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Some programming languages allow a program to operate differently or even have a different control flow from the source code, as long as it exhibits the same user-visible side effects, if undefined behavior never happens during program execution. Undefined behavior is the name of a list of conditions that the program must not meet.
In object-oriented computer programming, a null object is an object with no referenced value or with defined neutral (null) behavior.The null object design pattern, which describes the uses of such objects and their behavior (or lack thereof), was first published as "Void Value" [1] and later in the Pattern Languages of Program Design book series as "Null Object".
An undefined variable in the source code of a computer program is a variable that is accessed in the code but has not been declared by that code. [1] In some programming languages, an implicit declaration is provided the first time such a variable is encountered at compile time. In other languages such a usage is considered to be sufficiently ...
In some programming language environments (at least one proprietary Lisp implementation, for example), [citation needed] the value used as the null pointer (called nil in Lisp) may actually be a pointer to a block of internal data useful to the implementation (but not explicitly reachable from user programs), thus allowing the same register to be used as a useful constant and a quick way of ...
In the C and C++ languages, such non-portable constructs are generally grouped into three categories: Implementation-defined, unspecified, and undefined behavior. [3] The exact definition of unspecified behavior varies. In C++, it is defined as "behavior, for a well-formed program construct and correct data, that depends on the implementation."
An undefined value must not be confused with empty string, Boolean "false" or other "empty" (but defined) values. Depending on circumstances, evaluation to an undefined value may lead to exception or undefined behaviour, but in some programming languages undefined values can occur during a normal, predictable course of program execution.
Weak symbols are not mentioned by the C or C++ language standards; as such, inserting them into code is not very portable. Even if two platforms support the same or similar syntax for marking symbols as weak, the semantics may differ in subtle points, e.g. whether weak symbols during dynamic linking at runtime lose their semantics or not. [1]
A smart pointer typically uses reference counting to reclaim objects. Some other techniques include the tombstones method and the locks-and-keys method. [3] Another approach is to use the Boehm garbage collector, a conservative garbage collector that replaces standard memory allocation functions in C and C++ with a garbage collector. This ...