从cppref所说的约
value initialization
if T is a class type with no default constructor or with a user-provided or deleted default constructor,the object is default-initialized;
但由于该类类型已删除默认构造函数,该对象如何进行默认初始化？
据我所知,类类型的默认初始化需要访问默认构造函数.如果我们有：
struct A {
A() = delete;
int k;
};
然后A * a =新A;会失败,A * a = new A();也是如此.
但是A a {};没关系但为什么？根据cppreference
Otherwise,If the braced-init-list is empty and T is a class type with a default constructor,value-initialization is performed.
解决方法
我认为标准只是意味着“如果T是带有删除的默认构造函数的类类型,那么转到默认初始化”.它最终会失败,因为删除了为默认初始化选择的构造函数.它用于区分第二种情况,即“如果T是具有默认构造函数的类型,既不是用户提供也不是删除”,对于这种情况,首先执行零初始化,然后如果T具有非默认初始化-trivial默认构造函数.
A a{}is OK,but why?
因为当A是aggregate type聚合时,执行初始化.请注意,自C 11起,允许显式删除的构造函数用于聚合类型.
In all cases,if the empty pair of braces {} is used and T is an aggregate type,aggregate-initialization is performed instead of value-initialization.
和
An aggregate is one of the following types:
- array type
- class type (typically,struct or union),that has
- no private or protected non-static data members
- no user-provided
,inherited,or explicit (since C++17)constructors(explicitly defaulted or deleted constructors are allowed) (since C++11)
