Including c++ standard headers from C not allowed, what should I use instead?

Question

I'm trying to modernize an old code base that's written in a combination of C and C++.

My first task is to get it compiling with cl.exe v19.44.35214.

I'm getting a lot of this error in yvals_core.h:

#error: error STL1003: Unexpected compiler, expected C++ compiler.

After some digging, it looks like sometime in 2020-21 the standard library made the decision to prevent C files from using the C++ standard library.

My C isn't really that strong anymore, and I'm wondering what I should be using as replacements? The files I'm including that ultimately include yvals_core.h are:

• <stdio.h>
• <windows.h>
• <cassert>
• <cmath>

Answer 1

Replace <cmath> with <math.h> in all files. <cmath> is only in C++.

Additionally, compile with /TC for C files & /TP for C++ files.

Answer 2

it looks like sometime in 2020-21 the standard library made the decision to prevent C files using the C++ standard library.

Well, this seems very reasonable.

As also pointed out by 0ro2, if that files are the only include problem, then you can just replace <cmath> with <math.h> and <cassert> with <assert.h>.

HOWEVER

• Are you sure that the application doesn't use any C++ construct? Why is important to switch to a C compiler?

• Please remember that C++ is not a superset of C. A C program doesn't necessarily compile/execute correctly when compiled with a C++ compiler and, of course, vice versa. See here for some examples. The "downgrading process" may not be as easy as it seems and may hide subtle bugs.