c++ - 为什么编译器提示这不是一个 constexpr？

Question

我正在努力学习更多有关如何在实践中使用 C++ 常量表达式的知识，并创建了以下 Matrix 类模板以供说明之用:

#include <array>

template <typename T, int numrows, int numcols>
class Matrix{
public:
    using value_type = T;
    constexpr Matrix() : {}
   ~Matrix(){}

    constexpr Matrix(const std::array<T, numrows*numcols>& a) :
        values_(a){}

    constexpr Matrix(const Matrix& other) :
        values_(other.values_){

    }

    constexpr const T& operator()(int row, int col) const {
        return values_[row*numcols+col];
    }

    T& operator()(int row, int col){
        return values_[row*numcols+col];
    }

    constexpr int rows() const {
        return numrows;
    }

    constexpr int columns() const {
        return numcols;
    }


private:
    std::array<T, numrows*numcols> values_{};
};

我的想法是拥有一个简单的 Matrix 类，我可以将其用于小矩阵以在编译时评估 Matrix 表达式(请注意，我还没有实现通常的 Matrix 运算符来进行加法和乘法)。

当我尝试按如下方式初始化 Matrix 实例时:

constexpr std::array<double, 4> a = {1,1,1,1};
constexpr Matrix<double, 2, 2> m(a);

我从编译器 (MS Visual C++ 14) 收到以下错误:

error: C2127: 'm': illegal initialization of 'constexpr' entity with a non-constant expression

请注意确定我做错了什么......非常感谢任何帮助完成这项工作!

Answer 1

[basic.types]/p10指出:

A type is a literal type if it is:

• possibly cv-qualified void; or

• a scalar type; or

• a reference type; or

• an array of literal type; or

• a possibly cv-qualified class type (Clause [class]) that has all of the following properties:

  • it has a trivial destructor,

  • it is either a closure type ([expr.prim.lambda]), an aggregate type ([dcl.init.aggr]), or has at least one constexpr constructor or constructor template (possibly inherited ([namespace.udecl]) from a base class) that is not a copy or move constructor,

  • if it is a union, at least one of its non-static data members is of non-volatile literal type, and

  • if it is not a union, all of its non-static data members and base classes are of non-volatile literal types.

哪里[class.dtor]/p5说:

A destructor is trivial if it is not user-provided and if:

^(5.4) — the destructor is not virtual,

^(5.5) — all of the direct base classes of its class have trivial destructors, and

^(5.6) — for all of the non-static data members of its class that are of class type (or array thereof), each such class has a trivial destructor.

Otherwise, the destructor is non-trivial.

换句话说，要声明 Matrix 的 constexpr 实例，它必须是文字类型，并且要成为文字类型，其析构函数必须是 默认，或者完全删除，所以:

~Matrix() = default;

或: