C++ 变量作用域

一般来说有三个地方可以定义变量：

在函数或一个代码块内部声明的变量，称为局部变量。
在函数参数的定义中声明的变量，称为形式参数。
在所有函数外部声明的变量，称为全局变量。

作用域是程序的一个区域，变量的作用域可以分为以下几种：

局部作用域：在函数内部声明的变量具有局部作用域，它们只能在函数内部访问。局部变量在函数每次被调用时被创建，在函数执行完后被销毁。
全局作用域：在所有函数和代码块之外声明的变量具有全局作用域，它们可以被程序中的任何函数访问。全局变量在程序开始时被创建，在程序结束时被销毁。
块作用域：在代码块内部声明的变量具有块作用域，它们只能在代码块内部访问。块作用域变量在代码块每次被执行时被创建，在代码块执行完后被销毁。
类作用域：在类内部声明的变量具有类作用域，它们可以被类的所有成员函数访问。类作用域变量的生命周期与类的生命周期相同。

注意：如果在内部作用域中声明的变量与外部作用域中的变量同名，则内部作用域中的变量将覆盖外部作用域中的变量。

局部变量

在函数或一个代码块内部声明的变量，称为局部变量。它们只能被函数内部或者代码块内部的语句使用。下面的实例使用了局部变量：

实例

#include <iostream> using namespace std; int main () { // 局部变量声明 int a, b; int c; // 实际初始化 a = 10; b = 20; c = a + b; cout << c; return 0; }

全局变量

在所有函数外部定义的变量（通常是在程序的头部），称为全局变量。全局变量的值在程序的整个生命周期内都是有效的。

全局变量可以被任何函数访问。也就是说，全局变量一旦声明，在整个程序中都是可用的。下面的实例使用了全局变量和局部变量：

实例

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#include <iostream> using namespace std; // 全局变量声明 int g; int main () { // 局部变量声明 int a, b; // 实际初始化 a = 10; b = 20; g = a + b; cout << g; return 0; }

在程序中，局部变量和全局变量的名称可以相同，但是在函数内，局部变量的值会覆盖全局变量的值。下面是一个实例：

实例

#include <iostream> using namespace std; // 全局变量声明 int g = 20; int main () { // 局部变量声明 int g = 10; cout << g; return 0; }

当上面的代码被编译和执行时，它会产生下列结果：

初始化局部变量和全局变量

当局部变量被定义时，系统不会对其初始化，您必须自行对其初始化。定义全局变量时，系统会自动初始化为下列值：

数据类型初始化默认值

int	0
char	'\\0'
float	0
double	0
pointer	NULL

正确地初始化变量是一个良好的编程习惯，否则有时候程序可能会产生意想不到的结果。

块作用域指的是在代码块内部声明的变量：

实例

#include <iostream> int main() { int a = 10; { int a = 20; // 块作用域变量 std::cout << "块变量: " << a << std::endl; } std::cout << "外部变量: " << a << std::endl; return 0; }

以上实例中，内部的代码块中声明了一个名为 a 的变量，它与外部作用域中的变量 a 同名。内部作用域中的变量 a 将覆盖外部作用域中的变量 a，在内部作用域中访问 a 时输出的是20，而在外部作用域中访问 a 时输出的是 10。

当上面的代码被编译和执行时，它会产生下列结果：

块变量: 20
外部变量: 10

类作用域

类作用域指的是在类内部声明的变量：

实例

#include <iostream> class MyClass { public: static int class_var; // 类作用域变量 }; int MyClass::class_var = 30; int main() { std::cout << "类变量: " << MyClass::class_var << std::endl; return 0; }

以上实例中，MyClass 类中声明了一个名为 class_var 的类作用域变量。可以使用类名和作用域解析运算符 :: 来访问这个变量。在 main() 函数中访问 class_var 时输出的是 30。

C++ 变量作用域