从整数中减去指针

Question

    int *p = new int; 
    int a = 10; 
    cout << a+p;
    cout << a-p;

打印a+p，p+a，p-a给地址，a-p给我报错。为什么会这样？指针地址不是整数吗？如果是这样，a-p 不应该给我一个负值吗？

Answer 1

在指针运算方面，指针不等同于 "integer representing some address"。即使加法也不是那么简单:

您不能将指针添加到指针
写p+1并不代表"get address value from p variable and increase it by one"。实际地址增量取决于指向变量的大小，因此递增 int64_t *p 将给出与递增 int8_t *p

为什么我们有这样的"strange behavior"来做指针的加减法呢？简单回答：因为它在 C++ 标准中是这样定义的：

5.7 Additive operators

For addition, either both operands shall have arithmetic or unscoped enumeration type, or one operand shall be a pointer to a completely-defined object type and the other shall have integral or unscoped enumeration type.

For subtraction, one of the following shall hold:

— both operands have arithmetic or unscoped enumeration type; or

— both operands are pointers to cv-qualified or cv-unqualified versions of the same completely-defined object type; or

— the left operand is a pointer to a completely-defined object type and the right operand has integral or unscoped enumeration type.

但是这个限制背后是有原因的（实际上在标准的同一页上有描述）。指针算法用于以下用法：如果 p 指向数组中的某个元素，p+1 指向下一个元素，p-1 指向前一个元素， p1-p2 显示数字p1 和 p2 之间的元素。在这些术语中 p1+p2 没有任何意义，对于 1-p 也是如此，因此它们被视为语法错误。

从整数中减去指针

Subtracting pointer from an integer

c++

pointers

subtraction

pointer-arithmetic