如果基类是通过模板继承的，则在派生构造函数中初始化基类成员

Question

鉴于我知道我的基类中存在某个成员a，我如何使用模板派生类来引用它？即使我完全符合资格a，它也不起作用：

演示

#include <iostream>
#include <string_view>
#include <memory>
#include <string>

struct base {
    int a;
};

template <typename ImplType>
struct derived : public ImplType {

    derived()
        : a{2}
    {}

    auto print() {
        std::cout << ImplType::a << std::endl;
    }
};

int main() {
    derived<base> d{};
    d.print();
}

产量：

<source>:14:11: error: class 'derived<ImplType>' does not have any field named 'a'
   14 |         : a{2}
      |           ^

Answer 1

使用聚合基类初始化

#include <iostream>

struct base {
    int a;
};

template <typename ImplType>
struct derived : public ImplType {
    derived()
        : ImplType{2}
    {}

    auto print() {
        std::cout << ImplType::a << std::endl;
    }
};

int main() {
    derived<base> d{};
    d.print();
}