Contract Programming

Contracts are a breakthrough technique to reduce the programming effort for large projects. Contracts are the concept of preconditions, postconditions, errors, and invariants. Contracts can be done in C++ without modification to the language, but the result is clumsy and inconsistent.

Building contract support into the language makes for:

a consistent look and feel for the contracts
tool support
it's possible the compiler can generate better code using information gathered from the contracts
easier management and enforcement of contracts
handling of contract inheritance

The idea of a contract is simple - it's just an expression that must evaluate to true. If it does not, the contract is broken, and by definition, the program has a bug in it. Contracts form part of the specification for a program, moving it from the documentation to the code itself. And as every programmer knows, documentation tends to be incomplete, out of date, wrong, or non-existent. Moving the contracts into the code makes them verifiable against the program.

Assert Contract

The most basic contract is the assert. An assert inserts a checkable expression into the code, and that expression must evaluate to true:

assert(expression);

C programmers will find it familiar. Unlike C, however, an assert in function bodies works by throwing an AssertError, which can be caught and handled. Catching the contract violation is useful when the code must deal with errant uses by other code, when it must be failure proof, and as a useful tool for debugging.

Pre and Post Contracts

The pre contracts specify the preconditions before a statement is executed. The most typical use of this would be in validating the parameters to a function. The post contracts validate the result of the statement. The most typical use of this would be in validating the return value of a function and of any side effects it has. The syntax is:

in
{
    ...contract preconditions...
}
out (result)
{
    ...contract postconditions...
}
body
{
    ...code...
}

By definition, if a pre contract fails, then the body received bad parameters. An AssertError is thrown. If a post contract fails, then there is a bug in the body. An AssertError is thrown.

Either the in or the out clause can be omitted. If the out clause is for a function body, the variable result is declared and assigned the return value of the function. For example, let's implement a square root function:

long square_root(long x)
    in
    {
	assert(x >= 0);
    }
    out (result)
    {
	assert((result * result) <= x && (result+1) * (result+1) >= x);
    }
    body
    {
	return cast(long)std.math.sqrt(cast(real)x);
    }

The assert's in the in and out bodies are called contracts. Any other D statement or expression is allowed in the bodies, but it is important to ensure that the code has no side effects, and that the release version of the code will not depend on any effects of the code. For a release build of the code, the in and out code is not inserted.

If the function returns a void, there is no result, and so there can be no result declaration in the out clause. In that case, use:

void func()
   out
   {
	...contracts...
   }
   body
   {
	...
   }

In an out statement, result is initialized and set to the return value of the function.

In, Out and Inheritance

If a function in a derived class overrides a function in its super class, then only one of the in contracts of the function and its base functions must be satisfied. Overriding functions then becomes a process of loosening the in contracts.

A function without an in contract means that any values of the function parameters are allowed. This implies that if any function in an inheritance hierarchy has no in contract, then in contracts on functions overriding it have no useful effect.

Conversely, all of the out contracts needs to be satisfied, so overriding functions becomes a processes of tightening the out contracts.

Class Invariants

Class invariants are used to specify characteristics of a class that always must be true (except while executing a member function). They are described in Classes.

References