关于C++指针很好的介绍



通过指向类的NULL指针调用类的成员函数
试图用一个null指针调用类的成员函数，导致崩溃：

<code class="hljs cs has-numbering"><span style="font-size:18px;"><span class="hljs-preprocessor">#include <iostream></span><span class="hljs-keyword">using</span> namespace std;class A {    <span class="hljs-keyword">int</span> <span class="hljs-keyword">value</span>;<span class="hljs-keyword">public</span>:    <span class="hljs-keyword">void</span> <span class="hljs-title">dumb</span>() <span class="hljs-keyword">const</span> {cout << <span class="hljs-string">"dumb()\n"</span>;}    <span class="hljs-keyword">void</span> <span class="hljs-keyword">set</span>(<span class="hljs-keyword">int</span> x) {cout << <span class="hljs-string">"set()\n"</span>; <span class="hljs-keyword">value</span>=x;}    <span class="hljs-keyword">int</span> <span class="hljs-keyword">get</span>() <span class="hljs-keyword">const</span> {cout << <span class="hljs-string">"get()\n"</span>; <span class="hljs-keyword">return</span> <span class="hljs-keyword">value</span>;}};<span class="hljs-keyword">int</span> main(){    A *pA1 = <span class="hljs-keyword">new</span> A;    A *pA2 = NULL;    pA1->dumb();    pA1-><span class="hljs-keyword">set</span>(<span class="hljs-number">10</span>);    pA1-><span class="hljs-keyword">get</span>();    pA2->dumb();    pA2-><span class="hljs-keyword">set</span>(<span class="hljs-number">20</span>);<span class="hljs-comment">//崩溃</span>    pA2-><span class="hljs-keyword">get</span>();        <span class="hljs-keyword">return</span> <span class="hljs-number">0</span>;}</span></code>

为什么会这样？
通过非法指针调用函数，就相当于给函数传递了一个指向函数的非法指针！
但是为什么pA2->dumb()会成功呢？
因为导致崩溃的是访问了成员变量！！

使用已经释放的指针

<code class="hljs r has-numbering"><span style="font-size:18px;">struct X {    int data;};int foo() {    struct X *pX;    pX = (struct X *) malloc(sizeof (struct X));    pX->data = <span class="hljs-number">10</span>;         free(pX);        <span class="hljs-keyword">...</span>       <span class="hljs-keyword">return</span> pX->data;}</span></code>

使用未初始化的指针
如果你这样写，编译器会提示你使用了未初始化的变量p。

<code class="hljs perl has-numbering"><span style="font-size:18px;">void fooA(){    <span class="hljs-keyword">int</span> <span class="hljs-variable">*p</span>;    <span class="hljs-variable">*p</span> = <span class="hljs-number">100</span>;}</span></code>

那么如果我释放一个初始化的指针呢？

<code class="hljs cpp has-numbering"><span style="font-size:18px;"><span class="hljs-keyword">void</span> fooB(){    <span class="hljs-keyword">int</span> *p;    <span class="hljs-built_in">free</span>(p);}</span></code>

结果是一样的！！

释放已经释放的指针
直接看看代码：

<code class="hljs cpp has-numbering"><span style="font-size:18px;"><span class="hljs-keyword">void</span> fooA() {    <span class="hljs-keyword">char</span> *p;    p = (<span class="hljs-keyword">char</span> *)<span class="hljs-built_in">malloc</span>(<span class="hljs-number">100</span>);         <span class="hljs-built_in">cout</span> << <span class="hljs-string">"free(p)\n"</span>;    <span class="hljs-built_in">free</span>(p);    <span class="hljs-built_in">cout</span> << <span class="hljs-string">"free(p)\n"</span>;    <span class="hljs-built_in">free</span>(p);}</span></code>

这样的问题也许不会立即使你的程序崩溃，那样后果更加严重！！

没有调用子类的析构函数
之前的博客讲过，父类的析构函数最好声明为虚！！

<code class="hljs r has-numbering"><span style="font-size:18px;">ParentClass *pObj = new ChildClass;<span class="hljs-keyword">...</span>delete pObj;</span></code><ul class="pre-numbering" style="display: block;"><li><span style="font-size:18px;">1</span></li><li><span style="font-size:18px;">2</span></li><li><span style="font-size:18px;">3</span></li></ul>

上述代码会造成崩溃，如果父类的析构函数不声明为虚，那么不会调用继承类的析构函数，造成内存泄露。

内存溢出
当我们拷贝字符串的时候，我们常常会用到 memcpy函数。这里特别需要注意的就是字符串结尾的null字符：

<code class="hljs cpp has-numbering"><span style="font-size:18px;"><span class="hljs-keyword">char</span> *p = (<span class="hljs-keyword">char</span> *)<span class="hljs-built_in">malloc</span>(<span class="hljs-built_in">strlen</span>(str));<span class="hljs-built_in">strcpy</span>(p, str);</span></code>

为了躲过这个坑，只需要把 strlen(str) 改为 strlen(str)+1