Why does an unnamed struct inline variable not have the same address in every translation unit?

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thing.hpp:

struct
{
} inline thing;

other.cpp:

#include "thing.hpp"
#include <cstdio>

void test()
{
    std::printf("test: %p\n", &thing);
}

main.cpp:

#include "thing.hpp"
#include <cstdio>

extern void test();

int main()
{
    std::printf("main: %p\n", &thing);
    test();
}

Compiling with Clang 11 or GCC 9.3.0 in C++17 mode the output is something like this:

main: 0x404031
test: 0x404032

Per my understanding inline variables with the same definition should have the same address in every translation unit.
Giving the struct type a name results in the addresses being the same.

Why does thing not have the same address in different translation units?

c++17 c++
1 Answers

Let's ignore the inline specifier for a second. Imagine it was absent. Your code would still produce the same results. The reason is the linkage of the class type itself.

[basic.link]

8 A type is said to have linkage if and only if:

  • it is a class or enumeration type that is named (or has a name for linkage purposes ([dcl.typedef])) and the name has linkage; or
  • it is an unnamed class or unnamed enumeration that is a member of a class with linkage; or
  • it is a specialization of a class template (Clause [temp])35; or
  • it is a fundamental type; or
  • it is a compound type other than a class or enumeration, compounded exclusively from types that have linkage; or
  • it is a cv-qualified version of a type that has linkage.

A type without linkage shall not be used as the type of a variable or function with external linkage unless

  • the entity has C language linkage, or
  • the entity is declared within an unnamed namespace, or
  • the entity is not odr-used or is defined in the same translation unit.

[ Note: In other words, a type without linkage contains a class or enumeration that cannot be named outside its translation unit. An entity with external linkage declared using such a type could not correspond to any other entity in another translation unit of the program and thus must be defined in the translation unit if it is odr-used. Also note that classes with linkage may contain members whose types do not have linkage, and that typedef names are ignored in the determination of whether a type has linkage.  — end note ]

As you can see, the class type you use has no linkage. So under the quoted paragraph, because you odr-use the variable (which has external linkage), it must be defined in the same translation unit where it is used for the code to be well-formed. Which translates to different definitions in different translation units that include that header.

Ultimately, with or without the inline specifier, those definitions cannot be for the same object. So you end up seeing different addresses.

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