“int a = int（）;”必然给我一个零？ [英] Does "int a = int();" necessarily give me a zero?

问题描述

int a = int（）; 必须给我一个零吗？

如果 int 被替换为 char double ， bool 或指针类型？

> int a = int（）; 必须给我一个零吗？

该标准保证它给你零。

这被称为值初始化。对于 int 类型，价值初始化基本上是零初始化。

在语言标准中指定的位置在哪里？

< blockquote>

这些规则在第8.5节中的标准中有明确规定。
我将在这里将相关的内容引用到Q：

C ++ 03：8.5初始化程序

第7项：

初始化程序是一组空括号的对象， ），应进行值初始化。

价值初始化&零初始化在 8.5 Para 5 中定义为：

要 value-initialize 类型T的对象意味着：

- 如果T是带有用户声明的构造函数（12.1）的类类型（第9节），则默认
调用T的构造函数（如果T没有可访问的
默认构造函数，则初始化是不成形的）;

- 如果T是一个没有用户声明的构造函数的非联合类类型，那么T的每个非静态
数据成员和基类组件都被初始化;

- 如果T是数组类型，则每个元素都被初始化;

- 否则，对象为零初始化

要将类型T的对象初始化表示：

- 如果T是标量类型（3.9），则将对象设置为转换为T ;

- 如果T是非联合类类型，则每个非静态数据成员和每个基类子对象

都是零初始化的;

- if T是一个联合类型，对象的第一个命名数据成员是零初始化的;

- 如果T是一个数组类型，每个元素都是零初始化的;

- 如果T是参考类型，不执行初始化。

注意：粗体文字由我强调。

Does int a = int(); necessarily give me a zero?

How about if int is replaced by char, double, bool or pointer type?

Where is this specified in the language standard, please?

解决方案

Does int a = int(); necessarily give me a zero?

Yes, the standard guarantees that it gives you zero.
This is known as Value Initialization. For the type int, Value Initialization basically ends up being an Zero Initialization.

Where is this specified in the language standard, please?

The rules are clearly specified in the standard in section 8.5. I will quote the relevant ones to the Q here:

C++03: 8.5 Initializers
Para 7:

An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized.

Value Initialization & Zero Initialization are defined in 8.5 Para 5 as:

To value-initialize an object of type T means:

— if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is a non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized;
— if T is an array type, then each element is value-initialized;
— otherwise, the object is zero-initialized

To zero-initialize an object of type T means:

— if T is a scalar type (3.9), the object is set to the value of 0 (zero) converted to T;
— if T is a non-union class type, each nonstatic data member and each base-class subobject
is zero-initialized;
— if T is a union type, the object’s first named data member is zero-initialized;
— if T is an array type, each element is zero-initialized;
— if T is a reference type, no initialization is performed.

Note: The bold texts are emphasized by me.

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