Using `constexpr` with array initialization

Question

I know that in C++11, I can write

class foo {
    static constexpr const char *one = "one";
}

However, when I try to do the same for an array

class bar {
    static constexpr const float prim[4] = {2, 3, 5, 7};
}

(and reference it later on) I get an undefined reference linker error.

Is this just not possible for arrays or am I missing something in the syntax?

The declaration and usage of this example compiles fine on GCC 7.2 — Fureeish
– Fureeish, Commented Feb 6, 2018 at 16:13
Look at good answer there stackoverflow.com/questions/29397864/… — 3CEZVQ
– 3CEZVQ, Commented Feb 6, 2018 at 16:32

skypjack · Accepted Answer · 2018-02-06 17:14:00Z

3

Static constexpr data member declarations aren't definitions in C++11/14, therefore you cannot odr-use prim.
To work around it, put the following statement somewhere in your cpp file as you would do with any other non-constexpr static data member:

constexpr const float bar::prim[4];

In other terms, this returns an undefined reference:

struct bar {
    static constexpr const float prim[4] = {2, 3, 5, 7};
};

int main() {
    auto *foo = bar::prim;
}

This doesn't:

struct bar {
    static constexpr const float prim[4] = {2, 3, 5, 7};
};

constexpr const float bar::prim[4];

int main() {
    auto *foo = bar::prim;
}

Because in the second case you are actually defining prim other than declaring it and thus you can get its address, use it by means of a reference, and so on...

edited Feb 6, 2018 at 17:14

answered Feb 6, 2018 at 16:57

skypjack

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2 Comments

Robert Andrzejuk Over a year ago

Is this still required in C++17, Which allows inline variables to be initialized in the class?

skypjack Over a year ago

@RobertAndrzejuk In C++17 (I cite the standard) a function or static data member declared with the constexpr specifier is implicitly an inline function or variable. Therefore things changed with C++17 actually.

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