模板化继承类中的C ++成员变量初始化

时间:2016-08-11 01:44:25

标签: c++ compiler-errors c++14 variadic-templates template-meta-programming

我试图找出这段代码的错误。基本上type2继承自type1<T>, type1<T2>,我想从其中一个基类初始化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那里得到以下错误:

    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希望将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)) {}
                                                   ^

1 个答案:

答案 0 :(得分:1)

您无法在构造函数初始值设定项列表中初始化基类成员。

在Standardese中,T::value(std::move(arg.value))中的type2(T&& arg) : T::value(std::move(arg.value)) {}称为 mem-initializer T::value称为 mem-initializer-id 。根据{{​​3}},

  

除非 mem-initializer-id 命名构造函数的类,构造函数类的非静态数据成员,或该类的直接或虚拟基类, mem-initializer 格式不正确。

您可以调用基类的构造函数,并让它初始化该成员。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))即可。 [class.base.init]p2