computer programming Computer programming or coding is the composition of sequences of instructions, called computer program, programs, that computers can follow to perform tasks. It involves designing and implementing algorithms, step-by-step specifications of proc ...

, a postcondition is a condition or

predicate Predicate or predication may refer to: * Predicate (grammar), in linguistics * Predication (philosophy) * several closely related uses in mathematics and formal logic: **Predicate (mathematical logic) **Propositional function **Finitary relation, o ...

that must always be true just after the execution of some section of code or after an operation in a

formal specification In computer science, formal specifications are mathematically based techniques whose purpose is to help with the implementation of systems and software. They are used to describe a system, to analyze its behavior, and to aid in its design by verify ...

. 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 (mathematics), 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 ...

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 y^2 = x; in other words, a number whose ''square'' (the result of multiplying the number by itself, or y \cdot y) is . For example, 4 and −4 are square roots of 16 because 4 ...

of an input number might have the postconditions that the result be a number and that its

square In geometry, a square is a regular polygon, regular quadrilateral. It has four straight sides of equal length and four equal angles. Squares are special cases of rectangles, which have four equal angles, and of rhombuses, which have four equal si ...

be equal to the input.

Postconditions in object-oriented programming

In some software design approaches, postconditions, along with

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. If a precondition is violated, the effect of the ...

s and class invariants, are components of the software construction method

design by contract Design by contract (DbC), also known as contract programming, programming by contract and design-by-contract programming, is an approach for designing software. It prescribes that software designers should define formal, precise and verifiable ...

. 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

holds, the properties declared by the postcondition are assured.

Eiffel example

The following example written in

Eiffel Eiffel may refer to: Places * Eiffel Tower, in Paris, France, designed by Gustave Eiffel *:* Champ de Mars – Tour Eiffel station, Metro station serving the Eiffel Tower * Eiffel Bridge, Ungheni, Moldova, designed by Gustave Eiffel * Eiffel Bri ...

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. Offi ...

, 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.

References

{{reflist Programming constructs Formal methods Logic in computer science Mathematics of computing Articles with example Eiffel code

Postconditions in object-oriented programming

Eiffel example

Postconditions and inheritance

See also

References