什么时候需要使用初始化器列表初始化C ++类成员？ [英] When do I have to use initializer lists for initializing C++ class members?

问题描述

假设我有
std :: map< std :: string，std :: string> m_someMap 作为A类的私有成员变量

两个问题:(我唯一要问的原因是因为我遇到了代码那个）

1. 这一行的目的是什么：

     A :: A（）：m_someMap（）
     

   这是初步化的，但你必须这样做吗？
   我很困惑。




2. std :: map< std :: string，std :: string> m_someMap ，也是C＃定义int，double等始终初始化为defualt 0，对象为null（至少在大多数情况下）
   那么C ++中的规则是什么？对象是由defualt初始化为null和原语为垃圾？
   当然，我正在使用实例变量。

编辑：

也是，因为大多数人都指出这是一种风格选择，而不是必需的，那就是：

A :: A（）： m_someMap（），m_someint（0），m_somebool（false）

解决方案

m_somemap

1. 你不必。


2. 如果你省略，你会得到什么：一个空的的std ::地图< std :: string，std :: string> ，即该地图中没有元素的有效实例。

m_somebool

1. 您必须将其初始化为 true 或 false 如果您希望具有已知值。布尔是简单的旧数据类型，它们没有构造函数的概念。此外，C ++语言没有为非显式初始化的布尔值指定默认值。


2. 如果省略它，您会得到什么：具有未指定值的布尔成员。你不能这样做，后来使用它的价值。因此，您是强制推荐的政策，您可以初始化所有这种类型的值。

m_someint

1. 如果您希望具有已知值，则必须将其初始化为一个整数值。整数是普通旧数据类型，它们没有构造函数的概念。此外，C ++语言未指定非显式初始化整数的默认值。


2. 如果省略它，你会得到什么：具有未指定值的int成员。你不能这样做，后来使用它的价值。因此，您是一个强烈推荐的政策，您可以初始化所有这种类型的值。

let's say I have std::map< std::string, std::string > m_someMap as a private member variable of class A

Two questions: (and the only reason I'm asking is because I came across code like that)

1. What's the purpose of this line:

   A::A() : m_someMap()
   

   Now I know that this is intialization, but do you have to do this like that? I'm confused.

2. What's the default value of std::map< std::string, std::string > m_someMap, also C# defines that int, double, etc. is always initialized to defualt 0 and objects are to null (at least in most cases) So what's the rule in C++?? are object initialized by defualt to null and primitives to garbage? Of course I'm taking about instance variables.

EDIT:

also, since most people pointed out that this is a style choice and not necessary, what about:

A::A() : m_someMap(), m_someint(0), m_somebool(false)

解决方案

m_somemap

1. You don't have to.
2. What you get if you omit it: An empty std::map< std::string, std::string >, i.e., a valid instance of that map which has no elements in it.

m_somebool

1. You have to initialize it to true or false if you want it to have a known value. Booleans are "plain old data types" and they do not have the concept of a constructor. Moreover, the C++ language does not specify default values for non-explicitly-initialized booleans.
2. What you get if you omit it: a boolean member with an unspecified value. You must not do this and later use its value. Because of that, it is a strongly recommended policy that you initialize all values of this type.

m_someint

1. You have to initialize it to some integer value if you want it to have a known value. Integers are "plain old data types" and they do not have the concept of a constructor. Moreover, the C++ language does not specify default values for non-explicitly-initialized integers.
2. What you get if you omit it: an int member with an unspecified value. You must not do this and later use its value. Because of that, it is a strongly recommended policy that you initialize all values of this type.

这篇关于什么时候需要使用初始化器列表初始化C ++类成员？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！