Conflicting declaration in c++ - c++

Question

I have a cpp file as follows:
#include <iostream>
#include "i.h"
using namespace std;
typedef struct abc{
int a1;
int b1;
} abc_t, *abc;
void fun(abc x){
cout<<x->a1;
}
int main(){
abc val;
fun(val);
return 0;
}
The i.h file :
struct abc;
void fff(struct abc);
When I am compiling the code following error occurs:
t.cpp:8: error: conflicting declaration ‘typedef struct abc* abc’
t.cpp:5: error: ‘struct abc’ has a previous declaration as ‘struct abc’
t.cpp: In function ‘void fun(abc)’:
t.cpp:11: error: base operand of ‘->’ has non-pointer type ‘abc’
If I save the cpp file as c file and compile using c compiler then everything works fine.
What is the issue with c++ compiler?

Answer 1

You've declared abc as both a struct and a pointer to a struct by using typedef. It's the same as doing:
struct abc {...};
typedef abc abc_t; // ok, abc_t is now an alias for abc
typedef abc *abc; // error
Skip the typedef, abc_t and *abc and use the class (with all members public per default) abc as-is.
i.h
struct abc {
int a1 = 0;
int b1 = 0;
};
void fun(const abc& x);
i.cpp
#include <iostream>
#include "i.h"
void fun(const abc& x) {
std::cout << x.a1 << "\n";
}
main.cpp
#include <iostream>
#include "i.h"
int main(){
abc val;
fun(val);
return 0;
}

Answer 2

In C, this:
struct abc
{
int a1;
int b1;
};
creates a type struct abc (roughly speaking), but not a type abc.
That's why you use the typedef trick to create a type we can use without having to write struct everywhere:
typedef struct abc{
int a1;
int b1;
} abc_t;
Now you have the type abc_t, too, which is the same as struct abc. There is still no type abc.
So when you add on the declaration of a pointer called abc, that's valid, as the name is not taken yet.
In C++, the original declaration creates a type named abc. There is no need for the typedef trick, and your declaration of a pointer called abc is invalid because the name abc is taken.
Solution
You can disambiguate your names (and de-obfuscate the code) like so:
struct abc
{
int a1;
int b1;
};
typedef struct abc abc_t;
abc_t* ptr_to_abc;
Or, if you are writing C++ and don't need C compat, just this:
struct abc
{
int a1;
int b1;
};
abc* ptr_to_abc;

Answer 3

This
struct abc;
in C++ declares the type struct abc as well as the type abc, which then clashes with typedef'ing ...*abc again.
In C it just declares struct abc, so typedef'ing ...*abc does not produce a duplicate declaration.