Hi,

In VS2026, latest version, valid and activated license, I create a new project and each files were created from the solution explorer.

Whenever I create a global variable or define a global function in a header file that also contains a class definition, and the implementation of that class is in the corresponding cpp, that global variable or function trigger a "multiple implementation of" error on build.

That error doesn't occur if all the code is in the header...

If I exclude the files from the project, the project build correctly.

If I place them in a non-root, or non-src directory, the error still occurs if the files are included in the project. 

The project build if the files are not included (but then I wonder how they're found, as they're not in an "auto include" directory anymore...)

For the past 2 hours I tried toggling off/on different options, can't have the build pass if the files are included in the project... what am I missing?

I searched the forum and the web without success...

I'm starting to loose my mind... send help...

thank you a lot in advance for the help!

Tim@Visual Micro wrote on Sep 16^th, 2026 at 4:52pm:


Thanks for the quick reply, yes I use preprocessor directives to include only once the headers, there's no clash in the names, I can zip you the project If you desire, it's quite small

I've written initially a "dirty C" version with all the code in headers and I'm refactoring to classes, as of right now the problem occur in the first header I try to refactor.

As of right now, if that header contains also the class implementation, everything works. 

As soon as I keep the signature in the header and move the implementation in the cpp, I get the "multiple definition of" error. 

edit: I'm at my local hackerspace, I discussed the issue with a friend as I'm maybe a bit rusty in C++ (no pun intended), but there seems to be something going on with the build itself. To me, that it works depending on whether the sources are included in the vcxproj or not when the declaration and definition are separated is a sign of something, but couldn't figure the solution in the litany of project options... meanwhile I just keep everything in the header as it's working while in the vcxproj and allows to progress on the refactorings...