英文:
How to direct-initialize a derived class with an abstract base?
问题
Without the virtual method, the make_unique line in main compiles ok. But I'm lost as to how to direct-initialize a Derived like this when it's there. Is this possible?
struct Base
{
virtual void method();
};
struct Derived : public Base
{
int i;
};
int main() {
Base base{}; // this works
make_unique<Derived>(Base{}, 3); // this doesn't compile unless the virtual method is removed
}
I don't see any mention of virtual methods in the "Direct initialization" page: https://en.cppreference.com/w/cpp/language/direct_initialization
英文:
Without the virtual method, the make_unique line in main compiles ok. But I'm lost as to how to direct-initialize a Derived like this when it's there. Is this possible?
struct Base
{
virtual void method();
};
struct Derived : public Base
{
int i;
};
int main() {
Base base{}; // this works
make_unique<Derived>(Base{}, 3); // this doesn't compile unless the virtual method is removed
}
I don't see any mention of virtual methods in the "Direct initilization" page: https://en.cppreference.com/w/cpp/language/direct_initialization
答案1
得分: 6
使用virtual方法后，您的Derived类不再是一个聚合，因此无法进行这种形式的初始化。
来自cppreference：
聚合可以是以下类型之一：
- 数组类型
- 类类型（通常为
struct或union），具有- 无用户声明的构造函数
（直到C++11） - 无用户提供、继承或显式的构造函数
（自C++11以来）
（直到C++20） - 无用户声明或继承的构造函数
（自C++20以来） - 无私有或受保护的[直接（自C++17以来）]非静态数据成员
- 无基类
（直到C++17） - 无虚基类（自C++17以来）
- 无私有或受保护的直接基类
（自C++17以来） - 无虚成员函数
- 无默认成员初始化项
（自C++11以来）
（直到C++14）
- 无用户声明的构造函数
英文:
With the virtual method, your Derived class is not an aggregate anymore, so this form of initialization can't be done.
From cppreference:
An aggregate is one of the following types:
- array type
- class type (typically,
structorunion), that has- no user-declared constructors
(until C++11) - no user-provided, inherited, or explicit constructors
(since C++11)
(until C++20) - no user-declared or inherited constructors
(since C++20) - no private or protected [direct (since C++17)] non-static data members
- no base classes
(until C++17) - no virtual base classes (since C++17)
- no private or protected direct base classes
(since C++17) - no virtual member functions
- no default member initializers
(since C++11)
(until C++14)
- no user-declared constructors
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