为什么我的 constexpr 对象在我的函数中不是 constexpr？

Question

我编写了一个可以在编译时初始化和复制的类，并且在我的函数之外使用该对象也可以在编译时工作。现在我想将我的constexpr对象传递给一个函数来做一些计算，但是编译器产生一个错误，我的对象不是一个常量表达式。

我必须如何编写函数才能使用我的constexpr对象？

Using GCC 9.2, C++17, (CLion option to use C++20已激活)

这是我的课程，我的功能和主要内容。注意：并不是所有的定义都给出了，因为它会为一篇文章写很多代码。

template<std::size_t t1_skipPos, std::size_t t2_skipPos, typename T1, typename T2>
constexpr auto contraction(T1 tensor1, T2 tensor2){

    /*ERROR: tensor1 is not a constant expression*/
    auto sris_tensor1 = save_recreated_index_sequence<0, tensor1.indices_amount-2, 
         t1_skipPos, tensor1.indices_amount, DIM3>(tensor1.calculate_indices());

    return sris_tensor1;

}


template<typename T, typename ... Args>
class tensorBase{

    private:
        std::array<T, positive_natural_compiletime_pow<DIM3, std::tuple_size<Args...>::value>()> data;

    public:
        //std::vector<T> data = {};

        std::tuple<Args...> myTypeTup;

        std::size_t indices_amount =  std::tuple_size<Args...>::value;


    template<typename ... Element>
    constexpr tensorBase(Element&&... input) : data{input...} {};

    /* copy constructor */
    template<typename Tensor>
    constexpr tensorBase(const Tensor &oldObj){

        data = oldObj.get_data();

        myTypeTup = oldObj.myTypeTup;

        indices_amount = oldObj.indices_amount;

    }
/*
    template<typename Arr>
    constexpr tensorBase(Arr&& array) {
        data = array;
    };
*/
    constexpr auto calculate_indices() const{

        static_assert((std::tuple_size<Args...>::value <= 5), "tensor has to many indices");
        //auto l = std::tuple_size<Args...>::value;
        return cartesian_product<DIM3, std::tuple_size<Args...>::value>();
        //return cartesian_product_to_vec<DIM3, sizeof...(Args)+1>();
    };

    template<typename D>
    constexpr auto get_element(D&& val) const{
        return data[val];
    };

    constexpr auto get_data() const{
        return data;
    };
};

template<typename T, typename ... Args>
using tensor = tensorBase<T, std::tuple<Args...>>;

int main(){
    constexpr tensor<double, up_t, low_t, low_t> tensor1(11.0, 22.0, 33.0, 44.0, 55.0, 66.0, 77.0, 88.0, 99.0); /*OK*/
    constexpr tensor<double, up_t, low_t> tensor2(0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9);/*OK*/
    constexpr auto copyObj1 = tensor1; /*OK*/
    constexpr auto indices = tensor1.indices_amount; /*OK*/
    constexpr auto indices1 = tensor1.calculate_indices(); /*OK*/

    constexpr auto sris_outside = save_recreated_index_sequence
        <0, tensor1.indices_amount-1,0,tensor1.indices_amount,2>(tensor1.calculate_indices()); /*OK*/

    constexpr auto contract = contraction<0,0>(tensor1, tensor2); /*Error*/

    return 0;
}

错误信息：

   In instantiation of ‘constexpr auto contraction(T1, T2) [with long unsigned int t1_skipPos = 0; long unsigned int t2_skipPos = 0; T1 = tensorBase<double, std::tuple<up_t, low_t, low_t> >; T2 = tensorBase<double, std::tuple<up_t, low_t> >]’:

error: no matching function for call to ‘save_recreated_index_sequence<0, (tensor1.tensorBase<double, std::tuple<up_t, low_t, low_t> >::indices_amount - 2), 0, tensor1.tensorBase<double, std::tuple<up_t, low_t, low_t> >::indices_amount, DIM3>(std::array<std::tuple<long unsigned int, long unsigned int, long unsigned int>, 27>)’
       61 |     auto sris_tensor1 = save_recreated_index_sequence<0, tensor1.indices_amount-2, t1_skipPos, tensor1.indices_amount, DIM3>(tensor1.calculate_indices());
          |                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

note: candidate: ‘template<long unsigned int offset, long unsigned int N, long unsigned int skipPos, long unsigned int length, long unsigned int times, class Arr> constexpr auto save_recreated_index_sequence(Arr&&)’
       46 | constexpr auto save_recreated_index_sequence(Arr&& arr){
          |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 note:   template argument deduction/substitution failed:
    In file included from /home/martin/CLionProjects/tensor-library-v1/main.cpp:43:
 error: ‘tensor1’ is not a constant expression
       61 |     auto sris_tensor1 = save_recreated_index_sequence<0, tensor1.indices_amount-2, t1_skipPos, tensor1.indices_amount, DIM3>(tensor1.calculate_indices());
          |                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 note: in template argument for type ‘long unsigned int’
       61 |     auto sris_tensor1 = save_recreated_index_sequence<0, tensor1.indices_amount-2, t1_skipPos, tensor1.indices_amount, DIM3>(tensor1.calculate_indices());
          |                                                          ~~~~~~~~~~~~~~~~~~~~~~^~
error: ‘tensor1’ is not a constant expression
       61 |     auto sris_tensor1 = save_recreated_index_sequence<0, tensor1.indices_amount-2, t1_skipPos, tensor1.indices_amount, DIM3>(tensor1.calculate_indices());
          |                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 note: in template argument for type ‘long unsigned int’
       61 |     auto sris_tensor1 = save_recreated_index_sequence<0, tensor1.indices_amount-2, t1_skipPos, tensor1.indices_amount, DIM3>(tensor1.calculate_indices());
          |                                                                                                ~~~~~~~~^~~~~~~~~~~~~~
error: unable to deduce ‘auto’ from ‘sris_tensor1’
       63 |     return sris_tensor1;
          |            ^~~~~~~~~~~~
 In function ‘int main()’:
error: ‘constexpr const void contract’ has incomplete type
      162 |     constexpr auto contract = contraction<0,0>(tensor1, tensor2);
          |                    ^~~~~~~~

Answer 1

tensor1如果要在需要常量表达式的上下文中使用它，则需要作为模板参数传递。目前 (C++17) 只能作为参考：

template<std::size_t t1_skipPos, std::size_t t2_skipPos, auto& tensor1, auto& tensor2>
constexpr auto contraction(){

    auto sris_tensor1 = save_recreated_index_sequence<0, tensor1.indices_amount-2, 
         t1_skipPos, tensor1.indices_amount, DIM3>(tensor1.calculate_indices());

    return sris_tensor1;

}

//...

constexpr auto contract = contraction<0, 0, tensor1, tensor2>();

但是，这也需要您声明tensor1和tensor2as static，以便它们具有链接。否则，它们不能用作模板参数，并且会使函数无法与非constexpr变量一起使用。

请注意，C++20 添加了类类型的非类型模板参数，这可能会使引用以及链接的要求变得不必要（通过使用auto代替auto&），但这不适用于当前状态下的类C++20 草案，它要求该类型的所有非静态数据成员都是公共的。此要求最近已更改，因此当前的编译器版本在使用 时可能会实现先前草案版本的不太严格的要求-std=c++2a，其中您的类将是类类型的有效非类型模板参数。

您不能在需要常量表达式的上下文中使用函数参数。这个上下文出现在另一个常量表达式评估中的事实并不重要。一个constexpr函数必须始终也是一个有效的运行时函数。

但是，在所示代码的特定情况下，您可以改为创建类indices_amount的static成员并通过类型访问它，以便函数参数的状态永远不会在常量表达式上下文中使用：

static constexpr std::size_t indices_amount = std::tuple_size_v<Args...>;

//...

template<std::size_t t1_skipPos, std::size_t t2_skipPos, typename T1, typename T2>
constexpr auto contraction(T1 tensor1, T2 tensor2){

    /*ERROR: tensor1 is not a constant expression*/
    auto sris_tensor1 = save_recreated_index_sequence<0, T1::indices_amount-2, 
         t1_skipPos, T1::indices_amount, DIM3>(tensor1.calculate_indices());

    return sris_tensor1;

}

另请注意，将tensorBase参数包作为第二个模板参数似乎没有意义。它只填充了一个参数，也不能用于任何其他数字。例如，std::tuple_size<Args...>仅当包只有一个元素时才有效。我建议你删除所有...与Args该课程相关的s 。