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Editorial Team
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Asked: 2026-05-27T15:01:31+00:00

I’ve made a struct which does cached file manipulation for my application. I built

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I’ve made a struct which does cached file manipulation for my application. I built and tested to in a separate project before putting it into my current one.

Ever since I’ve moved it over, Xcode refuses to build it. Except when I don’t include the file from any Objective-C based header file.

I get one error when I try to include iostream:
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And more when I comment it out:
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Its file extension is .mm, however I have tried it with .cpp and .hpp, but all of them refuse to build unless I don’t #include it from the Objective-C header file.

I’ve also tried #import from iostream and the file itself in the Objective-C header file.

Any clues as to why this is happening?

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1 Answer

  1. Editorial Team

    As a matter of principle, you cannot include a C++ header file from an objective-C source file.

    After all, #including (or #importing) a file only means that the preprocessor replaces the #include directive by the contents of the #included file, before passing the result on to the “actual” compiler. The file extension of the header file is a matter of convention, only, it has no actual meaning.

    The error messages your are seeing are clearly the result of the file being compiled as [Objective-]C rather than [Objective-]C++.

    Solution: All the source files that include your C++ header file have to be either C++ (.cpp or .cc or a few other extensions) or Objective-C++ (.mm). All source files that include a header file that includes your C++ header file, also have to be C++ or Objective-C++.

    EDIT: I just saw that you are defining non-inline, non-template functions in your C++ file that you want to include. This is an unrelated problem, but it will lead to “multiple definition” errors sooner or later. Those function definitions belong in a .cpp, which shouldn’t get #included anywhere, only the struct/class definition belongs in a header.

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