是否为成员变量自动调用默认构造函数？ [英] Are default constructors called automatically for member variables?

问题描述

说我有这个类：

//Awesome.h
class Awesome
{
    public:
        Awesome();
    private:
        membertype member;
}

//Awesome.cpp
#include "Awesome.h"

Awesome::Awesome()
:member()
{
}

如果省略在 Awesome 的构造函数的初始化列表中的成员（）将构造函数 member 自动调用？只有当我在初始化列表中不包括成员时才会调用

If I omit the member() in the initialization list of the constructor of Awesome, will the constructor of member be called automatically? And is it only called when I don't include member in the initialization list?

推荐答案

如果没有为对象指定初始值设定项，对象就是
default-initialized;如果未执行初始化，则具有
自动或动态存储持续时间的对象具有不确定的值。 [注：带有静态或线程存储持续时间的
对象是零初始化的，
参见3.6.2。 -end note]

If no initializer is specified for an object, the object is default-initialized; if no initialization is performed, an object with automatic or dynamic storage duration has indeterminate value. [ Note: objects with static or thread storage duration are zero-initialized, see 3.6.2. —end note ]

更新未来引用：默认初始化的含义定义为<默认初始化类型为T的对象意味着：` - 如果T是一个
（可能是cv），则p>

Update for future references: Further the meaning of default initialized is defined as

-qualified）类类型（第9条），调用T的默认构造函数
（如果T没有
可访问的默认构造函数，则初始化是不成立的）;
- 如果T是一个数组类型，每个
元素是默认初始化的; - 否则，没有初始化是
。如果程序调用$ b的默认初始化$ b一个const限定类型T的对象，T是一个类型类型，具有
用户提供的默认构造函数。

To default-initialize an object of type T means:
— if T is a (possibly cv-qualified) class type (Clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is an array type, each element is default-initialized;
— otherwise, no initialization is performed.
If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.

此外，它从值初始化变化，参考以下内容： -

Further it varies from value initialized referring this:-

类型T意味着：
如果T是具有用户提供的
构造函数（12.1）的
（可能是cv限定的）类类型（条款9），则T的默认构造函数（和
，如果T没有可访问的缺省
构造函数，则初始化是错误的）;
- 如果T是一个（可能是cv限定的）非联合类
类型没有用户提供的构造函数，那么对象是
零初始化，如果T的隐式声明的默认构造函数
是非平凡的，那么该构造函数称为.Á - 如果T是array
type，then each element is value-initialized; - - 否则
对象为零初始化。

To value-initialize an object of type T means:
— if T is a (possibly cv-qualified) class type (Clause 9) with a user-provided 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 (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T’s implicitly-declared default constructor is non-trivial, that constructor is called.
— if T is an array type, then each element is value-initialized;
— otherwise, the object is zero-initialized.

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