clang error : note: candidate constructor (the implicit move constructor) not viable:

Question

我有一个导致 clang 发出错误的以下用例。

#include <iostream>
#include <type_traits>

class A
{
    public:
        int x;
        A(int g):x(g){}
};

class B : public A
{
    public:
        int y;
        B(int p):A(p),y(p) {}
};

class Holder
{
    template<typename T>
        Holder(typename std::enable_if<!std::is_same<T,A>::value
                     && std::is_base_of< A , T>::value >::type&& val)
        : b (std::forward<T>(val))
        {}


    private:
        B b;
};

int main()
{
    B b(10);

    Holder h(b);
}

错误如下

forward.cpp:35:12: error: no matching constructor for initialization of 'Holder'
    Holder h(b);
           ^ ~
forward.cpp:18:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'B' to 'const Holder' for 1st argument
class Holder
      ^
forward.cpp:18:7: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'B' to 'Holder' for 1st argument
class Holder
      ^
forward.cpp:21:9: note: candidate template ignored: couldn't infer template argument 'T'
        Holder(typename std::enable_if<!std::is_same<T,A>::value
        ^
1 error generated.

使用 clang++ --std=c++11 forward.cpp.

编译时

我在这里错过了什么？

我已经咨询了 this and this 个问题，但是它们似乎没有解决我的用例。

Answer 1

您忘记指定 std::enable_if 的第二个参数（默认为 void）

template<typename T>
Holder(typename std::enable_if<!std::is_same<T, A>::value
                 && std::is_base_of< A , T>::value, T>::type&& val)
    : b (std::forward<T>(val))
    {}

但无论如何，T 将处于不可推导的上下文中。你可以

template<typename T>
Holder(T&& val,
       typename std::enable_if<!std::is_same<typename std::decay<T>::type, A>::value
                               && std::is_base_of<A, typename std::decay<T>::type>::value,
                               std::nullptr_t>::type = nullptr)
    : b (std::forward<T>(val))
    {}

或

template<typename T,
         typename std::enable_if<!std::is_same<typename std::decay<T>::type, A>::value
                               && std::is_base_of<A, typename std::decay<T>::type>::value,
                               std::nullptr_t>::type = nullptr>
Holder(T&& val)
    : b (std::forward<T>(val))
    {}

clang error : note: candidate constructor (the implicit move constructor) not viable:

clang error : note: candidate constructor (the implicit move constructor) not viable:

c++

constructor

sfinae

c++11

clang++