什么是c ++中对象的动态初始化？ [英] What is dynamic initialization of object in c++?

问题描述

什么是c ++中对象的动态初始化？

请用简单的示例解释...

解决方案

动态初始化是指初始化值在编译时是未知的。

示例，

  int factorial（int n）
 {
 if（n< 0）return -1; //表示输入错误
 else if（n == 0）return 1; 
 else return n * factorial（n-1）; 
} 
 
 int const a = 10; // static initialization 
 // 10在编译时是已知的。其10！ 
 
 int const b = factorial（8）; //动态初始化
 // factorial（8）在编译时不知道，
 //而是在运行时计算。 
  

也就是说，静态初始化通常包含常量表达式已知在编译时），而动态初始化涉及非常量表达式。

  static int c; // this is also静态初始化（带零）！来自C ++标准（2003）的
  

§3.6.2/ 1说：

$ b具有静态存储持续时间的对象
（3.7.1）在任何其他初始化之前应为零初始化
（8.5）
发生。具有常量
表达式的零初始化和
初始化统称为
静态初始化;所有其他
初始化为动态
初始化。

• 静态初始化：使用常量表达式进行零初始化或初始化
任何其他初始化都是动态初始化。

还要注意，同一个变量在静态初始化后可以动态初始化。例如，请参阅以下代码：

  int d = factorial（8）; 
 int main（）
 {
} 
  

c $ c> d 是一个全局变量，它有静态存储。这意味着，根据§3.6.2.1，它在发生任何其他初始化之前发生的静态初始化阶段被初始化为0。然后，在运行时，它使用从函数 factorial（）返回的值进行动态初始化。

这意味着，全局对象可以初始化两次：一次是通过静态初始化（这是零初始化），稍后，在运行时，可以动态初始化。 / p>

What is dynamic initialization of objects in c++?

Please explain with an simple example...

解决方案

Dynamic initialization is that in which initialization value isn't known at compile-time. It's computed at runtime to initialize the variable.

Example,

int factorial(int n)
{
     if ( n < 0 )       return -1; //indicates input error
     else if ( n == 0 ) return 1;
     else               return n * factorial(n-1);
}

int const a = 10 ; //static initialization 
             //10 is known at compile time. Its 10!

int const b = factorial(8); //dynamic initialization 
                      //factorial(8) isn't known at compile time,
                      //rather it's computed at runtime.

That is, static-initialization usually involves constant-expression (which is known at compile-time), while dynamic-initialization involves non-constant expression.

static int c;//this is also static initialization (with zero)!

§3.6.2/1 from the C++ Standard (2003) says,

Objects with static storage duration (3.7.1) shall be zero-initialized (8.5) before any other initialization takes place. Zero-initialization and initialization with a constant expression are collectively called static initialization; all other initialization is dynamic initialization.

So there are two kind of initializations:

• Static initialization : Its either zero-initialization or initialization with a constant expression
• Any other initialization is dynamic initialization.

Also note that the same variable can be dynamically-initialized after it has been statically-initialized. For example, see this code:

int d = factorial(8);
int main()
{
}

Since d is a global variable, it has static storage. That means, according to §3.6.2.1 it's initialized to 0 at the static-initialization phase which occurs before any other initialization takes place. Then later, at runtime, it's dynamically-initialized with the value returned from the function factorial().

That means, global objects can be initialized twice: once by static initialization (which is zero-initialization) and later, at runtime, they can be dynamically-initialized.

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