default initialization

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C++
 
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初始化
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Initialization
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内联汇编
 
默认的初始值提供了一个新的对象.
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Provides the default initial value to a new object.
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目录

[编辑] 语法

T object ; (1)
new T ; (2)

[编辑] 解释

缺省的初始化是在三种情况下进行：
Original:
Default initialization is performed in three situations:
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1)
当声明一个变量，自动存储时间没有初始化
Original:
when a variable with automatic storage duration is declared with no initializer
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2)
创建一个新的表达没有初始化一个对象时，与动态存储时间的
Original:
when an object with dynamic storage duration is created by a new-expression without an initializer
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3)
一个基类的非静态数据成员时，没有提到在构造函数中初始化列表构造函数被调用.
Original:
when a base class or a non-static data member is not mentioned in a constructor 初始化列表 and that constructor is called.
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默认初始化的效果是：1
Original:
The effects of default initialization are:
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  • T是一个类类型，默认构造函数被称为为新对象提供的初始值.
    Original:
    If T is a class type, the 默认构造函数 is called to provide the initial value for the new object.
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  • T如果是一个数组类型的数组，每个元素默认初始化.
    Original:
    If T is an array type, every element of the array is default-initialized.
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  • 否则，什么也不做.....
    Original:
    Otherwise, nothing is done.
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如果T是一个const限定的类型，它必须是一个用户提供的默认构造函数的类类型.
Original:
If T is a const-qualified type, it must be a class type with a user-provided default constructor.
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[编辑] 注释

非类变量，自动和动态存储时间的默认初始化的对象不确定的值（静态和区域设置线程对象得到初始化为零），
Original:
Default initialization of non-class variables with automatic and dynamic storage duration produces objects with indeterminate values (static and thread-locale objects get 初始化为零)
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参考不能默认初始化.
Original:
Reference cannot be default-initialized.
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[编辑] 为例

#include <string>
struct T1 {};
class T2 { 
    int mem;
 public:
    T2() {} // "mem" not in initializer list
};
int n; // This is not default-initialization, the value is zero.
int main()
{
    int n;            // non-class: the value is undeterminate
    std::string s;    // calls default ctor, the value is "" (empty string)
    std::string a[2]; // calls default ctor, creates two empty strings
//    int& r;         // error: default-initializing a reference
//    const int n;    // error: const non-class type
//    const T1 nd;    // error: const class type with implicit ctor
    T1 t1; // ok, calls implicit default ctor
    const T2 t2; // ok, calls the user-provided default ctor 
                 // t2.mem is default-initialized
}


[编辑] 另请参阅

来自“http://zh.cppreference.com/mwiki/index.php?title=cpp/language/default_initialization&oldid=28917”