CTEC1335/2008F

Programming by Contract (PBC) - Introduction
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[also known as "Design by Contract" (DBC)]


- a way to design programs

- two functions enter into a "contract", where each has rights and 
  responsibilities
  
- the function doing the calling, the "caller", has the responsibility
  of passing valid data (arguments/parameters) to the function it calls,
  the "callee"
  
- the caller must check these "preconditions" using if statements or
  assertions before calling the callee
  
- the callee has the right to expect valid data from the caller

- because of the contract, the callee does not have to check for valid
  inputs (arguments/parameters)
  
- the callee has the responsibility of performing a service for the caller
  as designed, and the caller has a right to expect that this service
  will be performed correctly
  
- the callee has the responsibility of validating its outputs (return
  values), using ifs or assertions before returning back to the caller
  
- the caller has the right to expect valid return values from the callee

- the caller is freed from having to check for valid return values from
  the callee (due to the contract)

Link for more information:
http://en.wikipedia.org/wiki/Design_by_contract