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The braces are included in the comment in order to help distinguish this use of a comment from other uses.
Several modern programming languages include an assertion statement, which is essentially an assertion that is checked at runtime. If an assertion evaluates to false, an "assertion failure" results, which may cause execution to abort or cause the failure to be recognized in some other way. A statement is the minimal unit of structuring in imperative programming languages. ...
The use of assertions helps the programmer design, develop, and reason about a program. The use of assertion statements provides additional help during testing, for if an expected assertion evaluates to false, the programmer knows there is a bug somewhere.
The rest of this article focuses on the use of such assertion statements and compares them to error handling.
Usage In languages such as Eiffel, assertions are part of the design process, in others, such as C and Java, they are used only to check assumptions at runtime. In both cases, they can be checked for validity at runtime but can usually also be suppressed. Eiffel is an ISO-standardized object-oriented programming language designed for extensibility, reusability, reliability and programmer productivity. ...
C is a general-purpose, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. ...
Java is an object-oriented applications programming language developed by Sun Microsystems in the early 1990s. ...
Assertions in design by contract Assertions can be a form of documentation: they can describe the state part of the code expects to find before it runs (its preconditions), and the state the code expects to result in when it is finished running (postconditions); they can also specify invariants of a class. In Eiffel, such assertions are integrated into the language and are automatically extracted to document the class. This forms an important part of the method of design by contract. In logic a precondition is a condition that has to be met, before a main argument can have any value. ...
A postcondition is a fact that must always be true just after the execution of some section of code. ...
In computer science, optimising compilers and the methodology of design by contract pay close attention to invariant quantities in computer programs, where the set of transformations involved is the execution of the steps of the computer program. ...
In object-oriented programming, a class is a programming language construct that is used to group related instance variables and methods. ...
Eiffel is an ISO-standardized object-oriented programming language designed for extensibility, reusability, reliability and programmer productivity. ...
Design by contract, DBC or Programming by contract is a methodology for designing computer software. ...
This approach is also useful in languages that do not explicitly support it: the advantage of using assertion statements rather than assertions in comments is that assertions can be checked every time the program is run; if the assertion no longer holds, an error can be reported. This prevents the code from getting out of sync with the assertions (a problem that can occur with comments). An illustration of Java source code with prologue comments indicated in red and inline comments in green. ...
Assertions for run-time checking An assertion may be used to verify that an assumption made by the programmer during the implementation of the program remains valid when the program is executed. For example, consider the following Java code: Java is an object-oriented applications programming language developed by Sun Microsystems in the early 1990s. ...
int total = countNumberOfUsers(); if (total % 2 == 0) { // total is even } else { // total is odd assert(total % 2 == 1); } In Java, % is the remainder operator (not modulus) — if its first operand is negative, the result can also be negative. Here, the programmer has assumed that total is non-negative, so that the remainder of a division with 2 will always be 0 or 1. The assertion makes this assumption explicit — if countNumberOfUsers does return a negative value, it is likely a bug in the program. Java is an object-oriented applications programming language developed by Sun Microsystems in the early 1990s. ...
A major advantage of this technique is that when an error does occur it is detected immediately and directly, rather than later through its often obscure side-effects. Since an assertion failure usually reports the code location, one can often pin-point the error without further debugging.
Assertions are also sometimes placed at points the execution is not supposed to reach. For example, assertions could be placed at the default clause of the switch statement in languages such as C, C++, and Java. Cases that are intentionally not handled by the programmer will raise an error and abort the program rather than silently continuing in an erroneous state. C is a general-purpose, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. ...
C++ (pronounced see plus plus, IPA: ) is a general-purpose, high-level programming language with low-level facilities. ...
Java is an object-oriented applications programming language developed by Sun Microsystems in the early 1990s. ...
In Java, assertions have been a part of the language since version 1.4. Assertion failures result in raising an AssertionError. In C and C++, they are added on by a standard header defining assert (assertion) as a macro which signals an error in the case of failure, usually terminating the program. Java is an object-oriented applications programming language developed by Sun Microsystems in the early 1990s. ...
C is a general-purpose, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. ...
C++ (pronounced see plus plus, IPA: ) is a general-purpose, high-level programming language with low-level facilities. ...
Assertions during the development cycle During the development cycle, the programmer will typically run the program with assertions enabled. When an assertion failure occurs, the programmer is immediately notified of the problem. Many assertion implementations will also halt the program's execution — this is useful, since if the program continued to run after an assertion violation occurred, it might corrupt its state and make the cause of the problem more difficult to locate. Using the information provided by the assertion failure (such as the location of the failure and perhaps a stack trace), the programmer can usually fix the problem. Thus, assertions can simplify debugging. A stack trace (also called backtrace) is a report of the active stack frames instantiated by the execution of a program. ...
Static assertions Assertions that are checked at compile time are called static assertions. The D programming language implements the static assert: D is an object-oriented, imperative system programming language designed by Walter Bright of Digital Mars as a re-engineering of C/C++. He has done this by re-designing many C++ features, and borrowing ideas from other programming languages. ...
// Our algorithm depends on this static assert(t.sizeof == q.sizeof); The C programming language does not have static assertions, but it is possible to simulate their effect in the following way: Wikibooks has a book on the topic of C Programming The C programming language (often, just C) is a general-purpose, procedural, imperative computer programming language developed in the early 1970s by Dennis Ritchie for use on the Unix operating system. ...
// Our algorithm depends on this struct type_size_constraints { char require_t_size_equals_q_size[(sizeof(t) == sizeof(q)) ? 1 : -1]; }; This declares a structure with an illegal size if the condition is not met, so stopping the compiler. Such code could be confusing though to someone not familiar with this idiom so it should always be well-commented.
Static assertions are particularly useful in compile time template metaprogramming. Template metaprogramming is a programming technique in which templates are used by a compiler to generate temporary source code, which is merged by the compiler with the rest of the source code and then compiled. ...
Disabling assertions Assertions are usually implemented so that they can be enabled or disabled, usually on a program-wide basis; languages which distinguish different types of assertion – e.g. pre- and postconditions – usually allow these to be disabled independently. If assertions are disabled, assertion failures are ignored. Since assertions are primarily a development tool, they are often disabled when the program is released. Because some versions of the program will include assertions and some will not, it is essential that disabling assertions does not change the meaning of the program. In other words, assertions should be free of side effects. An alternative in the case of C or C++ is to redefine the macro assert to evaluate the expression even when assertions are disabled, though this reduces the savings due to suppressing assertions and may not be what other programmers expect. In computer science, a side-effect is a property of a programming language function that it modifies some state other than its return value. ...
C is a general-purpose, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. ...
C++ (pronounced see plus plus, IPA: ) is a general-purpose, high-level programming language with low-level facilities. ...
The removal of assertions from production code is almost always done automatically. It usually is done via conditional compilation, for example by using the preprocessor in C or C++ or by passing an option to the runtime engine, as in Java. Some people, however, object to the removal of assertions by citing an analogy that the execution with assertion in development stage and without it in practice is like practicing swimming in a pool with a lifeguard and then going swimming in the sea without a lifeguard. They add that assertions also could help make the program fail-safe. In computer science, a preprocessor is a program that processes its input data to produce output that is used as input to another program. ...
C is a general-purpose, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. ...
C++ (pronounced see plus plus, IPA: ) is a general-purpose, high-level programming language with low-level facilities. ...
Java is an object-oriented applications programming language developed by Sun Microsystems in the early 1990s. ...
Fail Safe is an episode from Season 5 of the science fiction television series Stargate SG-1. ...
Comparison with error handling It is worth distinguishing assertions from routine error handling. Assertions should be used to document logically impossible situations — if the "impossible" occurs, then something fundamental is clearly wrong. This is distinct from error handling: most error conditions are possible, although some may be extremely unlikely to occur in practice. Using assertions as a general-purpose error handling mechanism is usually unwise: assertions do not allow for graceful recovery from errors, and an assertion failure will often halt the program's execution abruptly. Assertions also do not display a user-friendly error message.
Consider the following example of using an assertion to handle an error:
int *ptr = malloc(sizeof(int) * 10); assert(ptr != NULL); // use ptr Here, the programmer is aware that malloc may return a NULL pointer if memory could not be allocated. This is possible: the operating system does not guarantee that every call to malloc will succeed, and the program should be prepared to handle the failure. An assertion is probably not the best choice here, because a malloc failure is not logically impossible — it is a legitimate possibility, albeit not one that will arise very often in practice. The assertion in this example does serve one useful purpose, however: it documents that the programmer has deliberately decided not to provide robust error handling for memory allocation failures.
See also The Assertion Definition Language (ADL) is a specification language providing a formal grammar to specify behaviour and interfaces for computer software. ...
Design by contract, DBC or Programming by contract is a methodology for designing computer software. ...
Hoare logic (also known as Floyd–Hoare logic) is a formal system developed by the British computer scientist C. A. R. Hoare, and subsequently refined by Hoare and other researchers. ...
Static analysis is the term applied to the analysis of computer software that is performed without actually executing programs built from that software (analysis performed on executing programs is known as dynamic analysis). ...
The Java Modeling Language (JML) follows the design by contract paradigm. ...
External links - Assertions: a personal perspective by Tony Hoare, 2001.
- Java:
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