为什么默认的默认构造函数取决于它是在课堂内还是外部声明的？
在下面的示例中，struct a没有默认的构造函数。因此，struct b和struct c从中继承的c 无法获得编译器生成的默认构造函数：
struct A {
A(int) {}
};
struct B : A {
B() = default; //#1
};
struct C : A {
C();
};
C::C() = default; //#2
＃1。 In struct B, defaulted default constructor is declared inside the class, and all compilers accept it, with only Clang showing a warning saying that explicitly defaulted default constructor is implicitly deleted [-Wdefaulted-function-删除]
＃2。但是在struct C之外声明的默认默认构造函数生成了编译器错误。演示： https://gcc.godbolt.org/z/z/ z/3EGC4RTQE
构造函数被默认：课堂内还是外部？
In the following example, struct A does not have default constructor. So both struct B and struct C inherited from it cannot get compiler-generated default constructor:
struct A {
A(int) {}
};
struct B : A {
B() = default; //#1
};
struct C : A {
C();
};
C::C() = default; //#2
#1. In struct B, defaulted default constructor is declared inside the class, and all compilers accept it, with only Clang showing a warning saying that explicitly defaulted default constructor is implicitly deleted [-Wdefaulted-function-deleted]
#2. But defaulted default constructor declared outside of struct C generates a compiler error. Demo: https://gcc.godbolt.org/z/3EGc4rTqE
Why does it matter where the constructor is defaulted: inside or outside the class?
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请注意，如果您实际尝试并实例化
b，那么您还会删除b :: b（）的错误： https://gcc.godbolt.org/z/jdkzv7zvdd差异的原因可能是当您声明
c cc时，构造函数，c的用户（假设定义在另一个翻译单元中）无法知道构造函数实际上已删除。充其量，这会导致一些令人困惑的链接错误，最坏的情况是在运行时崩溃。在b的情况下clang）将其声明为违约。Note that if you actually try and instantiate
Bthen you'll also get the error thatB::B()is deleted: https://gcc.godbolt.org/z/jdKzv7zvdThe reason for the difference is probably that when you declare the
Cconstructor, the users ofC(assuming the definition is in another translation unit) have no way of knowing that the constructor is in fact deleted. At best this would lead to some confusing linker error, at worst it'd just crash at runtime. In the case ofBall users ofBwill immediately be able to tell that the default constructor is deleted so it does no harm (even if it makes no sense as warned by clang) to declare it as defaulted.