一个关于 c++11 中引用的问题

[linux40]

template <size_t N>
struct tuple_helper_get {
    template <typename T, typename... Args> constexpr static
    auto get_ref(tuple_helper<T, Args...> &t) noexcept
        -> decltype(tuple_helper_get<N - 1>::get_ref
                    (static_cast<tuple_helper<Args...> &>(t)))
    {
        return tuple_helper_get<N - 1>::get_ref(
            static_cast<tuple_helper<Args...> &>(t));    //!required from ‘ static constexpr decltype (glgltinySTL::tuple_helper_get<(N - 1)>::get_ref(static_cast<glgltinySTL::tuple_helper<Args ...>&>(t))) glgltinySTL::tuple_helper_get<N>::get_ref(glgltinySTL::tuple_helper<Arg, Args ...>&) [with T = int; Args = {char, unsigned int}; long unsigned int N = 2ul; decltype (glgltinySTL::tuple_helper_get<(N - 1)>::get_ref(static_cast<glgltinySTL::tuple_helper<Args ...>&>(t))) = unsigned int&]’
    }
    //...
};

template <>
struct tuple_helper_get<1> {
    template <typename T1, typename T2> constexpr static
    T2 &get_ref(tuple_helper<T1, T2> &t) noexcept
    {
        return (t.t2);    //!错误：用类型为‘ unsigned int ’的右值初始化类型为‘ unsigned int&’的非常量引用无效
    }                     //!错误： body of constexpr function ‘ static constexpr T2& glgltinySTL::tuple_helper_get<1ul>::get_ref(glgltinySTL::tuple_helper<T1, T2>&) [with T1 = char; T2 = unsigned int]’ not a return-statement
    //...
};

template <size_t N, typename... Args> constexpr
auto get(tuple<Args...> &t) noexcept
    -> decltype(tuple_helper_get<N>::get_ref
                (static_cast<tuple_helper<Args...> &>(t)))
{
    return tuple_helper_get<N>::get_ref(
        static_cast<tuple_helper<Args...> &>(t));    //!required from ‘ constexpr decltype (glgltinySTL::tuple_helper_get<N>::get_ref(static_cast<glgltinySTL::tuple_helper<OtherArgs ...>&>(t))) glgltinySTL::get(glgltinySTL::tuple<Args ...>&) [with long unsigned int N = 2ul; Args = {int, char, unsigned int}; decltype (glgltinySTL::tuple_helper_get<N>::get_ref(static_cast<glgltinySTL::tuple_helper<OtherArgs ...>&>(t))) = unsigned int&]’
}

void test_tuple()
{
    glgltinySTL::pair<int, unsigned> p{ -1, 1 };
    glgltinySTL::tuple<int, char, unsigned> t{ -1, 'a', 1 };
    std::cout << glgltinySTL::get<0>(t) << std::endl;
    std::cout << glgltinySTL::get<1>(t) << std::endl;
    std::cout << glgltinySTL::get<2>(t) << std::endl;        //!required from here
    std::cout << glgltinySTL::get<0>(p) << std::endl;
    std::cout << glgltinySTL::get<1>(p) << std::endl;
}

明明每个函数都传递的引用，怎么变成右值了？而且，注意到其他几个都没事，可能是
template <size_t N>
struct tuple_helper_get {//...};的问题。。。

去掉了所有 constexpr 之后
```
In instantiation of ‘ static T2& glgltinySTL::tuple_helper_get<1ul>::get_ref(glgltinySTL::tuple_helper<T1, T2>&) [with T1 = char; T2 = unsigned int]’:
required from ‘ static decltype (glgltinySTL::tuple_helper_get<(N - 1)>::get_ref(static_cast<glgltinySTL::tuple_helper<Args ...>&>(t))) glgltinySTL::tuple_helper_get<N>::get_ref(glgltinySTL::tuple_helper<Arg, Args ...>&) [with T = int; Args = {char, unsigned int}; long unsigned int N = 2ul; decltype (glgltinySTL::tuple_helper_get<(N - 1)>::get_ref(static_cast<glgltinySTL::tuple_helper<Args ...>&>(t))) = unsigned int&]’
required from ‘ decltype (glgltinySTL::tuple_helper_get<N>::get_ref(static_cast<glgltinySTL::tuple_helper<OtherArgs ...>&>(t))) glgltinySTL::get(glgltinySTL::tuple<Args ...>&) [with long unsigned int N = 2ul; Args = {int, char, unsigned int}; decltype (glgltinySTL::tuple_helper_get<N>::get_ref(static_cast<glgltinySTL::tuple_helper<OtherArgs ...>&>(t))) = unsigned int&]’
required from here
错误：用类型为‘ unsigned int ’的右值初始化类型为‘ unsigned int&’的非常量引用无效
return t.t2;
^
```

直接看这里，上面不用看。。。

In instantiation of ‘ static T2& glgltinySTL::tuple_helper_get<1ul>::get_ref(glgltinySTL::tuple_helper<T1, T2>&) [with T1 = char; T2 = unsigned int]’:
required from ‘ static decltype (glgltinySTL::tuple_helper_get<(N - 1)>::get_ref(static_cast<glgltinySTL::tuple_helper<Args ...>&>(t))) glgltinySTL::tuple_helper_get<N>::get_ref(glgltinySTL::tuple_helper<Arg, Args ...>&) [with T = int; Args = {char, unsigned int}; long unsigned int N = 2ul; decltype (glgltinySTL::tuple_helper_get<(N - 1)>::get_ref(static_cast<glgltinySTL::tuple_helper<Args ...>&>(t))) = unsigned int&]’
required from ‘ decltype (glgltinySTL::tuple_helper_get<N>::get_ref(static_cast<glgltinySTL::tuple_helper<OtherArgs ...>&>(t))) glgltinySTL::get(glgltinySTL::tuple<Args ...>&) [with long unsigned int N = 2ul; Args = {int, char, unsigned int}; decltype (glgltinySTL::tuple_helper_get<N>::get_ref(static_cast<glgltinySTL::tuple_helper<OtherArgs ...>&>(t))) = unsigned int&]’
required from here
错误：用类型为‘ unsigned int ’的右值初始化类型为‘ unsigned int&’的非常量引用无效
return t.t2;
^


完整代码。。。

https://gist.github.com/anonymous/0d777cd07c516bc48cf3  显示 Gist 代码 

[htfy96]

你这个函数的 constexpr 有问题？根据标准一个 constexpr 函数不能拥有 a reference to a variable unless it was initialized with a constant expression or is a non-static data member of a temporary object whose lifetime begain within this expression

[htfy96]

你的那个 tuple 的构造函数不是 constexpr 函数，因而产生的 t 不是 constant expression ，所以 constexpr 函数不能正确处理关于 t 的引用

[linux40]

@htfy96 谢谢啦，等下我把 constexpr 去掉， tuple 不是 constexpr 的。。。

[linux40]

@htfy96 还是一样的错误。。。

[linux40]

@htfy96 只不过没有第二行错误了

[htfy96]

@linux40 贴个在 gist 的完整代码吧……

[linux40]

@linux40 好了

[htfy96]

@linux40

第 106 ～ 109 行：

template <typename T1, typename T2>
struct tuple_helper<T1, T2> {
T1 t1;
T1 t2;

应该改成
template <typename T1, typename T2>
struct tuple_helper<T1, T2> {
T1 t1;
T2 t2;

就正常了。

原来的写法貌似编译器尝试隐式转换出了一个右值……

[linux40]

@htfy96 汗，原来是这里错了，感谢了啊。。。

[linux40]

@htfy96 现在我意识到了 tuple 用这么简单的写法是不行的，首先要保证参数最多只有一个类类型，其次当参数只有一个类类型，且那个类类型是 tuple 时，要保证正确的函数调用。。。

[linux40]

@htfy96 呃，不对，是我没加逗号。。。