char和char *（p​​ointer） [英] char and char* (pointer)

问题描述

我想了解指针如何工作，所以我创建了这个小程序。首先我创建一个p指针，指向一个char。

I would like to understand how pointers work, so i created this small program. first of all i create a p pointer, which points to a char.

第一个问题是这一点。如果我创建一个指针，它的值是一个memoryaddress（如果我指向一个非指针对象），但这次是哈哈在我的例子。为什么它在char *中以这种方式工作？我怎么可以通过cin >> p增加价值？

The first question is at this point. If i create a pointer, the value of it is a memoryaddress (if i point it to a non-pointer object), but this time it is "haha" in my example. Why does it work this way in char*? And how i can add value to it with cin >> p?

我的第二个问题是，我创建了一个q char，它在我创建它的点上具有* p指针的值。但它的值和地址也是h，但为什么？它必须是这个char对象的内存地址！这是毫无意义的：D（mingw - gcc）

My second question is that, i created a q char, which has the value of the *p pointer at the point i created it. BUT its value and address are "h" too, but why? It must be the memory address of this char object! It's pointless :D (mingw - gcc)

int main() {
    char *p;
    cin >> p;                      //forexample: haha
    char q = *p;
    cout << "&q = " << &q << endl; //&q = h
    cout << "q = " << q << endl;   //q = h
    return 0;
}

更多：如果我先用char a [100] char * p = a;那么& q = h»ŢĹ，所以h和一些混乱。但它应该是一个memoryaddress！

MORE: If i allocate memory first with char a[100]; char *p=a; then &q = h»ŢĹ, so "h" and some mess. but it should be a memoryaddress! and my question is, why is not it address then?

推荐答案

想想 char * p; 从内存中的地址。

Think of char* p; as of address in memory. You did not initialize this pointer so it does not point to anything, you cannot use it.

要始终安全：

或者初始化指针到指针零：

To be safe always:
either initialize pointer to zero:

char *p = 0; // nullptr in C++11

或初始化到某些自动

void foo() {
  char a[100];
  char *p = a;
}

或全局内存：

char a[100];
void foo() {
  char *p = a;
}

或获取动态内存：

char* p = new char [100];

然后您可以使用p（如果不为NULL）读取数据并从p ...

Then you can use p (if not NULL) to read data and to read from p...

对于您误解的 operator>> （std :: istream& char * p）。这个运算符期望 p 指向一些内存（自动，全局，动态 - 不管），它不会自己分配内存。它只是从输入流读取字符，直到空格并将其复制到 p 指向的内存 - 但 p 到一些内存。

For your misunderstaning of operator >> (std::istream&, char* p). This operator expects that p points to some memory (automatic,global,dynamic - no matter) - it does not allocate memory by itself. It just reads characters from input stream until whitespace and copy it to the memory pointed by p - but p must already points to some memory.

取地址 char q; 。当然你可以取地址 q ：& q ，类型是 char * p 。但& q 与 p 不同，此 q = * p 只是复制 p 指向 q 指向的第一个字符，它不能更改 q - 其地址不可更改。对于 cout<< & q - 运算符<< （ostream& char; p）期望 p 指向NULL终止的字符串和& q 指向包含H的内存，但是这个字符后没有人知道 - 所以你会在屏幕上得到一些垃圾。使用 cout<< q 以打印单个字符。

For taking address of char q;. Of course you can take address of q: &q, and it type is char* p. But &q is different that p, and this q=*p just copies first character pointed by p to q, it cannot change address of q - its address is unchangeable. For cout << &q - operator << (ostream&, char* p) expects that p points to NULL terminated string - and &q points to memory containing "H" but what is after this character no one knows - so you will get some garbage on screen. Use cout << q to print single character.

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