Undefined reference for function specialization in namespace - c++

Question

I'm implementing a JSON archive system modelled after boost::archive. For each type you want to serialize, you define a non-intrusive function that accepts an archive and your object:
// archive.hpp
#pragma once
namespace Archive {
template <class A, class T>
void serialize(A& a, T& value);
}
struct ArchiveOut {
void add(const char* key, int& value) {}
// ... Implementations for basic types ...
template <class T>
void add(const char* key, T& value) {
ArchiveOut archive;
Archive::serialize(archive, value);
}
};
// main.cpp
#include "archive.hpp"
struct Child {
int id;
};
struct Parent {
int id;
Child child;
};
template <class A>
void Archive::serialize(A& a, Parent& v) {
a.add("id", v.id);
a.add("child", v.child);
}
template <class A>
void Archive::serialize(A& a, Child& v) {
a.add("id", v.id);
}
int main() {
Parent parent;
ArchiveOut archive;
Archive::serialize(archive, parent);
}
Right now, the system works for complex nested types but only if serialize is in the global namespace. Once it's moved into the Archive namespace I get a linker error:
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\DDP\AppData\Local\Temp\ccMWEvEu.o:test.cpp:(.text$_ZN10ArchiveOut3addI5ChildEEvPKcRT_[_ZN10ArchiveOut3addI5ChildEEvPKcRT_]+0x20): undefined reference to `void Archive::serialize<ArchiveOut, Child>(ArchiveOut&, Child&)
I know my specializations have the right signature since they match boost's, but maybe my initial prototype is wrong? I've tried digging through the boost internals but can't find where the initial serialize prototype is. I've also checked other answers and all of them are related to specializations not matching the function signature or not placing it in the right namespace. Could I get an explanation for this linker behavior?

Answer 1

You're using overloading where need partial specialization.
The problem is that a function can't be partial specialized.
Suggestion: use static functions inside partial specialized structs.
Something as follows (caution: code not tested)
// archive.hpp
namespace Archive {
template <typename A, typename T>
struct serial;
template <class A, class T>
void serialize(A& a, T& value)
{ serial<A, T>::func(a, value); }
}
// main.cpp
#include "archive.hpp"
// ...
namespace Archive {
template <class A>
struct serial<A, Parent>
{
static void func (A & a, Parent & v)
{
a.add("id", v.id);
a.add("child", v.child);
}
};
template <class A>
struct serial<A, Child>
{
static void func (A & a, Child & v)
{ a.add("id", v.id); }
};
}