inline variable is initialized more than once

Question

Im seeing some examples of inline const variable getting initialized (and destructed) 3 times with visual studio 2017. Is this is a bug with the linker ? or is

Answer 1

I get the bug in both debug and release (/Ox) mode using the MS C++ compiler version 19.16 (comes with, e.g., Visual Studio 15.9.4).

Inline.Hpp

#include   
inline struct Foo  
{ Foo() { std::cout << "Constructing a Foo at " << this << std::endl; } }  
Instance;  

Inline.cpp

#include "Inline.Hpp"  
int main() { return 0; }  

Inline2.cpp

#include "Inline.Hpp"    

After compiling and linking inline.cpp and inline2.cpp, the output on running is:

Constructing a Foo at 00BE4028  
Constructing a Foo at 00BE4028  

The compiler and linker correctly resolve the two inline definitions to a single object, but incorrectly call the constructor for each definition, instead of just once. This is a serious bug which renders the "inline variable" feature of C++17 unusable. The "workaround" is to regard inline variables as still unsupported by MS C++ as of version 19.16, even when the /std:c++17 switch is used.