C++ tuple linking issue: undefined reference - c++

Question

I suppose that there is something terribly wrong with this code. It will compile but does not get linked.
#include <iostream>
#include <tuple>
class Table_class
{
public:
constexpr static std::tuple<int, unsigned int, unsigned short> table[3]
= {std::make_tuple(1, 2, 3),
std::make_tuple(4, 5, 6),
std::make_tuple(7, 8, 9)};
};
int main()
{
std::cout << std::get<0>(Table_class::table[0]);
return 0;
}
The error that shows up is this:
[31m/tmp/ccDiIuPv.o: In function `main':
file.cpp:(.text+0x5): undefined reference to `Table_class::table'
collect2: error: ld returned 1 exit status
Compilation Failed
How can this be corrected?

Answer 1

It's not terribly wrong. Your code is (will be) perfectly legal in C++17. However, before C++17, static constexpr data members need to be defined outside of class, so find somewhere and add the following definition:
constexpr std::tuple<int, unsigned int, unsigned short> Table_class::table[3];
demo.
As usual, variable definition should not be in a header file.

Answer 2

It's a common source of linking errors.
You have to define the table outside the class definition, to make the linker find it.
A simplified example:
#include <iostream>
#include <tuple>
class Table_class
{
public:
constexpr static std::tuple<int, unsigned int, unsigned short> table = std::make_tuple(1, 2, 3);
};
constexpr std::tuple<int, unsigned int, unsigned short> Table_class::table;
int main()
{
std::cout << std::get<0>(Table_class::table);
return 0;
}