类定义中的引用变量 [英] Reference variable in class definition

问题描述

我正在学习C ++，我读到所有引用必须在声明时初始化，
，并且没有未初始化的引用。但是如果引用变量是一个类成员怎么办？

  class test 
 {
 int& k ; 
}; 
 
 int main（）
 {
 test * abc = new test; 
} 
  

此程序编译并正常运行（g ++，无警告）。然而， abc-> k 是一个引用，但是它初始化为什么？

解决方案

程序是不成形的，因为它构造一个无法初始化引用类型的非静态成员实体的类。

我相信gcc不能编译这个，但我只收到警告non -static reference'int& test :: k'in class without a constructor。

test -POD-struct类型，因为它包含引用成员。 （9 [class] / 4）

new test 默认初始化动态分配的类。 （5.3.4 [expr.new] / 15）

要默认初始化类型 / code>意味着调用隐式声明和隐式定义的默认构造函数。 （8.5 [dcl.init] / 5）

隐含定义的默认构造函数等价于一个带有空的默认构造函数mem- initialized-list 和一个空的函数体。 （12.1 [class.ctor] / 7）

更多：

隐式定义的默认构造函数执行
类的初始化集合，该集合将由用户编写的具有空的mem-initializer-list （12.6）的类的默认构造函数执行。 2）和一个空函数体。如果用户编写的默认构造函数是不成形的，那么程序就会失败。

$ b

否则，实体不被初始化。如果实体是const限定类型，或引用类型，[或...]程序是不成形的。（12.6.2 [class.base.init] / 4）

I am learning C++, and I read that all references must be initialized upon declaration, and there can be no "uninitialized references". But what if the reference variable is a class member?

class test
{
    int &k;
};

int main()
{
    test *abc = new test;
}

This program compiles and runs normally (in g++, no warnings). However, abc->k is a reference, but what is it initialized to? Or, is it an "uninitialized reference" of some sort, or something else?

解决方案

The program is ill-formed because it constructs a class that fails to initialize a non-static member entity of reference type.

I believe that gcc should fail to compiler this, but I only received the warning "non-static reference ‘int& test::k’ in class without a constructor".

test is a non-POD-struct type as it contains a reference member. (9 [class] / 4)

new test default-initializes the dynamically allocated class. (5.3.4 [expr.new] / 15)

To default-initialize an object of type test means to call the implicitly declared and implicitly defined default constructor. (8.5 [dcl.init] / 5)

The implicitly defined default constructor is equivalent to a default constructor with an empty mem-initialized-list and an empty function body. (12.1 [class.ctor] / 7)

Further more:

The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with an empty mem-initializer-list (12.6.2) and an empty function body. If that user-written default constructor would be ill-formed, the program is ill-formed.

If an entity is not name in a mem-initializer-list and the member is not of class type [with further restrictions] then the entity is not initialized.

Otherwise, the entity is not initialized. If the entity is of const-qualified type, or reference type, [or ...] the program is ill-formed." (12.6.2 [class.base.init] / 4)

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