Exception safety is the state of code working correctly when exceptions are thrown.[1] To aid in ensuring exception safety, C++ standard library developers have devised a set of exception safety levels, contractual guarantees of the behavior of a data structure's operations with regards to exceptions. Library implementers and clients can use these guarantees when reasoning about exception handling correctness. The exception safety levels apply equally to other languages and error-handling mechanisms.[2]
As David Abrahams writes, "nobody ever spoke of 'error-safety' before C++ had exceptions."[3] The term appeared as the topic of publications in JTC1/SC22/WG21, the C++ standard committee, as early as 1994.[4] Exception safety for the C++ standard library was first formalized for STLport by Abrahams, establishing the basic safety/strong safety distinction.[5] This was extended to the modern basic/strong/nothrow guarantees in a later proposal.[6]
Backgroundedit
Exceptions provide a form of non-local control flow, in that an exception may "bubble up" from a called function. This bubbling can cause an exception safety bug by breaking invariants of a mutable data structure, as follows:[7]
A step of an operation on a mutable data structure modifies the data and breaks an invariant.
An exception is thrown and control "bubbles up", skipping the rest of the operation's code that would restore the invariant
The exception is caught and recovered from, or a finally block is entered
The data structure with broken invariant is used by code that assumes the invariant, resulting in a bug
Code with a bug such as the above can be said to be "exception unsafe".[7]
Classificationedit
The C++ standard library provides several levels of exception safety (in decreasing order of safety):[8]
No-throw guarantee, also known as failure transparency: Operations are guaranteed to succeed and satisfy all requirements even in exceptional situations. If an exception occurs, it will be handled internally and not observed by clients.
Strong exception safety, also known as commit or rollback semantics: Operations can fail, but failed operations are guaranteed to have no side effects, leaving the original values intact.[9]
Basic exception safety: Partial execution of failed operations can result in side effects, but all invariants are preserved. Any stored data will contain valid values which may differ from the original values. Resource leaks (including memory leaks) are commonly ruled out by an invariant stating that all resources are accounted for and managed.
No exception safety: No guarantees are made.
Usually, at least basic exception safety is required to write robust code. Higher levels of safety can sometimes be difficult to achieve, and might incur an overhead due to extra copying. A key mechanism for exception safety is a finally clause, or similar exception handling syntax, which ensure that certain code is always run when a block is exited, including by exceptions. Several languages have constructs that simplify this, notably using the dispose pattern, named as using, with, or try-with-resources.
Exampleedit
Consider a smart vector type, such as C++'s std::vector or Java's ArrayList. When an item x is added to a vector v, the vector must actually add x to the internal list of objects and update a count field that says how many objects are in v. It may also need to allocate new memory if the existing capacity isn't sufficient.
Exception safety alternatives:
No-throw guarantee
Implemented by ensuring that memory allocation never fails, or by defining the insert function's behavior on allocation failure (for example, by having the function return a boolean result indicating whether the insertion took place).
Strong exception safety
Implemented by doing any necessary allocation first, and then swapping buffers if no errors are encountered (the copy-and-swap idiom). In this case, either the insertion of x into v succeeds, or v remains unchanged despite the allocation failure.
Basic exception safety
Implemented by ensuring that the count field is guaranteed to reflect the final size of v. For example, if an error is encountered, the insert function might completely deallocate v and reset its count field to zero. On failure, no resources are leaked, but v's old value is not preserved.
No exception safety
An insertion failure might lead to corrupted content in v, an incorrect value in the count field, or a resource leak.
Referencesedit
^Crichton, Alex (24 July 2015). "Rust RFC: Stabilize catch_panic". The Rust Programming Language. Retrieved 26 May 2022. Code is exception safe if it works correctly even when the functions it calls into throw exceptions.
^Lau, Ron (10 November 2020). "Exception safety in JS world". Medium.
^Colvin, Gregory (1994). "Exception Safe Exceptions" (PDF). C++ Standards Committee Papers. Retrieved 17 December 2021.
^Abrahams, David. "STLport: Exception Handling". www.stlport.org. Retrieved 17 December 2021.
^Abrahams, Dave; Colvin, Greg. "Making the C++ Standard Library Exception Safe" (PDF). C++ Standards Committee Papers. Retrieved 17 December 2021.
^ abCrichton, Alex (24 July 2015). "Rust RFC: Stabilize catch_panic". The Rust Programming Language. Retrieved 26 May 2022.
^Bjarne Stroustrup (1997). Appendix E: Standard-Library Exception Safety in "The C++ Programming Language"(PDF) (3rd ed.). Addison-Wesley. ISBN0-201-88954-4.
^Austern, Matt (30 May 1997). "Standard Library Exception Policy". C++ Standards Committee Papers. Retrieved 26 May 2022.
External linksedit
Herb Sutter: Exceptional C++: 47 Engineering Puzzles, Programming Problems, and Solutions, 2000
Jon Kalb: Exception-Safe Coding in C++, with C++Now! 2012 presentations on exception safety.
Related discussion on Stackoverflow: C++: do you (really) write exception safe code
January 01, 1970
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Exception safety is the state of code working correctly when exceptions are thrown 1 To aid in ensuring exception safety C standard library developers have devised a set of exception safety levels contractual guarantees of the behavior of a data structure s operations with regards to exceptions Library implementers and clients can use these guarantees when reasoning about exception handling correctness The exception safety levels apply equally to other languages and error handling mechanisms 2 Contents 1 History 2 Background 3 Classification 4 Example 5 References 6 External linksHistory editAs David Abrahams writes nobody ever spoke of error safety before C had exceptions 3 The term appeared as the topic of publications in JTC1 SC22 WG21 the C standard committee as early as 1994 4 Exception safety for the C standard library was first formalized for STLport by Abrahams establishing the basic safety strong safety distinction 5 This was extended to the modern basic strong nothrow guarantees in a later proposal 6 Background editExceptions provide a form of non local control flow in that an exception may bubble up from a called function This bubbling can cause an exception safety bug by breaking invariants of a mutable data structure as follows 7 A step of an operation on a mutable data structure modifies the data and breaks an invariant An exception is thrown and control bubbles up skipping the rest of the operation s code that would restore the invariant The exception is caught and recovered from or a finally block is entered The data structure with broken invariant is used by code that assumes the invariant resulting in a bug Code with a bug such as the above can be said to be exception unsafe 7 Classification editThe C standard library provides several levels of exception safety in decreasing order of safety 8 No throw guarantee also known as failure transparency Operations are guaranteed to succeed and satisfy all requirements even in exceptional situations If an exception occurs it will be handled internally and not observed by clients Strong exception safety also known as commit or rollback semantics Operations can fail but failed operations are guaranteed to have no side effects leaving the original values intact 9 Basic exception safety Partial execution of failed operations can result in side effects but all invariants are preserved Any stored data will contain valid values which may differ from the original values Resource leaks including memory leaks are commonly ruled out by an invariant stating that all resources are accounted for and managed No exception safety No guarantees are made Usually at least basic exception safety is required to write robust code Higher levels of safety can sometimes be difficult to achieve and might incur an overhead due to extra copying A key mechanism for exception safety is a finally clause or similar exception handling syntax which ensure that certain code is always run when a block is exited including by exceptions Several languages have constructs that simplify this notably using the dispose pattern named as using with or try with resources Example editConsider a smart vector type such as C s span class n std span span class o span span class n vector span or Java s span class n ArrayList span When an item span class n x span is added to a vector span class n v span the vector must actually add span class n x span to the internal list of objects and update a count field that says how many objects are in span class n v span It may also need to allocate new memory if the existing capacity isn t sufficient Exception safety alternatives No throw guarantee Implemented by ensuring that memory allocation never fails or by defining the span class n insert span function s behavior on allocation failure for example by having the function return a boolean result indicating whether the insertion took place Strong exception safety Implemented by doing any necessary allocation first and then swapping buffers if no errors are encountered the copy and swap idiom In this case either the insertion of span class n x span into span class n v span succeeds or span class n v span remains unchanged despite the allocation failure Basic exception safety Implemented by ensuring that the count field is guaranteed to reflect the final size of span class n v span For example if an error is encountered the span class n insert span function might completely deallocate span class n v span and reset its count field to zero On failure no resources are leaked but span class n v span s old value is not preserved No exception safety An insertion failure might lead to corrupted content in span class n v span an incorrect value in the count field or a resource leak References edit Crichton Alex 24 July 2015 Rust RFC Stabilize catch panic The Rust Programming Language Retrieved 26 May 2022 Code is exception safe if it works correctly even when the functions it calls into throw exceptions Lau Ron 10 November 2020 Exception safety in JS world Medium Dave Abrahams 2000 Exception Safety in Generic Components Generic Programming Lecture Notes in Computer Science Vol 1766 Springer pp 69 79 doi 10 1007 3 540 39953 4 6 ISBN 978 3 540 41090 4 Retrieved 2008 08 29 Colvin Gregory 1994 Exception Safe Exceptions PDF C Standards Committee Papers Retrieved 17 December 2021 Abrahams David STLport Exception Handling www stlport org Retrieved 17 December 2021 Abrahams Dave Colvin Greg Making the C Standard Library Exception Safe PDF C Standards Committee Papers Retrieved 17 December 2021 a b Crichton Alex 24 July 2015 Rust RFC Stabilize catch panic The Rust Programming Language Retrieved 26 May 2022 Bjarne Stroustrup 1997 Appendix E Standard Library Exception Safety in The C Programming Language PDF 3rd ed Addison Wesley ISBN 0 201 88954 4 Austern Matt 30 May 1997 Standard Library Exception Policy C Standards Committee Papers Retrieved 26 May 2022 External links editHerb Sutter Exceptional C 47 Engineering Puzzles Programming Problems and Solutions 2000 Jon Kalb Exception Safe Coding in C with C Now 2012 presentations on exception safety Related discussion on Stackoverflow C do you really write exception safe code Retrieved from https en wikipedia org w index php title Exception safety amp oldid 1195505741, wikipedia, wiki, book, books, library,
