通过静态局部变量看C，C++，C#，Java，PHP的特点
0 问题由来
对这个问题的思考来自于面向对象设计中的单例设计模式的实现。
C++中单例模式实现标准代码为：
#include <stdio.h>int init(){ printf("init()\n"); return 22;}int GetTheOnly(){ static int x = init(); return x;}int main(){ int only = GetTheOnly(); return 0;}</stdio.h>在获取实例函数GetTheOnly()中, 静态局部变量用户存储唯一实例，并且初始化时直接使用init()函数动态初始化。
看起来如此简单，但同样的代码作为C来编译却不能通过，编译器在编译 static int x = init()这一行时报错:
错误：初始值设定元素不是常量
可见，C语言中的静态局部变量初始化时必须以常量赋值，也就是说这个初值必须在编译器就能确定。
细想一下，以调用函数来初始化静态变量，C++必须保证init();只运行一次。为达此目的，C++编译器必须增加额外代码，我能想到的C++编译器对于static int x = init(); 可能增加的伪代码如下：
static int x; static char flag = 0; if(flag == 0){ x = init();flag = 1; } return x;
进而想看看其他几门语言对这个问题的处理，随后使用C#,Java,PHP进行了类似的试验，得出了小小的结论。
1 C#，Java根本就不支持静态局部变量两者只支持静态成员变量，不支持函数内的静态局部变量。想想也对，静态局部变量几乎总是可以使用静态成员变量来代替。
C#测试代码：
using System;namespace ConsoleApplication2{ class Program { static int x = 0; static int init() { Console.WriteLine("init()"); return 22; } static int GetTheOnly() { // static int x = 0; static local variable is NOT supported by C#. if(x == 0) { x = init(); } return x; } static void Main(string[] args) { GetTheOnly(); GetTheOnly(); } }}Java测试代码：
public class t{ public static int init(){ System.out.println("init()\n"); return 22; } private static int x = 0; public static int getTheOnly(){ //static int x = 0; This line cannot be compiled, static local variable is NOT supported by Java if(x==0){ x = init(); } return x; } public static void main(String[] args) { getTheOnly(); getTheOnly(); }}2 PHP对待静态局部变量与C相同，只支持以常量初始化
PHP测试代码：
<?phpfunction init(){ echo "init()\n"; return 22;}function getTheOnly(){ // static $x = init(); PHP only supports initializing static local vairalbel with constant. static $x = 0; if($x==0){ $x = init(); } return $x;}getTheOnly();getTheOnly();3 一点思考
通过静态局部变量这一非常小的语言细节，可以发现这几门语言的特点。
C++ 编译器是勤劳全面的全才，尽量为用户提供更多的语言功能，而做到这些必须偷偷为用户生成代码，从而导致C++语言的复杂性和“冰山效应”。（想想C++的多重继承，栈上对象，复制构造。。。）
Java和C#则注重易用性，避免二义性，对于同样功能，只给用户一个正确的选择。（想想单继承、对象只能建立在堆上，垃圾回收）
C则始终保持其简洁高效透明，编译器老实巴交，看到了代码也基本上就能预测到生成的汇编。
PHP的结构化部分模仿C的语法，所以很多特性与之类似，然其毕竟是解释性语言，特别是变量名，类名等本身就可以作为变量的解释性语言特性，让其变得异常灵活。面向对象部分则模仿Java的语法，同时又充分体现了解释语言的特点。
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