Default constructors

This page has been machine-translated from the English version of the wiki using Google Translate. The translation may contain errors and awkward wording. Hover over text to see the original version. You can help to fix errors and improve the translation. For instructions click here. Click here for the English version of this page

[编辑] 语法

[编辑] 解释

• 创建对象或数组是静态的，本地线程，并自动存储时间被宣布没有初始化，（T obj;或T arr[10];）

  Original:

  creating objects or arrays of static, thread-local, and automatic storage duration that are declared without an initializer, (T obj; or T arr[10];)

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 动态存储时间创建对象时，新的表达在编写时没有一个初始（new T;）

  Original:

  creating objects of dynamic storage duration when the new-expression is written without an initializer (new T;)

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 创建阵列的的动态存储时间的表达new T[n]

  Original:

  creating arrays of dynamic storage duration with the expression new T[n]

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 创造价值的初始化临时对象与演员表达T().

  Original:

  creating value-initialized temporary objects with the cast expression T().

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.

[编辑] 隐式声明默认构造函数

[编辑] 删除隐式声明的缺省构造函数

• T具有引用类型的成员（无大括号或等于initializer（C++11 起））.

  Original:

  T has a member of reference type (without a brace-or-equal initializer（C++11 起）).

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• T有const成员（没有括号或等于initializer（C++11 起）的）或用户定义的默认构造函数.

  Original:

  T has a const member (without a brace-or-equal initializer（C++11 起）) or a user-defined default constructor.

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• T有一个成员（没有括号或等于initializer（C++11 起）的），其中有一个删除默认的构造函数，其默认的构造函数是含糊不清或无法从这个构造函数.

  Original:

  T has a member (without a brace-or-equal initializer（C++11 起）), which has a deleted default constructor, or its default constructor is ambiguous or inaccessible from this constructor.

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• T有直接或虚基类，有删除默认的构造函数，或者是含糊不清或无法从这个构造函数.

  Original:

  T has a direct or virtual base which has a deleted default constructor, or it is ambiguous or inaccessible from this constructor.

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• T直接或虚基类，它有一个删除析构函数或析构函数，此构造函数是无法从.

  Original:

  T has a direct or virtual base which has a deleted destructor, or a destructor that is inaccessible from this constructor.

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• T与至少一种变体与非平凡的默认constructorunion的部件是一个（C++11 起）.

  Original:

  T is a union with at least one variant member with non-trivial default constructor（C++11 起）.

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• T是一个union和它的变体成员都const.

  Original:

  T is a union and all of its variant members are const.

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.

[编辑] 常用的默认构造函数

• T没有虚成员函数

  Original:

  T has no virtual member functions

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• T没有虚基类

  Original:

  T has no virtual base classes

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• T没有括号或等于初始值设定项（C++11 起）的非静态成员

  Original:

  T has no non-static members with brace-or-equal initializers （C++11 起）

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 每一个直接基础T有一个简单的默认构造函数

  Original:

  Every direct base of T has a trivial default constructor

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 每类类型的非静态成员常用的默认构造函数

  Original:

  Every non-static member of class type has a trivial default constructor

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.

[编辑] 隐式定义的默认构造函数

[编辑] 为例

struct A {
    int x;
    A(int x = 1) : x(x) {} // user-defined default ctor
};
struct B : A {
    // B::B() is implicitly-defined, calls A::A()
};
struct C {
    A obj;
    // C::C() is implicitly-defined, calls A::A()
};
struct D : A {
    D(int y) : A(y) {}
    // D::D() is not declared because another constructor exists
};
struct E : A
{
    E(int y) : A(y) {}
    E() = default; // explicitly defaulted, calls A::A()
};
 
struct F {
    int& ref; // reference member
    const int c; // const member
    // Bad::Bad() is deleted
};
 
int main()
{
    A a;
    B b;
//  D d; // compile error
    E e;
//  F f; // compile error
}