模板继承中的 C++ 成员变量初始化 class

Question

我想弄清楚这段代码有什么问题。基本上 type2 继承自 type1<T>, type1<T2>，我想从基础类.

之一初始化 value 成员

#include <utility>

template <typename T>
struct type1 {
    using base_type = T;

    template <typename... Args> type1(Args&&... args) : value(std::forward<Args>(args)...) {}

    T value;
};

template <typename... Ts>
struct type2 : public Ts... {
    template <typename T>
    type2(T&& arg) : T::value(std::move(arg.value)) {}
};

int main()
{
    type2<type1<int>, type1<double>> x(type1<int>(10));
    return 0;
}

但是我从 clang 得到以下错误：

    Error(s):

source_file.cpp:15:25: error: typename specifier refers to non-type member 'value' in 'type1<int>'
    type2(T&& arg) : T::value(std::move(arg.value)) {}
                        ^~~~~
source_file.cpp:20:38: note: in instantiation of function template specialization 'type2<type1<int>, type1<double> >::type2<type1<int> >' requested here
    type2<type1<int>, type1<double>> x(type1<int>(10));
                                     ^
source_file.cpp:9:7: note: referenced member 'value' is declared here
    T value;
      ^
1 error generated.

为什么 clang 说 typename specifier refers to non-type member 'value' in 'type1<int>'？ Gcc 想将（也可能是 clang）value 视为一种类型：

Error(s):

source_file.cpp: In instantiation of ‘type2<Ts>::type2(T&&) [with T = type1<int>; Ts = {type1<int>, type1<double>}]’:
source_file.cpp:20:54:   required from here
source_file.cpp:15:51: error: no type named ‘value’ in ‘struct type1<int>’
     type2(T&& arg) : T::value(std::move(arg.value)) {}
                                                   ^

Answer 1

您不能在构造函数初始化列表中初始化基 class 的成员。

在标准语中，type2(T&& arg) : T::value(std::move(arg.value)) {} 中的 T::value(std::move(arg.value)) 称为 mem-initializer，而 T::value 称为 内存初始化程序 ID。根据[class.base.init]p2、

Unless the mem-initializer-id names the constructor's class, a non-static data member of the constructor's class, or a direct or virtual base of that class, the mem-initializer is ill-formed.

可以调用基class的构造函数，让它初始化成员。在这种特定情况下，您只需将 T::value(std::move(arg.value)) 更改为 T(std::move(arg.value))。 Demo.

模板继承中的 C++ 成员变量初始化 class

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