是* P是左值或右值 [英] Is *p an lvalue or rvalue

问题描述

在以下code，是 *一个右值或左值？

In the following code, is *a an rvalue or an lvalue?

#include <stdio.h>

void main()
{
    int b=2;
    int *a=NULL;
    a=&b;
    *a=3;
    printf("%d",*a);

}

推荐答案

有一个左值，并且可以在r值的上下文中使用。见例子;

It is an l-value and can be used in a context of r-value. See the example;

int i, j, *p, *q;
p = &i;
q = &j;

*p = 1; // *p is l-value
*q = 2; // *q is l-value

*p = *q // *q is r-value

由于指向的查尔斯·贝利在他的评论： - 子前pression * q 仍然是一个左值，它经历了一个L值r值转换，因为它是在上下文中使用，其中的R值是需要。

As pointed by Charles Bailey in his comment:- The sub-expression *q is still an l-value, it undergoes an l-value to r-value conversion because it is used in a context where an r-value is required.

现在人们可能会想到的为什么 * Q 仍然是左值？结果
因为R值的定义 R值​​是不持续时间超过使用它的前pression临时值。的结果
在的这种情况下，值* Q 是持续超过前pression * p = * Q 。

Now one may think Why *q is still l-value?
Because by definition of r-value an r-value is a temporary value that does not persist beyond the expression that uses it.
In this case value of *q is persists beyond the expression *p = *q.

注意：

*操作符 - 的差额/间接运算符 - 对指针变量进行操作，并在指针地址​​返回 L值等同于值

" * " operator--the deference/indirection operator -- operates on pointer variable, and returns an l-value equivalent to the value at pointer address.

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