CGAL 4.13 - Manual
Checks: Pre- and Postconditions, Assertions, and Warnings

Much of the CGAL code contains checks. Some are there to check if the code behaves correctly, others check if the user calls routines in an acceptable manner. We describe the different categories of checks (Section Categories of checks), the usage of checks (Section Using checks), and a more selective means of controlling checks (Section Controlling checks at a finer granularity). Finally, a statement about exception handling is given (Section Exception handling).

It is forbidden to call std::abort, std::exit or assert directly from CGAL, as these do not allow the user code to react after the error (application processes are killed). Thus, the default behavior of all checks is to throw exceptions for reporting failures.

# Categories of checks

There are five types of checks.

• Preconditions check if a routine has been called in a proper fashion and the input adheres to the specifications given by the author of the function. If a precondition fails, it is the responsibility of the caller (usually the user of the library) to fix the problem.
• Postconditions check if a routine does what it promises to do. If a postcondition fails it is the fault of this routine, so the author of the code is responsible.
• Assertions are other checks that do not fit in the above two categories, e.g. they can be used to check invariants.
• Warnings are checks for which it is not so severe if they fail.
• Static assertions are compile-time assertions, used e.g. to verify the values of compile-time constants or compare types for (in)equality.
• Destructor assertions These are checks which can be made within normal clean up of an object. A special macro CGAL_destructor_assertion is provided to ensure these checks are not made when the object is being cleaned up during exception handling.

The according macro names all have the format CGAL_<check_type> where <check_type> can be one of

• precondition
• postcondition
• assertion
• warning
• static_assertion
• destructor_assertion

Failures of the first three types are errors and lead to a halt of the program, failures of the last one only lead to a warning. Checks of four categories can be marked with one or both of the following attributes:

• Expensive

checks take considerable time to compute. "Considerable" is an imprecise phrase. Checks that add less than 10 percent to the execution time of their routine are not expensive. Checks that can double the execution time are. Somewhere in between lies the border line.

• Exactness

checks rely on exact arithmetic. For example, if the intersection of two lines is computed, the postcondition of this routine may state that the intersection point lies on both lines. However, if the computation is done with doubles as the number type, this may not be the case, due to roundoff errors.

By definition, static assertions are both inexpensive and unaffected by precision management. Thus, the categories do not apply for static assertions.

The format is one of

• CGAL_<check_type>
• CGAL_expensive_<check_type>
• CGAL_exactness_<check_type>
• CGAL_expensive_exactness_<check_type>

By default, all standard checks (without any attribute) are enabled, while expensive and exactness checks are disabled. How this can be changed and how checks are actually used in the code are described in the next section.

Additionally, we provide macros CGAL_error() and CGAL_error_msg(MSG_TEXT) which are equivalent to always-failing assertions. However, they cannot be disabled.

# Using checks

The checks are implemented as preprocessor macros;

i.e., CGAL_<check_type>(<Cond>) realizes a check of type <check_type> that asserts the condition <Cond>. For example,

CGAL_precondition( first != last);

checks the precondition that a given iterator range is not empty. If the check fails, an error message similar to

CGAL error: precondition violation!
Expr: first != last
File: <file name>
Line: <source code line>

is written to the standard error stream and the program is aborted. If an additional explanation should be given to the user, macros CGAL_<check_type>_msg(<Cond>,<Msg>) can be used. The text in <Msg> is just appended to the failure message given above.

In case a check is more complicated and the computation does not fit into a single statement, the additional code can be encapsulated using CGAL_<check_type>_code(<Code>). This has the advantage that the computation is not done if the corresponding category is disabled. For an example, suppose an algorithm computes a convex polygon. Thus we want to check the postcondition that the polygon is indeed convex, which we consider an expensive check. The code would look like this.

CGAL_expensive_postcondition_code( bool is_convex; )
CGAL_expensive_postcondition_code( /* compute convexity */ )
CGAL_expensive_postcondition_code( /* ... */ )
CGAL_expensive_postcondition_msg ( is_convex, \
"The computed polygon is NOT convex!" );

As already mentioned above, the standard checks are enabled by default. This can be changed through the use of compile-time flags.

By setting the flag CGAL_NO_<CHECK_TYPE> all checks of type <CHECK_TYPE> are disabled, e.g. adding -DCGAL_NO_ASSERTIONS to the compiler call switches off all checks for static and dynamic assertions. To disable all checks in the library, the flag CGAL_NDEBUG can be set. Note that the standard flag NDEBUG sets CGAL_NDEBUG, but it also affects the assert macro.

To enable expensive and exactness checks, respectively, the compile-time flags CGAL_CHECK_EXPENSIVE

and CGAL_CHECK_EXACTNESS

have to be supplied. However, exactness checks should only be turned on if the computation is done with some exact number type.

# Controlling checks at a finer granularity

The macros and related compile-time flags described so far all operate on the whole library. Sometimes the user may want to have a more selective control. CGAL offers the possibility to turn checks on and off on a per-package basis. Therefore a package-specific term is inserted in the macro names directly after the CGAL prefix, e.g., CGAL_kernel_assertion(<Cond>). Similarly, the uppercase term is used for the compile-time flags; e.g., CGAL_KERNEL_NO_WARNINGS switches off the warnings in only the kernel. Other packages have their own specific terms as documented in the corresponding chapters of the reference manual.

For a new package you will first have to create a suitable header file with all macro definitions. This is done with the shell script cgal_create_assertions.sh, to be found in the in the scripts directory.

The following command will create a file optimisation_assertions.h:

sh cgal_create_assertions.sh optimisation

You should place the generated file in the proper directory (and possibly rename it). Then you can use the checks in the following fashion.

#include <CGAL/optimisation\_assertions.h>
void optimisation\_foo( int i)
{
CGAL_optimisation_precondition_msg( i == 42, "Only 42 allowed!");
// ...
}

The documentation of your new package has to name the term chosen to be part of the package-specific macros in order to enable the user to selectively turn off and on the checks of your package. For example, in the documentation of the optimisation package you can find a sentence similar to the following.

The optimisation code uses the term OPTIMISATION for the checks; e.g., setting the compile time flag CGAL_OPTIMISATION_NO_PRECONDITIONS switches off precondition checking in the optimisation code.

# Suppress warnings using CGAL_assume.

Moderns compilers, when their optimizers are activated, sometimes emit warnings about events that may occur. For example:

warning: array subscript is above array bounds [-Warray-bounds]


or:

warning: 'res' may be used uninitialized in this function [-Wmaybe-uninitialized]


Most false positives could be removed if the compiler knew that an integer variable is within given bounds, or that a Boolean variable is true. If CGAL_NDEBUG is not defined, then a CGAL_assertion is enough to instruct the compiler that a given condition is fulfilled. But, usually when the compiler optimizers are activated, CGAL_NDEBUG is also defined, to speed up the generated binary code. In that case, CGAL_assume can be used.

CGAL_assume is identical to CGAL_assertion when CGAL_NDEBUG is not defined. But, even if CGAL_NDEBUG is defined, its semantic uses compiler-specific instructions, such as __assume from MSVC, or __builtin_unreachable recognized by both clang and g++.

# Exception handling

Some parts of the library use exceptions, but there is no general specific policy concerning exception handling in CGAL. It is nevertheless good to target exception safety, as much as possible. Good references on exception safety are: Appendix E of [9] (also available at http://www.research.att.com/~bs/3rd_safe0.html), and [1] (also available at http://www.boost.org/more/generic_exception_safety.html). Any destructor which might throw an exception, including a destructor which uses the CGAL_destructor_assertion macro, should be marked with the CGAL_NOEXCEPT(false) macro. This macro provides future compatibility with C++11 and above, which provides the noexcept keyword.

# Requirements and recommendations

Requirements:

• Write pre- and postcondition checkers for your functions wherever possible.
• Use the CGAL preprocessor macros (Sections Using checks and Controlling checks at a finer granularity ) exclusively throughout your code (instead of, for example, the assert macro or the std::abort or std::exit functions) for all checks to assure that all CGAL invariant tests can be handled in a uniform way.