Having a constexpr static string gives a linker error

Question

The following program gives me a link-time error:

#include <iostream>

struct Test { static constexpr char text[] = "Text"; };

int main()
{
    std::cout << Test::text << std::endl; // error: undefined reference to `Test::text'
}

The error message is

/tmp/main-35f287.o: In function `main':
main.cpp:(.text+0x4): undefined reference to `Test::text'
main.cpp:(.text+0x13): undefined reference to `Test::text'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Ok. Let's try to fix that: I add a definition outside the struct body:

#include <iostream>

struct Test { static constexpr char text[] = "Text"; };
constexpr char Test::text[] = "Text";

int main()
{
    std::cout << Test::text << std::endl;
}

Clang gives me the following error message.

main.cpp:4:35: error: static data member 'text' already has an initializer
    constexpr char Test::text[] = "Text";
                                  ^
main.cpp:3:50: note: previous initialization is here
    struct Test { static constexpr char text[] = "Text"; };

Oh, well, I thought, now I know what you want:

#include <iostream>

struct Test { static constexpr char text[]; };
constexpr char Test::text[] = "Text";

int main()
{
    std::cout << Test::text << std::endl;
}

And again an error:

main.cpp:3:41: error: declaration of constexpr static data member 'text' requires an initializer
    struct Test { static constexpr char text[]; };

And there the dog bites its own tail. :(

Is there a way to use compile-time constant character arrays that are declared inside a class? The reason, I want the data inside a class, is that I need a type traits class that helps me do template stuff.

Did you mean simply `struct Test { static const char text[] = "Text"; }; `? — πάντα ῥεῖ, Jan 14 '16 at 14:49
You have tried two options - initializer outside the class, both inside and outside. Now, there is a third alternative... — Bo Persson, Jan 14 '16 at 14:51
This works fine: `struct Test { static constexpr auto text = "Text"; };` — vincentp, Jan 14 '16 at 14:53
@vincentp Nice. This is the most concise way of writing it. Avoids duplication. Worthy to be an accepted answer. — Ralph Tandetzky, Jan 14 '16 at 15:12
@vincentp follows up with your comment, I asked this question http://stackoverflow.com/questions/34793392/different-behavior-observed-with-constexpr-auto-char-array-variable — Danh, Jan 14 '16 at 15:33

marcinj · Accepted Answer · 2016-01-14T15:16:27.027

Should work:

#include <iostream>

struct Test { static constexpr char text[] = "Text"; };
constexpr char Test::text[];

int main()
{
    std::cout << Test::text << std::endl;
}

In standard (n4140 §9.4.2/3) you can find:

A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. [ Note: In both these cases, the member may appear in constant expressions. —end note ] The member shall still be defined in a namespace scope if it is odr-used (3.2) in the program and the namespace scope definition shall not contain an initializer.

Only one initialization initialization assignment. – Nathaniel Johnson Jan 14 '16 at 14:54 — Nathaniel Johnson, Jan 14 '16 at 14:54
Works for me! Thank you! – Ralph Tandetzky Jan 14 '16 at 14:55 — Ralph Tandetzky, Jan 14 '16 at 14:55

vincentp · Answer 2 · 2016-01-14T15:58:27.330

2

As said in comments, this version works fine:

struct Test { static constexpr auto text = "Text"; };

But text will be a const char* instead of a char[].

edited Jan 14 '16 at 15:58

answered Jan 14 '16 at 15:49

vincentp

1,433
9
12

Why does it not work to make a `const char[5]` out of it? I mean why is there so much of a difference? This is annoying. That makes it unusable for my purposes. – Ralph Tandetzky Jan 14 '16 at 16:24

Having a constexpr static string gives a linker error

2 Answers2

Linked