Should VSC C/C++ extention take compile_commands for absolute truth?

[bhubavenski]

🤖 compile_commands.json is not absolute truth (C++ extension behavior)

Hello!
I’d like to bring up a topic for discussion. Below, I’ve structured my thoughts and observations into a few sections: background, description, idea, reproduction, and conclusion.

Note: this is a personal reflection, structured with the help of ChatGPT.

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🧭 A bit of background

I recently started learning C++ and exploring the official VS Code extension for it. When I got to the topic of compile_commands.json, I read that the extension uses this file to determine how each file is supposed to be compiled.
I wanted to test whether this is an absolute truth, or if there are exceptions.

(Please don’t judge too harshly if this seems naive — I’m still learning! 😊)

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📌 compile_commands.json is not absolute truth

If you’re using the C++ Standard Library but your file has a .c extension, the compiler will, by default, treat it as a C file.
That means it will use the C runtime and include only the C Standard Library (e.g. <stdio.h>), but not the C++ Standard Library (e.g. <iostream>).

However, if you explicitly tell the compiler to treat the .c file as C++ using the -x c++ flag, for example:

g++ -x c++ -std=c++17 -o hello.exe src/hello.c

then the compilation will succeed and link against the C++ Standard Library.

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❗ Problem with IntelliSense

Even though the actual compilation works fine, IntelliSense may still report errors and fail to recognize C++ headers. That’s because:

• IntelliSense does not fully trust compile_commands.json;
• and it applies the "cStandard": "c17" setting from c_cpp_properties.json when it sees a .c file.

So even if the compiler is using C++, IntelliSense continues to treat the file as plain C, purely based on its extension.

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❓ Discussion question

Shouldn’t IntelliSense fully honor compile_commands.json and treat files according to the flags specified there?
If a file is compiled with g++ -x c++, is it really correct for IntelliSense to treat it as C just because it ends in .c?

[bobbrow]

It sounds like a bug in the way we handle compile_commands.json. Given that the majority of C++ developers use one of the expected file extensions for C++ source files, it may not be a bug that we end up prioritizing. I would recommend that you change your source files' extensions so that you get the best support from the various tools you'll end up using for C++ development (many of which will bake in some of the same assumptions we do about file extensions).