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Addison–Wesley. p. 88. ISBN 0-201-88954-4. "CWE-476: NULL Pointer Dereference". MITRE. "Class Poco::NullPointerException". docs.pocoproject.org. Retrieved 17 August...

null reference (null pointer), however, which throws a java.lang.NullPointerException. The space occupied by unreferenced memory objects is recovered automatically...

= null; i.intValue(); // throws a NullPointerException } catch (NullPointerException error) { // Variable might be null } catch (ArithmeticException error)...

makeSound(); will throw a NullPointerException. Therefore, additional code may be necessary to test for null objects. The null object pattern solves this...

_bar: String as Bar } var foo: Foo = null // the below will evaluate to null and not return a NullPointerException var bar = foo.Bar Safe navigation operator...

object references. "NullPointerException". Oracle. Archived from the original on May 6, 2014. Retrieved May 6, 2014. "Exceptions in Java". Artima.com...

multi-objective genetic programming. NpeFix: An automatic repair tool for NullPointerException in Java, available on Github. Astor: A generate-and-validate program...

always desirable. String myString = null; if (myString.equals("foobar")) { /* ... */ } // This causes a NullPointerException in Java With Yoda conditions, the...

following limits on programs: NULL checks are inserted to prevent segmentation faults Pointer arithmetic is limited Pointers must be initialized before use...

to: if (k == null || v == null) { throw new NullPointerException(); } if (m.containsKey(k)) { return m.put(k, v); } else { return null; } m.replace(k...

class of error in programs written in languages that support pointers that can be null, or that can be set to an arbitrary value, or that support arrays...

dereference the null reference must result in an exception being thrown at run-time, specifically a NullPointerException. (It would not make sense to dereference...

createMock(Collection.class); EasyMock .expect(mockCollection.remove(null)) .andThrow(new NullPointerException()) .atLeastOnce(); In the Java Swing API, the LayoutManager...

methodThrowingExceptions(Object obj) { if (obj == null) { // Throws exception of NullPointerException type throw new NullPointerException(); } // Will...

integer, zero, or a positive integer, or throws a NullPointerException (if one or both objects are null). Similarly, in the .NET framework, any class that...

null pointer to his design of the ALGOL W language and called it a "mistake": I call it my billion-dollar mistake. It was the invention of the null reference...

Valley Hockey League in the 2014–2015 season. Teams "StackTrace" and "NullPointerException" both representing Wissahickon High School earned first and second...

Norton Power Eraser, a tool that scans a computer system for threats Null pointer exception, a form of run-time error in many programming languages National...

setting the environment to an invalid BoundedGrid, it will cause a NullPointerException. Instead of the discontinued case studies, the College Board created...

("double-bang") is the not-null assertion operator, used to override null safety so as to allow a null pointer exception. In the ML programming language...

Streaming JEP 352: Non-Volatile Mapped Byte Buffers JEP 358: Helpful NullPointerExceptions JEP 359: Records (Preview) JEP 361: Switch Expressions (Standard)...

even cause it to fail. For example, SIGSEGV generates a java::lang::NullPointerException, java::lang::Runtime::exec() makes use of SIGCHLD, while other signals...

developer from introducing null-pointer exceptions, a common, but difficult to debug, error. With Dart 3.0, all code must follow sound null safety. Snapshot files...

in an undetermined state and potentially cause write to cause a null pointer exception. To avoid this, read returns the special value EOF (end-of-file)...

for the function or method associated with that frame; if an exception is thrown, a pointer in the layout directs the runtime to the appropriate handler...

a pointer fails, the result is the nullptr constant, whereas if the destination is a reference (which cannot be null), the cast throws an exception. Objects...

are among the reported users of Infer. Infer performs checks for null pointer exceptions, resource leaks, annotation reachability, missing lock guards,...

unusable. This means that, for example, a pointer to 0 is never a valid pointer and can be used as a special null pointer value. Unlike the previously mentioned...

for null-terminated strings, with associated handling in its standard library. The language supports low-level access to computer memory via pointers, procedure-like...

function pointer or exception handler to point to the shellcode, which is subsequently executed; By overwriting a local variable (or pointer) of a different...

never again be used, no code will try to access the null pointer, and because the pointer is null, its memory is not deleted when it goes out of scope...

may lead to unexpected results. Null pointer dereference – A null pointer dereference will often cause an exception or program termination in most environments...

concept of a null pointer to denote a pointer that does not refer to any valid data. The macro NULL is often used in place of a null pointer, relying on...

2019. Retrieved April 14, 2019. BillWagner. "Unsafe code, pointers to data, and function pointers". Microsoft Learn. Archived from the original on July 4...

termination of the code that made the invalid reference. A null pointer is usually represented as a pointer to address 0 in the address space; many operating systems...

The null value is a primitive value that represents the null, empty, or non-existent reference. The type Null has exactly one value, called null.: local...

a crash when the invalid pointer is used. Whether a pointer is null or not is another example of the predicate problem; null may be a flag indicating...

to initialize a pointer, also in a specification statement, is to use the NULL function: REAL, POINTER, DIMENSION(:) :: vector => NULL() ! compile time...

to capture machine (CPU/FPU) generated software interrupts (such as null pointer dereferences and divide-by-zero operations), and to handle these situations...

machine independently. Pointer arithmetic may be accomplished by aliasing a binary variable with a pointer as in DCL P POINTER, N FIXED BINARY(31) BASED(ADDR(P));...

with non-nullable types that checks so the variables of nullable types that has been set as non-nullable are not null. If is null then an exception is thrown...

include: Race conditions Exceptions Resource leaks Memory leaks Security attack vulnerabilities (e.g., SQL injection) Null pointers Uninitialized memory Buffer...

*)malloc(sizeof(SExpression)); if (b == NULL) return NULL; b->type = eVALUE; b->value = 0; b->left = NULL; b->right = NULL; return b; } SExpression *createNumber(int...

or may be defined to have a specific "default" value such as 0 or a null pointer (as in Java). In C++ a std::vector object supports the store, select...

memory: It is common to use pointers to access and modify memory. If such a pointer is a null pointer, dangling pointer (pointing to memory that has...

:= b; Pointers in Pascal are type safe; i.e. a pointer to one data type can only be assigned to a pointer of the same data type. Also pointers can never...

compatible, an exception will be thrown (when dealing with references) or a null pointer will be returned (when dealing with pointers). A Java typecast...

bugs; Swift has features addressing some common programming errors like null pointer dereferencing and provides syntactic sugar to help avoid the pyramid...

originated, but also Ada, Vala, and Rust. The technique was developed for exception-safe resource management in C++ during 1984–1989, primarily by Bjarne...

condition-testing statements (if, while) assume that zero (and hence a NULL pointer or a null string terminator '\0' also) is false and all other values are true...

within the bounds of the formal language. In C and C++, constructs such as pointer type conversion and union — C++ adds reference type conversion and reinterpret_cast...

to the fields a and b in the record, and to initialise the pointer c to the null pointer ("NIL" in Pascal), the statements would be: new(NodePtr); ....

and passes a pointer to it as a hidden first parameter; the callee populates the memory and returns the pointer, popping the hidden pointer when returning...

the base class of the exception class hierarchy. Error, Exception, and RuntimeException – the base classes for each exception type. Thread – the class...

standardized. The original behaviour of the functions, which was to return a null pointer when an error occurred, is accessible via placement syntax. Usage of...

type(animal) is Bulldog, else an exception Bulldog b = (Bulldog)animal; // if (animal is Bulldog), b = (Bulldog)animal, else b = null b = animal as Bulldog; //...

mem_buffer = NULL; if (setjmp(exception_env)) { // if we get here there was an exception printf("first failed, exception type: %d\n", exception_type); } else...

NOP or null statement if it has no effect. Null statements may be required by the syntax of some languages in certain contexts. In Ada, the null statement...

better. In C and C++, 0 is often used to mean null pointer even though the C standard library defines a macro NULL and modern C++ includes a keyword nullptr...

Agriculture for Natural Resources and Environment or USA(NRE) Null Reference Exception (Null_pointer#Null_dereferencing) This disambiguation page lists articles...

dangling pointers and no buffer overruns. There is no null but optional types can be safely represented using unions with the None type. Exception safety...

either a pointer or a reference can be used. C++ supports goto statements, which may lead to spaghetti code programming. With the exception of the goto...

are initialized by language specific rules (to undefined values, 0, NULL pointers, ...). Variables that are defined in other source/object files must...

"10" echo $adder(null); // throws an exception because an incorrect type was passed $adder = getAdder([]); // would also throw an exception By default, scalar...

being more weakly-typed regarding pointers. Specifically, C allows a void * pointer to be assigned to any pointer type without a cast, while C++ does...

"Warning" (class 01), "N" denotes "No data" (class 02), and "X" denotes "Exception" (all other classes). Real DBMSs are free to define additional values...

Atomic Alignment Requirements". N3322 - "Allow Zero Length operations on Null Pointers (Including in the Library)". N3323 - "How Do You Add One To Something...

an exception propagates from the rest of the try or not. The following C# code ensures that the stream stream is closed. FileStream stream = null; try...

type. If both are pointer or reference types, or one is a pointer type and the other is a constant expression evaluating to 0, pointer or reference conversions...

natural type for the "null pointer" value (a pointer which does not point to any object) of languages like Java: in Java, the null type is the universal...

If resource acquisition can fail without throwing an exception, such as by returning a form of null, it must also be checked before release, such as: from...

keyword that will return a null reference if the cast is invalid, or by using a C-style cast that will throw an exception if the cast is invalid. See...

concept of a null pointer distinct from any valid pointer, and often used as an error return. Some languages allow most types to be nullable, for example...

0 (integer types), 0.0 (floating-point types), false (boolean type), or null (reference types), etc. class Point { private: int x; int y; public: Point(int...

tracking a given plain pointer. This introduces the possibility of having two (or more) smart pointers tracking the same plain pointer (which causes corruption...

bills. The issue stems from these systems confusing their name for a null pointer. Depending on the system, this may cause the system to not show their...

illustrated in the following C snippet where minimal evaluation prevents both null pointer dereference and excess memory fetches: bool isFirstCharValidAlphaUnsafe(const...

a pointer-to-base-class. Objects that may be deleted in this way need to inherit a virtual destructor. A destructor should never throw an exception; all...

Client(Service service) { if (service == null) { throw new IllegalArgumentException("service must not be null"); } this.service = service; } } Dependency...

Note that by definition byte memory accesses are always aligned. A memory pointer that refers to primitive data that is n bytes long is said to be aligned...

counter (PC). Although any register can be used as a stack pointer, R6 is the stack pointer (SP) used for hardware interrupts and traps. R5 is often used...

variables. If a variable is not initialized then it defaults to NULL value. The optional exception-handling part is used to handle run-time errors. Only the...

an exception catch (ArgumentNullException e) { Console.Error.WriteLine($"No number was entered: {e.Message}"); return 1; } catch (FormatException e) {...

is checking that a reference about to be processed is not null, which avoids null-pointer failures. Other uses include using a Boolean field for idempotence...

void* (a void pointer), from which objects must be cast explicitly. Although the code is compiled without error, it throws a runtime exception (java.lang...

errors: Dangling pointers, which occur when a piece of memory is freed while there are still pointers to it, and one of those pointers is dereferenced...

use ptr ... Here, the programmer is aware that malloc will return a NULL pointer if memory is not allocated. This is possible: the operating system does...

smart pointers. In the constructor of the smart pointer, an object can be requested from the pool, and in the destructor of the smart pointer, the object...

type and p->im is not defined because the value of p is the null pointer): complex *p = NULL; complex abs_p = sqrt(*p >> 4 + p->im); If the type declaration...

instruction 0x7A throw Throw an exception. Object model instruction 0xFE 0x12 unaligned. (alignment) Subsequent pointer instruction might be unaligned...

form new(std::nothrow), which specifies that no exception should be thrown; instead, a null pointer is returned to signal an allocation error. The new...

QUEUE_EXCEPTION("Internal error!"); goto err; } PQfreemem(data); } else { if (JS_DefineProperty(cx, o, "data", OBJECT_TO_JSVAL(NULL), NULL, NULL, JSPROP_ENUMERATE)...

required_argument, NULL, 0}, {"append", no_argument, NULL, 0}, {"delete", required_argument, NULL, 0}, {"verbose", no_argument, NULL, 0}, {"create", required_argument...

Rather than allow any value to be implicitly nullable (such as JavaScript's undefined or a null pointer), Elm's standard library defines a Maybe a type...

Strict type checking, even across modules, and index checking at runtime, null pointer checking, and the safe type extension concept largely allow programming...

entire array. This is one of the few exceptions to the rule that the name of an array is converted to a pointer to the first element of the array, and...

standardized by the POSIX specification. Similar(following the logic) to pointers, the new file description is merely an alias to the old one, with both...

with the system call a structure containing, among others, a function pointer to the signal handling routine. Some predefined signals (such as SIGKILL)...

cases are nullable types (option types), where the return value can be null to indicate failure. For exception handling, one can return a nullable type, or...

example. Let null be the empty list: (reset (begin (shift k (cons 1 (k (void)))) ;; (1) null)) The context captured by shift is (begin [*] null), where [*]...

"Java and scala's type systems are unsound: the existential crisis of null pointers". ACM SIGPLAN Notices. 51 (10): 838–848. doi:10.1145/3022671.2984004...

instructions. SP (Stack pointer): Points to the top of stack in memory. It is automatically updated during PUSH and POP operations. BP (Base Pointer): Points to the...

overwrite the return pointer. However, any overflow that occurs in a buffer from a previous stack frame will still overwrite a return pointer and allow for malicious...

consumed thus far from the input is stored through the next pointer, which must be a pointer to int. This is not a conversion and does not increase the...

character strings, the standard library uses the convention that strings are null-terminated: a string of n characters is represented as an array of n + 1...

default value of a union is null, but if all types in the union are non-nullable, then a switch expression need not check for null. Cyclone, a dialect of C...

a mutable string and the latter is an explicit pointer to a mutable string (unless it is a null pointer), in Java it is only possible to have a StringBuilder...

which acquires the resource and returns a pointer to it; the resource is released by calling fclose on a pointer to the FILE object. In code: #include <stdio...

guaranteed to not be null and point to valid memory. The raw pointer types *const and *mut opt out of the safety guarantees, thus they may be null or invalid;...

high-level and type-safe. Ada has no generic or untyped pointers, nor does it implicitly declare any pointer type. Instead, all dynamic memory allocation and...

requested section. If an entry is found, INI mapping uses its value as a pointer to another part of the Registry. It then looks up the requested INI setting...

between threads. In Microsoft's Component Object Model (COM), interface pointers must be marshalled when crossing COM apartment boundaries. In the .NET...

following code demonstrates a more advanced use of pointers – passing a pointer to a pointer. An int pointer named a is defined on line 9 and its address is...

most one of the members can be stored in a union object at any time. A pointer to a union object, suitably converted, points to each of its members (or...

example, if the expected type is int*, then a null pointer should be passed as (int*)NULL. Writing just NULL would result in an argument of type either int...

no pointers single inheritance and interfaces supports multi-threaded programming integrated statements for database access event handling exception handling...

literals, function, which is simply a pointer to a stack-allocated function, and delegate, which also includes a pointer to the relevant stack frame, the surrounding...

exception routine in supervisor mode can also save the user stack pointer A7, which would total 8 address registers. However, the dual stack pointer (A7...

Old Frame Pointer Omission, Exceptions, Fixups, Object Maps to and from Source, Section Headers, Token Ring IDs, Xdata, Pdata, New Frame Pointer Omission...

Time-of-check to time-of-use (TOCTOU) is a form of unprotected critical section. Null pointer dereference. Using an uninitialized variable. Using an otherwise valid...

fingers would go under the palm, but if it was a pencil, a 1-handshape (pointer finger out) would go under the palm. Attraction (grammar) Case government...

is empty (root node is Null). In the case of a perfect binary tree of height h, there are 2h+1−1 nodes and 2h+1 Null pointers as children (2 for each...

setAccessible(true); unsafe = (Unsafe)theUnsafe.get(null); } catch (Exception e) { throw new RuntimeException("Unable to access Unsafe", e); } } public NativeMemory(long...

generate a runtime exception. If messages are sent to nil (the null object pointer), they will be silently ignored or raise a generic exception, depending on...

sufficiently large to handle array indices and the difference between pointers. ^d Perl 5 does not have distinct types. Integers, floating point numbers...

engineering team at Facebook with open-source contributors. Targets null pointers, leaks, API usage and other lint checks. Available as open source on...

language or running environment), for example dereferencing a null pointer. As such, null pointer checks are unnecessary for the purpose of stopping the program...

It is guaranteed that argc is non-negative and that argv[argc] is a null pointer. By convention, the command-line arguments specified by argc and argv...

optical media using Windows Explorer. Improved mouse pointer ballistics via a new Enhance pointer precision setting in the Mouse Control Panel. DualView...

cleaning up references after use, typically by setting the reference to null once it is no longer needed and, if necessary, by deregistering any event...

[citation needed] New features provided by ThumbEE include automatic null pointer checks on every load and store instruction, an instruction to perform...

generalize pointers to elements of an array (which indeed can be used as iterators), and their syntax is designed to resemble that of C pointer arithmetic...

server and renegotiates with an invalid signature algorithms extension, a null-pointer dereference occurs. This can cause a DoS attack against the server. A...

evaluated expression is the return value of the subroutine. In other cases a Null value is returned if there is no explicit return statement: in Python, the...

traverseNode := null; } } // A match is found if we arrive at a leaf node and have used up exactly x.length elements return (traverseNode != null && traverseNode...

including pointer initialization Expanded the ability to use initialization expressions for data objects Initialization of pointers to NULL() Clearly...

JavaScript, but mutable in C++. A null pointer passed as a string to WinRT by C++ is converted to an empty string In .Net, null being passed as a string to...

specification) Portable arithmetic results Bit and Boolean data types Pointers and syntax for getting and freeing storage The SCREEN SECTION for text-based...

functions, dynamic_cast operator, non-placement new/delete operators, exceptions, pointer to member functions, references to functions, C++ standard libraries...

time, the amount of time added is determined by the section the clock pointer is in, meaning the riddle is worth less time the longer it takes to solve...

instructions Comments Control flow Foreach loops While loops For loops Do-while Exception handling Enumerated types Algebraic data types Generators Anonymous functions...

C++ also featured a dynamic_cast operator for casting a reference or pointer down to a more specific type in the class hierarchy, similar to a Java...

mechanisms, by associating an IErrorInfo object with an HRESULT and storing (a pointer to) an IErrorInfo object in thread-local storage. The IErrorInfo mechanism...

is constant, and whether the data pointed to by the pointer is constant. Applying const to a pointer itself, as in SomeClass* const ptr, means that the...

programming Object-oriented constructors Operators Ternary conditional operator Null coalescing operators Safe navigation operators Modulo operators Evaluation...

INT_NONE) { SuperUser = TRUE; WriteMemory(++StackPointer, ProgramCounter); ProgramCounter = InterruptPointer; } switch (ReadMemory(ProgramCounter++)) { /*...

expressions Type inference for local variables Generics Non-null types Assisted memory management Exception handling Graphical user interfaces can be developed...

computation. Those techniques can be tailored for specific bugs, such as null pointer dereferences. Shotgun debugging: a process of attempting to correct a...

instructions Comments Control flow Foreach loops While loops For loops Do-while Exception handling Enumerated types Algebraic data types Generators Anonymous functions...

is found, some lookup functions raise an exception, while others return a default value (such as zero, null, or a specific value passed to the constructor)...

documentation to be null-terminated, however this is not always the case (e.g. DM&P Vortex86DX3 and AMD Ryzen 7 6800HS are known to return non-null-terminated...

recently) defined word to the oldest, until a sentinel value, usually a NULL pointer, is found. A context switch causes a list search to start at a different...

CPU via the keyboard controller and saving the system registers, stack pointer and often the interrupt mask in the real-time clock chip's RAM. This allowed...

integer between -2,147,483,648 and 2,147,483,647. Null: The Null data type contains only one value, null. This is the default value for the String data type...

patterns that will lead to unexpected behavior (Uninitialized variable, null pointers, etc.) Methods, procedures and functions doing Insert, Update, Delete...

modern desktop paradigm of Windows, Icons, Menus, and Pointers (WIMP) was created. Although pointers had already been invented, Smalltalk was the first system...

considered equal if and only if their references, typically represented as pointers or integers, are equal. Some languages do this automatically: for example...

was a variant of the Thumb2 16/32-bit instruction set. It integrated null pointer checking; defined some new fault mechanisms; and repurposed the 16-bit...

8-bit registers (which can be combined into three 16-bit pointers) and a larger 16-bit stack pointer. In order to make up somewhat for the lack of registers...

common solution employs a double-wide cas instruction to update both the pointer and an adjacent counter; unfortunately, such an instruction requires a...

during runtime. An out of memory error occurs when the heap pointer and the stack pointer meet. C provides the malloc() library function to allocate heap...

able to create a pointer to these functions by taking their address, and invoke the function by means of that pointer. If two pointers to the same function...

semantically defined (because p is a null pointer, the operations p->real and p->im have no meaning): complex *p = NULL; complex abs_p = sqrt (p->real * p->real...

Master: "When the people of Cheng go out to collect jade, they carry a south-pointer with them so as not to lose their way." Long before any knowledge of electromagnetism...

Pointer Name Number of bits Description Pointer field 8 Present at the start of the TS packet payload signaled by the payload_unit_start_indicator bit...

represent "null reference" and "non-null reference to an object" as in other languages. assuming that "x" and "y" are the objects (and not pointers). Can be...

огн҄ьнѫ 'into a furnace, a fiery one'. The long form, by contrast, acts as a pointer and is definite, 'the good man'. The combination of long-form adjective...

forwarded (assuming that the 'unblocked' information can be forwarded in null time) Time 6-10: The blue packet goes through the network. Wormhole flow...

numbers that are less than 100. NIL is the ALGOL 68 analogue of the null pointer in other languages. The notation x OF y accesses a member x of a STRUCT...

the compiler that every time foo() is called, the passed pointer bar is definitely not null, and points to an array of at least 100 ints. Some C99 features...

arena soccer, indoor football, indoor soccer or simply as six-a-side. Six-pointer – game between teams both competing for a title, promotion or relegation...

LoadLibrary(TEXT("Example.dll")); if (hinstLib == NULL) { printf("ERROR: unable to load DLL\n"); return 1; } // Get function pointer addNumbers = (importFunction)...

code experience will need to brush up on programming concepts such as pointers, structures, and passing by reference to overcome some of the obstacles...

Open Cascade Technology is available under the LGPL-2.1-only with added exception. Versions before that were licensed under the "Open Cascade Technology...

following weaknesses have been described : It only finds basic bugs (e.g. null pointer dereferencing). It is only as precise as the specification and specifications...

pointed to by addresses in registers or memory. Even the PC and the stack pointer could be affected by the ordinary instructions using all of the ordinary...

enable software programs to recover from otherwise fatal errors such as null pointer dereference and divide by zero. Comparing to the failure oblivious computing...

redefining it: (set! + (let ((original+ +)) (lambda args (apply (if (or (null? args) (string? (car args))) string-append original+) args)))) (+ 1 2 3)...

and a preprocessor Modules and namespaces Manual memory management Pointers and NULL Regular expressions Default function parameters Seed7 has many libraries...

convert any object into a double type value, but which throws an exception in the case of a null value) CInt, CStr, CByte, CDbl, CBool, CDate, CLng, CCur, CObj...

of the Variant operator (⍠). Because C does not bound-check indices, a pointer to the interior of any array can be defined that will symbolically act...

setPrototypeOf(myObject, null); As of ECMAScript 5 (ES5), the prototype can also be bypassed by using Object.create(null): const myObject = Object.create(null); Object...

Object* original = new Object(); Object* copy = NULL; copy = original; makes it clear that it is a pointer to the object being copied, not the object itself...

a TRIPOS port by MetaComCo, written in BCPL. BCPL does not use native pointers, so the more advanced functionality of the operating system was difficult...

reentrant) cannot be specified. address constants can't be identified as pointers (such as for access to a data structure) as opposed to, say, access to...

follows: Pointer to the next line (two bytes) Binary equivalent of line number (two bytes, unsigned) Tokenized source statement (variable length) Null (one...

program automatically. A null byte embedded in a file name will interrupt the listing after loading by BASIC. If there are three null bytes, that makes it...

languages follow the convention that any text not executed by the instruction pointer (e.g., Befunge) or otherwise assigned a meaning (e.g., Brainfuck), is considered...

the hand is held up near the forehead with the palm facing the self. The pointer finger is then extended. If the person is nodding and has their eyebrows...

"abcdef" "abc" + "def" ' returns "abcdef" "abc" & Null ' returns "abc" "abc" + Null ' returns Null // Example in D "abc" ~ "def"; // returns "abcdef"...

indexes when an entire index key is NULL, when any key segment is NULL or when just the first key segment is NULL. Indexes can also have conditional columns...