Sunday, May 6, 2012

Error Handling in STL

The design goal of the STL was the best performance rather than the most security. Error checking wastes time, so almost none is done. This is fine if you can program without making any errors, but it can be a catastrophe if you can't. Before the STL was adopted into the C++ standard library, discussions were held regarding whether to introduce more error checking. The majority
decided not to, for two reasons:
  1. Error checking reduces performance, and speed is still a general goal of programs. As mentioned, good performance was one of the design goals of the STL.
  2. If you prefer safety over speed, you can still get it, either by adding wrappers or by using special versions of the STL. But you can't program to avoid error checking to get better performance when error checking is built into all basic operations. For example, when every subscript operation checks whether a range is valid, you can't write your own subscripts without checking. However, it is possible the other way around.

As a consequence, error checking is possible but not required inside the STL. The C++ standard library states that any use of the STL that violates preconditions results in undefined behavior. Thus, if indexes, iterators, or ranges are not valid, the result is undefined. If you do not use a safe version of the STL, undefined memory access typically results, which causes some nasty side effects or even a crash. In this sense, the STL is as error prone as pointers are in C.

Finding such errors could be very hard, especially without a safe version of the STL.

In particular, the use of the STL requires that the following be met:
  • Iterators must be valid. For example, they must be initialized before they are used. Note that iterators may become invalid as a side effect of other operations. In particular, they become invalid for vectors and deques if elements are inserted or deleted, or reallocation takes place.
  • Iterators that refer to the past-the-end position have no element to which to refer. Thus, calling operator * or operator -> is not allowed. This is especially true for the return values of the end() and rend() container member functions. 
  • Ranges must be valid:
    • Both iterators that specify a range must refer to the same container.
    • The second iterator must be reachable from the first iterator.
  • If more than one source range is used, the second and later ranges must have at least as many elements as the first one.
  • Destination ranges must have enough elements that can be overwritten; otherwise, insert iterators must be used.

The following example shows some possible errors:

Note that these errors occur at runtime, not at compile time, and thus they cause undefined behavior.

There are many ways to make mistakes when using the STL, and the STL is not required to protect you from yourself. Thus, it is a good idea to use a "safe" STL, at least during software development.

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