Why does the compiler complain about this not being a constexpr?

Question

I am trying to learn a bit more on how to use C++ constant expressions in practice and created the following Matrix class template for illustration purposes:

#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_{}; };

The idea is to have a simple Matrix class, which I can use for small matrices to evaluate Matrix expressions at compile time (note that I have not yet implemented the usual Matrix operators for addition and multiplication).

When I try to initialize a Matrix instance as follows:

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

I am getting the following error from the compiler (MS Visual C++ 14):

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

Note sure what I am doing wrong...any help to make this work would be greatly appreciated!

Maybe std::array doesn't have a constexpr copy constructor? — Kerrek SB
– Kerrek SB, Commented Jun 29, 2016 at 15:17
As a sidenote, there is no need to store numrows_ and numcols_ as member variables. Since you have the values as template parameters already, just return those. — Nasser Al-Shawwa
– Nasser Al-Shawwa, Commented Jun 29, 2016 at 15:23
@Piotr Skotnicki yes that did the trick. Thanks! If you post this as an answer I will accept it. But I don't understand why defining a destructor triggers the error? — BigONotation
– BigONotation, Commented Jun 29, 2016 at 15:26
@nasser-sh as suggested I cleaned up the code by removing unnecessary member variables for row and column size — BigONotation
– BigONotation, Commented Jun 29, 2016 at 20:39

Piotr Skotnicki · Accepted Answer · 2016-06-29 15:43:59Z

[basic.types]/p10 states that:

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.

where [class.dtor]/p5 says that:

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.

In other words, to declare a constexpr instance of Matrix, it must be a literal type, and to be a literal type, its destructor must be either defaulted, or removed altogether, so:

~Matrix() = default;

or:

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