Editing Precondition (section)

==In object-oriented programming==
Preconditions in [[Object-oriented programming|object-oriented]] software development are an essential part of [[design by contract]]. Design by contract also includes notions of [[postcondition]] and [[class invariant]]. 

The precondition for any routine defines any constraints on object state which are necessary for successful execution. From the program developer's viewpoint, this constitutes the routine caller's portion of the contract. The caller then is obliged to ensure that the precondition holds prior to calling the routine. The reward for the caller's effort is expressed in the called routine's [[postcondition]].<ref>[[Bertrand Meyer|Meyer, Bertrand]], ''[[Object-Oriented Software Construction]], second edition,'' [[Prentice Hall]], 1997, p. 342.</ref>

===Eiffel example===
The routine in the following example written in [[Eiffel (programming language)|Eiffel]] takes as an argument an integer which must be a valid value for an hour of the day, i. e., 0 through 23, inclusively. The precondition follows the keyword <code>require</code>. It specifies that the argument must be greater than or equal to zero and less than or equal to 23. The tag "<code>valid_argument:</code>" describes this precondition clause and serves to identify it in case of a runtime precondition violation.

<syntaxhighlight lang="eiffel">
    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
</syntaxhighlight>

===Preconditions and inheritance===
In the presence of inheritance, the routines inherited by descendant classes (subclasses) do so with their preconditions in force. This means that any implementations or redefinitions of inherited routines also have to be written to comply with their inherited contract. Preconditions can be modified in redefined routines, but they may only be weakened.<ref>Meyer, 1997, pp. 570–573.</ref> That is, the redefined routine may lessen the obligation of the client, but not increase it.