为什么我不能在 C++ 中为整数指针分配一个整数值？

Question

My code is

int null = 0;
int *p = null;

It gives me an error: cannot initialize a variable of type 'int *' with an lvalue of type 'int'.

In C it would work.

EDIT:

How can I make it work in this spirit?

It's from an exercise (2.32) from the Primer C++ book:

Is the following code legal or not? If not, how might you make it legal?

int null = 0, *p = null;

Answer 1

change int *p = null; to int *p = (int*)null;

Answer 2

If you are using C++11 then use nullptr :

int *p = nullptr;

If you are not using C++11 then use NULL instead:

int *p = NULL;

Do not define your own null value at all, since C/C++ already has one of its own. NULL has no type, it is simply defined as 0 , and an integer literal 0 can be assigned to a pointer:

int *p = 0;

Answer 3

You should use nullptr in C++11. C++ doesn't do automatic conversion between int and x* (where x is type), so using an int typed variable to define null won't work. In pre-C++11, you can use integral literals, like 0 . (You can still use integral constants after the new standard, but there are good reasons for using nullptr over NULL . What are the advantages of using nullptr? )

You can read more about it nullptr here: http://en.cppreference.com/w/cpp/language/nullptr .

Answer 4

我认为 *p 不是 C 的正确标识符。这是因为一个好的标识符不应该以符号开头，而应该以字母开头。