C++中引用和指

引用就是别名，我引用你，我改变了，你也跟着改变。可能没学过C++的人更多的疑问就是，有了指针为何又要有引用呢？二者的区别何在？

A pointer can be re-assigned any number of times while a reference can not be re-seated after binding.

指针可以被重新赋值，而引用不可以

<code class="hljs rust has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> x = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> y = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *p;
p =  &x;
p = &y;
*p = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">assert</span>(x == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">assert</span>(y == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>);

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> x = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> y = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> &r = x;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

Pointers can point nowhere (NULL), whereas reference always refer to an object.

指针可以指向空，引用不能为空

You can’t take the address of a reference like you can with pointers.

你不能获得一个引用的地址

A pointer has its own memory address and size on the stack (4 bytes on x86), whereas a reference shares the same memory address (with the original variable) but also takes up some space on the stack. Since a reference has the same address as the original variable
itself, it is safe to think of a reference as another name for the same variable. Note: What a pointer points to can be on the stack or heap. Ditto a reference. My claim in this statement is not that a pointer must point to the stack. A pointer is just a variable
that holds a memory address. This variable is on the stack. Since a reference has its own space on the stack, and since the address is the same as the variable it references. More on stack vs heap. This implies that there is a real address of a reference that
the compiler will not tell you.

<code class="hljs perl has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">x</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> &r = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">x</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">*p</span> = &<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">x</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">*p2</span> = &r;
assert(p == p2);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

You can have pointers to pointers to pointers offering extra levels of indirection. Whereas references only offer one level of indirection.

可以指针指向指针，不能引用引用引用。(绕口令，但我相信你能明白)

<code class="hljs rust has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> x = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> y = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *p = &x;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *q = &y;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> **pp = &p;
pp = &q;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//*pp = q</span>
**pp = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">assert</span>(y == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">assert</span>(x == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

Const references can be bound to temporaries. Pointers cannot (not without some indirection):

<code class="hljs cs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> &x = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//legal C++</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *y = &<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//illegal to dereference a temporary</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

Use references in function parameters and return types to define useful and self-documenting interfaces.

函数的参数最好使用引用，如果返回值有意义，最好也返回引用。但是不要返回局部变量的引用

Use pointers to implement algorithms and data structures

在算法和数据结构中最好使用指针