Undefined symbols for architecture x86_64 linking constexpr std::array

Question

I am getting the error in while linking my object files:

#include <cstdint>
#include <array>

enum SystemType : uint8_t { AC, DC, HCP, EFF };

template<SystemType TYPE> struct System;

template<> 
struct System<AC> { 
public:     
static constexpr size_t number_of_sockets = 2;  
static constexpr std::array<size_t, number_of_sockets> object_per_socket { { 12, 6 } }; 
};

I am using it as below to allocate data into a vector.

terminal->no_obj_per_system.assign(
            Sytem<AC>::object_per_socket.begin(),
            Sytem<AC>::object_per_socket.end());

I am using clang on mac Os.

There is a typo in this expression Sytem::objet_per_socket.begin(), There should be System instead of Sytem — Vlad from Moscow, Jun 07 '17 at 00:12
Your problem is almost surely what you haven't posted. Please post a complete (and minimal -- just enough code to trigger the error) example: i.e. all the relevant files, along with the actual command lines you execute and the actual text of the errors you got. — , Jun 07 '17 at 00:12
@aaramoon The problem is that you have to define the static data member of the type std::array outside the class definition. — Vlad from Moscow, Jun 07 '17 at 00:14
So, where did you define those objects? Also wow that's a lot of typos; pay more attention and take more care. — Lightness Races in Orbit, Jun 07 '17 at 00:14
@VladfromMoscow: it is static constexpr ? needs an initializer. not ? — apramc, Jun 07 '17 at 00:19

score 5 · Accepted Answer · answered Jun 07 '17 at 01:03

In C++14 and earlier, static data members must have an out-of-class definition in exactly one translation unit if they are odr-used; this rule still applies to constexpr members.

This question contains an example with references from the C++14 Standard.

If there is no out-of-class definition provided, and the variable is odr-used, then it is ill-formed with no diagnostic required, which explains why some people don't get a compilation error. To be on the safe side you should provide a definition, there's no harm doing so even in the case where the variable is not odr-used.

The definition would look like, (NOT in a header file):

constexpr std::array<size_t, System<AC>::number_of_sockets>  System<AC>::object_per_socket;

In C++17 there is a new feature "inline variables", which allows variables to be defined in header files with similar semantics to inline functions, i.e. you're allowed to have multiple matching definitions across translation units, and the compiler/linker will select one as needed. constexpr variables will be implicitly inline, so your original code will be correct in C++17. Recent versions of gcc and clang should accept the code with -std=c++1z flag.

Thanks for the response, I am not quite clear about one thing, should I move the initialization into another file, or should I declare the variable somewhere else ? — apramc, Jun 07 '17 at 01:18
@aaramoon copy and paste the line from my answer into exactly .cpp file, don't change your class definition — M.M, Jun 07 '17 at 01:32

Undefined symbols for architecture x86_64 linking constexpr std::array

1 Answers1