Safe STL

Safe STL

Cay S. Horstmann

Impatient? Download SAFESTL.ZIP by clicking here. Unzip and read SAFESTL.TXT.

1. Legalese

These materials build upon the Standard Template Library (STL) code which is

Copyright (c) 1994 Hewlett-Packard Company

That material is provided pursuant to the following permission notice.

Permission to use, copy, modify, distribute and sell this software and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation. Hewlett-Packard Company makes no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.

The Safe STL enhancements are

(C) Horstmann Software Design Corp. 1995. All Rights Reserved.

You may freely use, copy, modify, or distribute these Safe STL enhancements and this documentation for any purpose, provided (1) you charge NO FEE for distributing these copyrighted materials and (2) you include both the copyright notice and permission notice from Hewlett-Packard Company and this copyright notice and permission notice. Horstmann Software Design Corporation specifically disclaims that these materials might do anything useful at all. If they work for you and don't destroy anything, you are in luck. There is no free technical support available for these materials.

2. Background

STL, the Standard Template Library designed by Alexander Stepanov and Meng Lee, is slated to become a part of the ANSI/ISO C++ Standard. Reaction to STL has been mixed. Some programmers applaud its elegance and power, others find flaws with the interface, naming conventions, multithread support or safety. While STL may not be the perfect container class library, it is here to stay. I designed this small but useful enhancement to make STL safer to use. Safe STL catches many typical STL programming errors at runtime (and a few at compile time).

3. What it does

When you compile and link your code with Safe STL, iterators are instrumented to be checked whenever they are used. There are four kinds of checks.
  1. When an iterator is dereferenced, it must be valid (i.e. belong to a container and not be past the end).
  2. When an iterator is incremented, it must be valid before the increment and valid or past the end after the increment. When an iterator is decremented, it must be valid or past the end before the decrement and valid after the decrement.
  3. To compute the difference of two iterators or to compare two iterators with <, <=, >, >=, the iterators must point to the same container.
  4. When a container is accessed through an iterator (like c.erase(i)), then the iterator must actually point inside the container.
Furthermore, container operations (such as assignment, insertion, removal, etc.) update all iterators attached to a container.

If an error is detected, then an assertion failure is raised. (See section 6 to see how to change that behavior.)

Of course, checking and updating the iterators takes time. For release versions of your code, you would probably want to go back to the unchecked version of STL.

The interface of Safe STL is purposefully left the same as that of regular STL. It would be an easy matter to add public member functions to the iterators that check their status, but I believe that it is best not to develop yet another nonstandard template library.

Safe STL works better than a debugger or BoundsChecker for a number of reasons. If your code dies because of a pointer error that is the consequence of an STL usage error, the debugger will break deep in the bowels of STL code which is usually less than illuminating. If your code gets in an infinite loop because of an STL usage error, BoundsChecker will not actually complain. Some STL usage errors cause subtle inconsistencies in the STL data structures that will not lead to immediate failure. See the examples in section 4 for more information.

4. Some typical errors that Safe STL catches

5. How to use it

Get the complete version of regular STL by ftp from Put all regular STL files in a directory, e.g. /stl. Download SAFESTL.ZIP by clicking here. Unzip and put the .h files
into a separate directory, e.g. /safestl. Then put that directory into the directory search path, BEFORE the directory that contains the regular STL. For example,
   bcc -I/safestl;/stl whatnot.cpp
Then compile and link your application.

Note: You must rebuild the ENTIRE application. You cannot have mixed object files that share STL containers and iterators, some compiled with the regular and others with the safe headers. I realize that may be impossible if some of your code is compiled into libraries whose interface references an STL container or iterator.

Once your application is debugged, simply remove /safestl from the include path and rebuild the application.

You will notice that Safe STL generates a large number of WARNINGS. Unfortunately, there is little I can do about that without modifying the original STL source extensively. Some warnings come from int/unsigned mismatches--they also occur in regular STL. Others have the form "functions with property X cannot be inline expanded". I don't want them inline expanded, but I have no choice--the Borland compiler does not support templates defining member functions of nested classes. If the Safe STL code becomes popular, I may rewrite it to eliminate most warnings, but deviating further from the regular STL code.

6. Changing the error handling

When an STL usage error is detected, an assertion failure is triggered, using the standard assert macro. It works, and it is portable.

Why didn't I throw an exception when detecting a failure? That would have changed the interface of the STL classes, which I didn't want to do. Had the operations thrown exceptions upon failure, then you might write code catching them. That code would no longer work when you switch back to regular STL.

However, there is one major disadvantage to using assert. The debugger will not give you a stack trace at the point of failure, so you can't see what part of your code caused the problem. (The file and line number reported by assert are in the Safe STL header.) If your debugger can break on exception throw (like Turbo Debugger), you can view the stack and locate the offending code. A brutal but effective way is to turn the assertion failure into an exception. Before including the Safe STL headers,

   #define assert(X) if (!(X)) throw #X;
After the Safe STL headers, undefine it again.

7. Supported platforms

I have tested most examples from the HP STL site that compile with regular STL (some require vendor-specific extensions), using both the Borland 4.53 and Symantec 7.2 compilers. If your compiler supports the HP STL distribution but chokes on Safe STL, please let me know (see section 8). If you made a Safe STL version of your compiler and would like to make it available to the public, I'd be happy to include it with this package, provided of course that your modifications are freely distributable.

8. Bug reports

Bug reports and suggestions for improvement are very welcome. Please direct them to

PLEASE, NO REQUESTS FOR HANDHOLDING. If you can't compile the sample programs of the HP ftp site with regular and safe STL, or you don't know how to set up the files and the compiler, then I cannot and will not try to fix it for you.

I am particular interested in

Before sending a bug report, please check if your bug has been fixed already. The most current version of this code is available on the World Wide Web: