A curious case with references and static class members

Question

Take the following piece of code:

#include <type_traits>
#include <iostream>

template <class T>
void print(T &&t){
    std::cout << t << std::endl;
}

template<class T, T val>
struct Foo{
    static constexpr T value = val;
};

int main(){
    print(Foo<int, 123>::value);
}

It refuses to compile under Clang 3.3 and GCC 4.8.1 ("undefined reference to Foo<int, 123>::value"), which puzzles me because Foo does exactly the same as std::integral_constant, and the same code runs fine with integral_constant. It also fails with plain lvalue reference in the print function. Any explanation regarding this behaviour?

The issue is also present with this quite minimal example:

template<class T>
struct Bar{
    static const bool value = true;
};

Your example compiles and runs [on Ideone](http://ideone.com/BJLEuL). Maybe a compiler issue? — djf, Jun 30 '13 at 16:51
I edited in the compiler versions; Ideone probably uses older stuff. — András Kovács, Jun 30 '13 at 16:53
Now this is quite interesting: http://www.compileonline.com throws the error but has gcc 4.7.2 just like Ideone. — András Kovács, Jun 30 '13 at 17:03
Whether using `const` or `constexpr`, it compiles with `-O2` on g++4.8. But not without. Haha — Yohan Danvin, Jun 30 '13 at 17:23
Also, if you use `cout` directly (no print(..)) it always compiles on g++. — Yohan Danvin, Jun 30 '13 at 17:31
This might be a [duplicate](http://stackoverflow.com/questions/14547986/what-am-i-allowed-to-do-with-a-static-constexpr-in-class-initialized-data-memb). I think by binding it to a reference (universal -> `const&`, not rvalue) you odr-use the variable therefore it has to be defined. — dyp, Jun 30 '13 at 17:42
Why does it compile with integral_constant, though? I just copied in its definition from the libstdc++ sources and it's not working in my source. — András Kovács, Jun 30 '13 at 18:06
Maybe the libstdc++ provides a definition somewhere else? As a workaround you can use `+Foo::value` (unary plus) to force an rvalue ([thanks litb](http://stackoverflow.com/a/3182557/688659)). — gx_, Jun 30 '13 at 18:32

Stefano Falasca · Accepted Answer · 2013-07-01T13:04:15.810

4

As the compiler says, there is no reference to the static variable, you must add

template<class T, T val>
constexpr T Foo<T, val>::value;

after the definition of the class Foo

edited Jul 01 '13 at 13:04

answered Jul 01 '13 at 11:25

Stefano Falasca

8,837
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Thanks, this seems to be a nice enough solution. – András Kovács Jul 01 '13 at 13:14

A curious case with references and static class members

1 Answers1