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scoping.html
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<title>scoping</title>
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<body>
<h1>作用域</h1>
<h2>局部变量</h2>
<pre><code>将函数变量尽可能置于最小作用域内, 并在变量声明时进行初始化.
</code></pre>
<ul>
<li>
<p>C++ 允许在函数的任何位置声明变量. 我们提倡在尽可能小的作用域中声明变量, 离第一次使用越近越好. 这使得代码浏览者更容易定位变量声明的位置, 了解变量的类型和初始值. 特别是，应使用初始化的方式替代声明再赋值, 比如:</p>
<pre><code>int i;
i = f(); // 坏——初始化和声明分离
int j = g(); // 好——初始化时声明
</code></pre>
</li>
<li>
<p>GCC 正确实现了 <code>for (int i = 0; i < 10; ++i)</code> (<code>i</code> 的作用域仅限 <code>for</code> 循环内), 所以其他 <code>for</code> 循环中可以重新使用 <code>i</code>. 在 <code>if</code> 和 <code>while</code> 等语句中的作用域声明也是正确的, 如:</p>
<p>while (const char* p = strchr(str, ‘/’)) str = p + 1;</p>
</li>
<li>
<p>如果变量是一个对象, 每次进入作用域都要调用其构造函数,每次退出作用域都要调用其析构函数， 则应该将变量声明放在循环外从而提高效率.</p>
<pre><code>// 低效的实现
for (int i = 0; i < 1000000; ++i)
{
Foo f; // 构造函数和析构函数分别调用 1000000 次!
f.DoSomething(i);
}
// 高效的实现
Foo f; // 构造函数和析构函数只调用 1 次
for (int i = 0; i < 1000000; ++i)
{
f.DoSomething(i);
}
</code></pre>
</li>
</ul>
<h2>静态和全局变量</h2>
<ul>
<li>禁止使用 <code>class</code> 类型的静态或全局变量: 它们会导致很难发现的bug和不确定的构造和析构函数调用顺序.</li>
<li>静态生存周期的对象, 包括全局变量, 静态变量, 静态类成员变量,以及函数静态变量, 都必须是原生数据类型 (POD : Plain Old Data): 只能是int, char, float, 和 void, 以及 POD 类型的数组/结构体/指针.</li>
<li>
<p>永远不要使用函数返回值初始化静态变量; 不要在多线程代码中使用非 <code>const</code>
的静态变量.</p>
<p>不幸的是, 静态变量的构造函数, 析构函数以及初始化操作的调用顺序在 C++标准中未明确定义, 甚至每次编译构建都有可能会发生变化, 从而导致难以发现的bug. 比如, 结束程序时, 某个静态变量已经被析构了, 但代码还在跑, 其它线程很可能试图访问该变量, 直接导致崩溃.
所以, 我们只允许 POD 类型的静态变量. 本条规则完全禁止 <code>vector</code> (使用 C数组替代), <code>string</code> (使用 <code>const char*</code>),及其它以任意方式包含或指向类实例的东东, 成为静态变量. 出于同样的理由,我们不允许用函数返回值来初始化静态变量.</p>
</li>
<li>如果你确实需要一个<code>class</code> 类型的静态或全局变量, 可以考虑在 <code>main()</code>函数或<code>pthread\_once()</code>内初始化一个你永远不会回收的指针.</li>
</ul>
</body>
</html>
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