Prefer `Expects()` for expressing preconditions

Reason

To make it clear that the condition is a precondition and to enable tool use.

Example

int area(int height, int width)
{
    Expects(height > 0 && width > 0);            // good
    if (height <= 0 || width <= 0) my_error();   // obscure
    // ...
}

Note

Preconditions can be stated in many ways, including comments, if-statements, and assert(). This can make them hard to distinguish from ordinary code, hard to update, hard to manipulate by tools, and might have the wrong semantics (do you always want to abort in debug mode and check nothing in productions runs?).

Note

Preconditions should be part of the interface rather than part of the implementation, but we don't yet have the language facilities to do that. Once language support becomes available (e.g., see the contract proposal) we will adopt the standard version of preconditions, postconditions, and assertions.

Note

Expects() can also be used to check a condition in the middle of an algorithm.

Note

No, using unsigned is not a good way to sidestep the problem of ensuring that a value is non-negative.

Enforcement

(Not enforceable) Finding the variety of ways preconditions can be asserted is not feasible. Warning about those that can be easily identified (assert()) has questionable value in the absence of a language facility.