In
computer programming
Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as anal ...
, 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. Postconditions are sometimes tested using
assertions within the code itself. Often, postconditions are simply included in the documentation of the affected section of code.
For example: The result of a
factorial
In mathematics, the factorial of a non-negative denoted is the product of all positive integers less than or equal The factorial also equals the product of n with the next smaller factorial:
\begin
n! &= n \times (n-1) \times (n-2) ...
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
In mathematics, a square root of a number is a number such that ; in other words, a number whose ''square'' (the result of multiplying the number by itself, or ⋅ ) is . For example, 4 and −4 are square roots of 16, because .
...
of an input number might have the postconditions that the result be a number and that its
square
In Euclidean geometry, a square is a regular quadrilateral, which means that it has four equal sides and four equal angles (90-degree angles, π/2 radian angles, or right angles). It can also be defined as a rectangle with two equal-length a ...
be equal to the input.
Postconditions in object-oriented programming
In some software design approaches, postconditions, along with
preconditions and class
invariants, are components of the software construction method
design by contract.
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 holds, the properties declared by the postcondition are assured.
Eiffel example
The following example written in
Eiffel
Eiffel may refer to:
Places
* Eiffel Peak, a summit in Alberta, Canada
* Champ de Mars – Tour Eiffel station, Paris, France; a transit station
Structures
* Eiffel Tower, in Paris, France, designed by Gustave Eiffel
* Eiffel Bridge, Ungheni, ...
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
Inheritance is the practice of receiving private property, titles, debts, entitlements, privileges, rights, and obligations upon the death of an individual. The rules of inheritance differ among societies and have changed over time. Offici ...
, 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 contracts. Postconditions can be modified in redefined routines, but they may only be strengthened.
[Meyer, 1997, pp. 570–573.] That is, the redefined routine may increase the benefits it provides to the client, but may not decrease those benefits.
See also
*
Precondition
*
Design by contract
*
Hoare logic
Hoare logic (also known as Floyd–Hoare logic or Hoare rules) 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 lo ...
*
Invariants maintained by conditions
*
Database trigger
References
{{reflist
Programming constructs
Formal methods
Logic in computer science
Mathematics of computing
Articles with example Eiffel code