Tombstones are a mechanism to detect

dangling pointer Dangling pointers and wild pointers in computer programming are pointers that do not point to a valid object of the appropriate type. These are special cases of memory safety violations. More generally, dangling references and wild references are ...

s and mitigate the problems they can cause in computer programs. Dangling pointers can appear in certain computer

programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming ...

s, e.g. C,

C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...

and

assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...

s. A tombstone is a structure that acts as an intermediary between a pointer and its target, often heap-dynamic data in

memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered, ...

. The pointer – sometimes called the

handle A handle is a part of, or attachment to, an object that allows it to be grasped and manipulated by hand. The design of each type of handle involves substantial ergonomic issues, even where these are dealt with intuitively or by following tra ...

– points only at tombstones and never to its actual target. When the data is deallocated, the tombstone is set to a

null Null may refer to: Science, technology, and mathematics Computing * Null (SQL) (or NULL), a special marker and keyword in SQL indicating that something has no value * Null character, the zero-valued ASCII character, also designated by , often use ...

(or, more generally, to a value that is illegal for a pointer in the given runtime environment), indicating that the variable no longer exists. This mechanism prevents the use of invalid pointers, which would otherwise access the memory area that once belonged to the now deallocated variable, although it may already contain other data, in turn leading to corruption of in-memory data. Depending on the

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...

, the CPU can automatically detect such an invalid access (e.g. for the null value: a ''null pointer dereference error''). This supports in analyzing the actual reason, a programming error, in

debugging In computer programming and software development, debugging is the process of finding and resolving '' bugs'' (defects or problems that prevent correct operation) within computer programs, software, or systems. Debugging tactics can involve in ...

, and it can also be used to abort the program in production use, to prevent it from continuing with invalid data structures. In more generalized terms, a tombstone can be understood as a marker for "this data is no longer here". For example, in

filesystems In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...

it may be efficient when deleting files to mark them as "dead" instead of immediately reclaiming all their data blocks. The downsides of using tombstones include a

computational overhead In computer science, overhead is any combination of excess or indirect computation time, memory, bandwidth, or other resources that are required to perform a specific task. It is a special case of engineering overhead. Overhead can be a decidin ...

and additional memory consumption: extra processing is necessary to follow the path from the pointer to data through the tombstone, and extra memory is necessary to retain tombstones for every pointer throughout the program. One other problem is that all the code that needs to work with the pointers in question needs to be implemented to use the tombstone mechanism. Among popular programming languages, C++ implements the tombstone pattern in its standard library as a weak pointer using std::weak_ptr. Built–in support by programming languages or the compiler is not necessary to use this mechanism.

References

Programming constructs {{Compu-lang-stub

See also

References