constexpr array in template class undefined at link time

[Clifford Heath]

Hi there,

I'm returning to C++ after an absence of 15 years,
and discovering the new features. The attached
cut-down code snippet compiles ok, but I an get
undefined symbol on link.

What should I do to resolve this problem?

Clifford Heath.

$ g++ --std=c++11 -o foo foo.cpp
Undefined symbols for architecture x86_64:
"Base<3>::lengths", referenced from:
Base<3>::write(int) in foo-3d4042.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see
invocation)
$

#include <stdio.h>

template <int unused>
class Base
{
public:
static constexpr unsigned char lengths[8] = { 1, 3, 2, 3, 4, 2, 3, 2 };

void write(int x)
{ // dummy function body
printf("%d\n", x+lengths[x]);
}
};

class MyBase : public Base<3>
{
};

int
main()
{
MyBase base;
base.write(2);
}

[Bo Persson]

Using C++11 you would have to also define the array to allocate storage
for it. It is static const *integral* types that can (most often) be
only declared in the class declaration.

With C++17 you could get away with that if you declare it `inline
constexpr`.


Bo Persson

[Clifford Heath]

Thanks Bo. I had figured that. But this is a template - there doesn't
seem to be anywhere I can put a declaration. How would you write it? I'm
still a bit of a newb with templates.

Clifford Heath.

[Alf P. Steinbach]

The ODR has a special rule for static members of class templates.

You can define that member in the header file.


Cheers & hth.,

- Alf

[Clifford Heath]

The header file does not define the template instance,
so it cannot define the array.

What's the point of having a template if you have to
redefine its members for each instance?

Thanks, I'll find another way to do it.

Clifford Heath.

[Kalle Olavi Niemitalo]

Clifford Heath <no....@please.net> writes:

> The header file does not define the template instance,
> so it cannot define the array.

template <int unused>
constexpr unsigned char Base<unused>::lengths[];

[Clifford Heath]

I tried many variations of this. None worked.
It's now a local static in the method that needs it,
instead of beside the enum whose members it matches.
Sigh.

C++ was always a train wreck, but now it's like
someone nuked the intergalactic railway convention.

[Öö Tiib]

Oh. Its just a tool. With gcc version 7.2.0 here your first
post code seems to compile and run ... I go try with clang
on mac too.

[Öö Tiib]

Also compiled and ran ... with clang 3.8.0.

[Juha Nieminen]

Clifford Heath <no....@please.net> wrote:
> template <int unused>
> class Base
> {
> public:
> static constexpr unsigned char lengths[8] = { 1, 3, 2, 3, 4, 2, 3, 2 };
>
> void write(int x)
> { // dummy function body
> printf("%d\n", x+lengths[x]);
> }
> };

This works ok with clang:

//--------------------------------------------------------------------
template <int unused>
constexpr unsigned char Base<unused>::lengths[8];
//--------------------------------------------------------------------

If it doesn't work for your compiler for some reason, try the C++98 way:

//--------------------------------------------------------------------

template <int unused>
class Base
{
public:

static const unsigned char lengths[8];

void write(int x)
{ // dummy function body
printf("%d\n", x+lengths[x]);
}
};

template <int unused>
const unsigned char Base<unused>::lengths[8] = { 1, 3, 2, 3, 4, 2, 3, 2 };
//--------------------------------------------------------------------

[Jorgen Grahn]

I haven't kept track of this thread, but noone promised that the C++11
(and beyond) tools would solve /your/ problem. And you're not obliged
to use them all.

You don't have to use /any/ of them. I design my code like I always
have, i.e. the way I know will probably be successful. If a C++11
feature seems to make it easier or better, I'll give it a try. Not
the best way to learn C++11 -- but the best way to get things done!

Constexpr always seemed tricky; hard to know when it's usable, except
in trivial situations.

/Jorgen

--
// Jorgen Grahn <grahn@ Oo o. . .
\X/ snipabacken.se> O o .