Address of a constexpr in a template parameter-CodePudding

Can someone solve the mystery of why do I get linking errors if I take the address of a variable that should go in a template parameter only if I make the variable const, constexpr or static, but not oherwise?

The piece(s) of code below are part of a larger project, but I extracted the exact same structure:

CMakeLists.txt

project(fun)
add_library(fun SHARED fun.cpp)
add_executable(main main.cpp)
target_link_libraries(main fun)

fun.cpp

#include "fun.h"
BlaaUser testBlaa(int)
{
    return BlaaUser();
}

fun.h

#pragma once 

#include "types.h"

extern BlaaUser testBlaa(int);

main.cpp

#include "fun.h"

int main()
{
    auto b = testBlaa(2);
}

types.h

#pragma once

struct P
{
    char n;
};

template<const P* S>
struct Blaa
{
    Blaa() {}
    char blaa{S->n};
};

//constexpr
//static
//const
P p{};  // <-- Here, right now it works. If I uncomment any of the specifiers, I get errors, see below

using BlaaUser = Blaa<&p>;

If I add any kind of specifier I get the following errors:

Scanning dependencies of target fun
[ 25%] Building CXX object CMakeFiles/fun.dir/fun.cpp.o
[ 50%] Linking CXX shared library libfun.so
[ 50%] Built target fun
Scanning dependencies of target main
[ 75%] Building CXX object CMakeFiles/main.dir/main.cpp.o
In file included from /home/fld/work/tmpl_test/main.cpp:1:
/home/fld/work/tmpl_test/fun.h:5:17: warning: ‘BlaaUser testBlaa(int)’ used but never defined
    5 | extern BlaaUser testBlaa(int);
      |                 ^~~~~~~~
[100%] Linking CXX executable main
/usr/bin/ld: CMakeFiles/main.dir/main.cpp.o: in function `main':
main.cpp:(.text 0x12): undefined reference to `testBlaa(int)'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/main.dir/build.make:104: main] Error 1
make[1]: *** [CMakeFiles/Makefile2:97: CMakeFiles/main.dir/all] Error 2
make: *** [Makefile:103: all] Error 2

I sort of have the feeling that it has to do with the address of an object (as per the standard: For pointers to objects, the template arguments have to designate the address of a complete object with static storage duration and a linkage (either internal or external) ) but I'm just a bit confused because if everything goes in one file all is ok, the problem comes when it's split up in libraries ...

Does anyone have any explanations?

CodePudding user response：

The additional specifiers (e.g const, constexpr) makes the template parameter have internal linkage, which subsequently makes testBlaa(int) have internal linkage. It works if you do

extern const P p{}

right? Maybe Why does the following method get internal linkage? is similar?