处理超出范围的指数

Question

For a container class, what should the expected behavior be when the user of the class tries to set a value that's outside the class's size?

The method in question is a "replace" method (like operator[] ), rather than an "add" method (like operator+= ).

I could throw an out_of_range exception or I could just resize the container to accommodate the addition.

Answer 1

超出范围的异常在这里更合适，因为“替换”语义通常意味着调用者假定/断言在指定的索引处存在数据，而没有。

Answer 2

For std::vector::operator[] , a domain error causes undefined behavior. For std::map::operator[] , a domain error creates a new entry. I guess the design questions are:

• How expensive is it to extend the container? That is, can you create just one item (like map::operator[] ), or do you have to create all of the intervening items (like vector::operator[] )

• How expensive is the range check, compared to the access function? In map , the range check is free once you've run the access function. In vector the range check approximately doubles the cost off the access.

Since you are designing the container, feel free to impose whichever pattern suits you and your customers. Just be sure to document the behavior.

Answer 3

In general, I prefer throwing out-of-range exception. If the users really want to expand the storage, offer them a method to do so. This will eliminate many unintentional expansions, which could lead to more serious bugs. This is just my opinion.

Answer 4

One point not yet mentioned is that it may sometimes be useful to provide for a special one-over case to extend the class size. The idea is that a class may have an invariant that items[0] to item[length-1] have all been validly assigned. If one attempts to write to item[K], with K>=length+1, it may be necessary to create new items [length..K-1] with unknown values, breaking the class invariant. On the other hand, if one writes to item[length], one could increment length by one and maintain the class invariant.