用空花括号初始化

Question

第一次尝试，一切正常：

class Base {
 public:
    Base() {std::cout << "default ctor!\n"; }
};
...
Base b{};
Base b_one = {};

另一种实现方式（添加explicit）：

class Base {
 public:
    explicit Base() {std::cout << "default ctor!\n"; }
};
...
Base b{};
Base b_one = {};  // error! Why?

我在 cppreference 上读到，在这两种情况下都将使用默认初始化并且没有区别。

来自列表初始化：

Otherwise, If the braced-init-list is empty and T is a class type with a default constructor, value-initialization is performed.

从值初始化：

if T is a class type with no default constructor or with a user-provided or deleted default constructor, the object is default-initialized;

Answer 1

I have read on cppreference that in both cases default initialization would be used and no diffences.

不，它们不一样。准确地说，Base b{};是direct-list-initialization, while Base b_one = {}; is copy-list-initialization；对于复制列表初始化，只能调用非explicit构造函数。

（强调我的）

direct-list-initialization (both explicit and non-explicit constructors are considered)

copy-list-initialization (both explicit and non-explicit constructors are considered, but only non-explicit constructors may be called)

用空花括号初始化

Initialization with empty curly braces

c++

c++11

constructor

initialization

explicit