Trying to incorporate a C++ file in CubeIDE - c++

Question

I am trying to use a C++ header file in my project but it is giving a syntax error. I have tried to convert the project to a C++ project but the error still persists. I don't know exactly and very much confused on how to use extern C command and #ifdef. My C++ header file goes like this:
#pragma once
#define _USE_MATH_DEFINES
#include <complex.h>
#include <math.h>
template <class NumberFormat, size_t DFT_Length>
class SlidingDFT
{
private:
///////////////rest of the code goes here/////
Now if i include this file in my Main.c file and instantiate an object it gives an error: syntax error
Any help will be highly appreciated! Thanx in advance.

Answer 1

The issue is that you have C++ code (template class) in a header file that is being included in a C file. The C compiler doesn't know how to deal with that.
You can and should wrap your C++ code like this:
#ifdef __cplusplus
template <class NumberFormat, size_t DFT_Length>
class SlidingDFT
{
private:
///////////////rest of the code goes here/////
.
.
.
#endif
That will keep you C++ code from being seen by the C compiler.
The other issue you mentioned is extern "C" linkage for functions and when to use it. When you have functions that need to be called from plain C code, they will need to have C linkage. Here are some things to know.
Functions defined in a .c module will have C linkage.
Functions defined in a .cpp module by default will have C++ linkage and will not be able to be called (easily) by C code.
You need to define any functions in a C++ module that you intend to call from C with C linkage. There are required steps in both the .cpp and .h files.
C linkage in .cpp/.h files:
In the .cpp file, define your functions like this:
extern "C" void myfunc()
{
//code
}
Then in the .h file do this:
#ifdef _cplusplus
extern "C" {
#endif
void myfunc();
#ifdef _cplusplus
}
#endif
Now your .h file can be included in both .c and .cpp modules AND the function myfunc() can be called in both as well.