square be equal to the input .
Postconditions in object-oriented programming
In object-oriented programming, postconditions, along with preconditions
Precondition
In computer programming, a precondition is a condition or predicate that must always be true just prior to the execution of some section of code or before an operation in a formal specification....
In computer science, a predicate is called an invariant to a sequence of operations provided that: if the predicate is true before starting the sequence, then it is true at the end of the sequence.-Use:...
Design by contract , also known as programming by contract and design-by-contract programming, is an approach to designing computer software...
.
The postcondition for any routine is a declaration of the properties which are guaranteed upon completion of the routine's execution. As it relates to the routine's contract, the postcondition offers assurance to potential callers that in cases in which the routine is called in a state in which its precondition
Precondition
In computer programming, a precondition is a condition or predicate that must always be true just prior to the execution of some section of code or before an operation in a formal specification....
holds, the properties declared by the postcondition are assured.
Eiffel is an ISO-standardized, object-oriented programming language designed by Bertrand Meyer and Eiffel Software. The design of the language is closely connected with the Eiffel programming method...
example
The following example written in Eiffel sets the value of a class attribute hour based on a caller-provided argument a_hour. The postcondition follows the keyword ensure. In this example the postcondition guarantees, in cases in which the precondition holds (i.e., when a_hour represents a valid hour of the day), that after the execution of set_hour, the class attribute hour will have the same value as a_hour. The tag "hour_set:" describes this postcondition clause and serves to identify it in case of a runtime postcondition violation.
set_hour (a_hour: INTEGER)
-- Set `hour' to `a_hour'
require
valid_argument: 0 <= a_hour and a_hour <= 23
do
hour := a_hour
ensure
hour_set: hour = a_hour
end
Postconditions and inheritance
In the presence of inheritance, the routines inherited by descendant classes (subclasses) do so with their contracts, that is their preconditions and postconditions, in force. This means that any implementations or redefinitions of inherited routines also have to be written to comply with their inherited contract. Postconditions can be modified in redefined routines, but they may only be strengthened. That is, the redefined routine may increase the benefits it provides to the client, but may not decrease those benefits.
In computer programming, a precondition is a condition or predicate that must always be true just prior to the execution of some section of code or before an operation in a formal specification....
Hoare logic is a formal system with a set of logical rules for reasoning rigorously about the correctness of computer programs. It was proposed in 1969 by the British computer scientist and logician C. A. R. Hoare, and subsequently refined by Hoare and other researchers...
In computer science, a predicate is called an invariant to a sequence of operations provided that: if the predicate is true before starting the sequence, then it is true at the end of the sequence.-Use:...
A database trigger is procedural code that is automatically executed in response to certain events on a particular table or view in a database. The trigger is mostly used for keeping the integrity of the information on the database...
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