NULL, nullptr, 0,’ \0′,’0

NULL, nullptr,’ \0′,’0′,’0′, they all look different, but in fact the first four zeros are different, only the last one is different. Let’s take a look

0

This is just a number zero, a constant, and the ACsill code is zero.

printf("%d\n".'\ 0'); The results of0
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‘\ 0’

This is the end of the string, which is actually the number zero, and this is the number zero in character form, which is what you think of as a string.

‘0’

This is the character 0, ASCII code 48, which is completely different from the number 0, not to be confused.

printf("%d\n".'0'); The results of48
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NULL

This is implicitly converted to a pointer to the number 0, so you can see that an rvalue is a pointer type, so why don’t you just assign it to 0, because if you assign it to a direct value 0, an lvalue is an integer, an rvalue is a pointer type, and the compiler will warn you, so the direct macro defines NULL, and 0 is implicitly converted to a pointer. And NULL is a better read than 0, so assignments of 0 and NULL work the same way, but when you see 0, you wonder why. When you see NULL, you immediately know that the assignment is NULL.

#define NULL ((void *)0)
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nullptr

This is essentially 0, but it is a pointer type 0 in c++. Why not use NULL? Let’s look at the definition of NULL in c++

#ifndef NULL
#ifdef _cpluscplus                       
#define NULL 0                        
#else
#define NULL ((void*)0)             
#endif
#endif
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Can see NULL in c + + is 0, no implicitly converted to a pointer type, this is because the c + + is a strongly typed language, does not support an implicit conversion, although with the effect of the number 0 is consistent, but using a NULL value to a pointer type, in as a parameter, just have a function overloading, according to the shape of a type is int, another for int *, If you want to call a function of type pointer and pass NULL, you will find that the function whose parameter is int is called. So c++11 uses nullptr as the new pointer type 0 instead of NULL. Nullptr is a member of the nullptr_t class template that supports Pointers of any type. In c++, it is best to use nullptr as a pointer to a nullptr.