ptrA = ptrB 是否等同于 ptrA = &*ptrB？，空值是否共享一个地址空间？

Is ptrA = ptrB equivalent to ptrA = &*ptrB?, and do nulls share an address space?

本文关键字：ptrA 是否 ptrB 一个 地址空间 等同于 空值 共享      更新时间：2023-10-16

我在指针重新分配，指针比较以及nulls是否分别在Q1，Q2和Q3中共享地址空间有几个问题。

int main()
{
int var2 = 2;
int varnull = NULL;
int* myPtr_simple = &var2;
//Q1) are these 2 statements equivalent in terms of reassigning 
//both myPtr_long and myPtr_short to what myPtr_simple points to?
int* myPtr_long = &*myPtr_simple;
int* myPtr_short = myPtr_simple;
//Q2) are these 2 statements equivalent?
std::cout<<(myPtr_short==myPtr_long)<<std::endl; //TRUE
std::cout<<(*myPtr_short==*myPtr_long)<<std::endl; //TRUE
//if var2 is non-null
//(address spaces of var2, myPtr_short, myPtr_long are equal)
std::cout<<(&*myPtr_short==&*myPtr_long)<<std::endl; //TRUE
std::cout<<(&*myPtr_short==&var2)<<std::endl; //TRUE
std::cout<<(&var2==&*myPtr_long)<<std::endl; //TRUE
//Q3) if varnull is null
//(myPtr_short and myPtr_long share address space, but not same as varnull...why?)
myPtr_long = NULL;
myPtr_short = NULL;
std::cout<<(&*myPtr_short==&*myPtr_long)<<std::endl; //TRUE
std::cout<<(&*myPtr_short==&varnull)<<std::endl; //FALSE
std::cout<<(&varnull==&*myPtr_long)<<std::endl; //FALSE
return 0;
}

Q1(这两个陈述是否等于将myptr_long和myptr_short重新分配到myptr_simple指向的内容？

int* myPtr_long = &*myPtr_simple;
int* myPtr_short = myPtr_simple;

是的，这些效果相同。

Q2(这两个语句等效？

std::cout<<(myPtr_short==myPtr_long)<<std::endl; //TRUE
std::cout<<(*myPtr_short==*myPtr_long)<<std::endl; //TRUE

不，绝对不是。myPtr_short==myPtr_long检查myPtr_short和myPtr_long是否是同一对象的指针。(这不是完全正确的，但现在足够近。(*myPtr_short==*myPtr_long检查两个引用的对象是否具有相同的值。

int i = 0, j = 0;
int* pi = &i;
int* pj = &j;

现在，pi == pj不是正确的。pi和pj指向不同的对象。

但是*pi == *pj是正确的，因为i和j碰巧具有值0，而0 == 0显然是正确的。

Q3(如果varnull为null(myptr_short和myptr_long共享地址空间，但与varnull不一样...为什么？(

myPtr_long = NULL;
myPtr_short = NULL;
std::cout<<(&*myPtr_short==&*myPtr_long)<<std::endl; //TRUE
std::cout<<(&*myPtr_short==&varnull)<<std::endl; //FALSE

&*myPtr_short和&*myPtr_long仅myPtr_short和myPtr_long。它们都是NULL，因为那是您刚分配给它们的值。

&varnull 不是 NULL：它是对象的有效指针。您用来初始化该对象的值是无关紧要的，因为这不会检查varnull的值。

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