C + + notes: The difference between null and 0, the origin of nullptr

At some point, we need to assign the pointer to a null pointer to prevent the wild pointer. Some people prefer to use NULL as a null pointer constant, for example: int* p = NULL;. There is also a direct use of the 0 value as a null pointer constant, for example: int* p = 0;. The former may think: null as a null pointer constant, the name is very image, readability is strong. The latter may feel that NULL is not a keyword for C/+ + languages, but a macro defined in the standard library header file <stddef.h>, so to use NULL, you may need to include the <stddef.h> header file directly or briefly, which is more cumbersome. question one: What is the difference between a null and a constant 0 value? To figure this out, we use a question and answer form to describe it. Q: What exactly is null? Answer: null is a macro. Q: What is the value of it? A: the C + + standard stipulates that its value is a null pointer constant (null pointer constant), which is defined by the implementation. #1, #2 Q: What kind of values can be called NULL pointer constants? A: C language constants 0 and (void*) 0 are null pointer constants, C + + (ignoring c++11) constant 0 is, and (void*) 0 is not. #3, #4 Q: Where is the null macro defined? A: Usually in the C standard library <stddef.h> header file, but other header files may also be defined. Q: How is a null macro defined in the General compiler's <stddef.h> header file? A: In the case of GCC or clang compilers, the definition of NULL is roughly as follows (slightly simplified):

#if defined (__cplusplus)0    //  C + + uses 0 as a null value #else# Define NULL ( (void *)0)    //  C with a value of ((void *) 0) as null #endif 

Q: Why C (void*) 0 is a null pointer constant, but not in C + +? A: Because any type of pointer in the C language can (implicitly) be converted to the void* type and vice versa, the void* type in C + + cannot be implicitly converted to another type pointer (for example: Int*p = (void*) 0; compile with C + + compiler will error). #5, #6 Q: Since the C + + standard, constant 0 is available as a null pointer constant, why not use 0 uniformly? A: Personally feel that because (void*) 0 more can reflect the meaning of pointers, and the constant 0 more time is used as an integer. Therefore, the null definition in the C language is selected (void*) 0. (for reference only) question two: Why do you want to introduce nullptr in c++11? Consider the case of such a function overload:

#include <stddef.h>void foo (int) {}     //  #1void foo (Char *) {}   //  #2int  main () {    //  Call # # or # # #? }

Literally, NULL is a null pointer constant, and we might think that since it's a pointer, you should call # #. In fact, it is called # # because the null extension in C + + is constant 0, which is an int type. The root cause is: constant 0 is both an integer constant and a null pointer constant. To solve this ambiguity, the C++11 standard introduces the keyword nullptr, which acts as a null pointer constant. #7例如:

void foo (int) {}     //  #1void foo (char*) {}   // #2 int Main () {    //  It will call # # without any objection}

notes: [#1] c99:7.17-p3:    The macros are        null    which expands to an impl ementation-defined null pointer constant; And ...  [#2] c++03:18.1-p4:    The macro NULL is a implementation-defined C + + NULL pointer constant in This International standard (4.10).  [#3] c99:6.3.2.3-p3:    A integer constant expression with the value 0 , or such an expression cast to type void *, is called a null pointer constant. [#4] c++03:4.10-p1:    a n Ull pointer constant is a integral constant expression (5.19) Rvalue of integer type that evaluates to zero. [#5] C9 9:6.3.2.3-p1:    A pointer to void is converted to or from A pointer to any incomplete or object type. A pointer to any incomplete or object type is converted to a pointer to void and back again; The result shall compare equal to the original pointer. [#6] c++03:4.10-p2:    An rvalue of type "pointer To CV T, "WHere's a object type, can be converted to a rvalue of type "pointer to CV void."  [#7] c++11:4.10-p1:    A null pointer constant is an integral constant expression (5.19) Prvalue of Integ ER type, evaluates to zero or a prvalue of type std::nullptr_t.  Reference:(1) C99/C++03/C++11 standard document (2) nullptr proposal Document: N2431:http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2431.pdf

C + + notes: The difference between null and 0, the origin of nullptr