Why does the extern “C” group C++ class (not header) here?

Question

I was searching SVM libraries and encountered BudgetedSVM.

In the source code, I found an unusual usage, just like this:

#sample.h

#ifndef SAMPLE_H
#define SAMPLE_H

//no header included or namespace declared here

#ifdef __cplusplus
extern "C" {
#endif

//no header included or namespace declared too

class Sample: public Parent
{
public:
    Sample();
    ~Sample();

    type0 fun(type1 val1, type2 val2);
    ...
};

#ifdef __cplusplus
}
#endif

#endif // SAMPLE_H

As seen, no extra header or namespace is needed in the header, which all are in the cpp file.

Here are my thoughts:

1. Why does extern "C", which usually is used for C interfaces, group the C++ class? Is the usage in this way good for something?

2. Even if type0, type1 and type2 appeared, their own headers are not included here, but in the cpp file (e.g. sample.h). When I call the class Sample, however, I have to include these headers (e.g. type0.h, type1.h, type2.h), which seems inconvenient.

Answer 1

According to C++ Standard C language linkage is ignored in determining the language linkage of the names of class members and the function type of class member functions.

So such grouping looks meaningless, unless type0, type1 or type2 are function pointers defined inline, in this case they will have C linkage.

In case type0, type1 or type2 are user defined types and their definitions are placed in separate headers it violates the good C/C++ style rule - all headers shall be self-contained and require no additional inclusions to compile.

To answer the main question ('Why?') - it looks like C language linkage was just blindly copy-pasted from another header (possibly pure C, where it makes sense). Or it maybe done deliberately - by "uniformity" reasons.