使用花括号初始化程序的默认参数 [英] Default argument using curly braces initializer

问题描述

我的这段代码似乎运行良好：

  class foo {/ *一些成员变量和函数* /}; 
 void do_somthing（foo x = {}）{} 
 int main（）{
 do_somthing（）; 
} 
  

我以前使用 void do_somthing（foo x = foo（））{} 默认为 x 自变量，但我以这种方式看到 = {} 在某些书或在线示例中（不记得了）。使用它完全可以吗？两种方法之间有什么区别？

解决方案

foo x = foo（）是复制初始化，

从另一个对象初始化一个对象

和 foo（ ）是值初始化。

这是使用空的初始化程序构造变量时执行的初始化。

foo x = {} 是汇总初始化。

如果初始化子句的数量少于
成员和基数的数量（自C ++ 17起），则从braced-init-list

初始化聚合。或初始值设定项列表完全为
为空，其余成员和基数（自C ++ 17起）由其缺省初始值设定项（如果在类定义中提供，则
初始化，否则由
初始化） ++ 14）通过空列表执行
值初始化。

在这种情况下（两个值都已初始化）结果是相同的。

在这种情况下，值初始化的效果是：

如果T是具有默认构造函数的类类型，该构造函数既不是用户提供的，也不是删除的d（也就是说，它可能是具有隐式定义或默认默认构造函数的类），该对象被零初始化

最后，在这种情况下，零初始化的影响是：

如果T是标量类型，则对象的初始值为整数$ b $

如果T是非联合类类型，则所有基类和非静态数据
成员都将初始化为零。 ，并且所有填充都初始化为零
位。

I have this snippet of code which seems to work well:

class foo{/* some member variables and functions*/};
void do_somthing(foo x={}){}
int main(){
  do_somthing();
}

I used to use void do_somthing(foo x=foo()){} to default the x argument but I see this way ={} in some book or online example(can not remember). Is it totally ok to use it? Is there any difference between the two methods?

解决方案

foo x=foo() is copy initialization,

Initializes an object from another object

and foo() is value initialization.

This is the initialization performed when a variable is constructed with an empty initializer.

foo x={} is aggregate initialization.

Initializes an aggregate from braced-init-list

If the number of initializer clauses is less than the number of members and bases (since C++17) or initializer list is completely empty, the remaining members and bases (since C++17) are initialized by their default initializers, if provided in the class definition, and otherwise (since C++14) by empty lists, which performs value-initialization.

So the result is the same in this case (both value-initialized).

And the effects of value initialization in this case are:

if T is a class type with a default constructor that is neither user-provided nor deleted (that is, it may be a class with an implicitly-defined or defaulted default constructor), the object is zero-initialized

Finally the effects of zero initialization in this case are:

If T is a scalar type, the object's initial value is the integral constant zero explicitly converted to T.

If T is an non-union class type, all base classes and non-static data members are zero-initialized, and all padding is initialized to zero bits. The constructors, if any, are ignored.

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