structure name not declared and swap function swaps without pass by refrence - c++

Question

I have this code,which takes input of 3 students from division a and b each.
those 2 divions are sorted and merged in a 3rd array according to birth dates of students.
the swap function ,I have not passed anything by refrence still its swapping and sort output is correct !!!.
NOTE:the line below #include..
void swap(struct a,struct b)
it should be
void swap(struct student a,struct student b)
but without changing that the program is runnong and giving correct outputs !! how ??
#include<iostream>
#include<string.h>
using namespace std;
void swap(struct a,struct b);
void findweek(struct student ar[10],int l,int bd1,int bm1);
struct student
{
//m is for month and b is for birthdate prn=prn number,name=name of student
int m,bd;
char prn[10],name[10];
};
int main()
{
//2 divisions a and b declared and will be merged into c
struct student a[3], b[3], c[6];
//division a input
for(int i=0;i<3;i++)
{
cout<<" Enter name of student "<<endl;
cin>>a[i].name;
cout<<"Enter prn no. "<<endl;
cin>>a[i].prn;
cout<<"Enter birth day "<<endl;
cin>>a[i].bd;
cout<<"Enter birth month "<<endl;
cin>>a[i].m;
}
//sorting of a
for(int i=0;i<3;i++)
{
for(int j=i;j<3;j++)
{
if(a[i].m>a[j].m)
{
swap(a[i],a[j]);
}
else if(a[i].m==a[j].m)
{
if(a[i].bd>a[j].bd)
{
swap(a[i],a[j]);
}
}
}
}
//division b input
for(int i=0;i<3;i++)
{
cout<<" Enter name of student "<<endl;
cin>>b[i].name;
cout<<"Enter prn no. "<<endl;
cin>>b[i].prn;
cout<<"Enter birth day "<<endl;
cin>>b[i].bd;
cout<<"Enter birth month "<<endl;
cin>>b[i].m;
}
//sorting of b
for(int i=0;i<3;i++)
{
for(int j=i;j<3;j++)
{
if(b[i].m>b[j].m)
{
swap(b[i],b[j]);
}
else if(b[i].m==b[j].m)
{
if(b[i].bd>b[j].bd)
{
swap(b[i],b[j]);
}
}
}
}
cout<<"-----------------------"<<endl;
cout<<"Division A"<<endl;
int count=0;
for(int i=0;i<3;i++)
{ //c has merged array , a being filled first
c[i]=a[i];
count++;
cout <<c[i].name<<"\t"<<c[i].prn<<"\t"<<c[i].bd<<"|"<<c[i].m<<endl;
}
cout<<"Division B"<<endl;
for(int i=0;i<3;i++)
{
//resume filling the array from count
c[count]=b[i];
cout <<c[count].name<<"\t"<<c[count].prn<<"\t"<<c[count].bd<<"|"<<c[count].m<<endl;
count++;
}
int bd1,bm1;
cout<<"Enter date to find birthdays in that week "<<endl;
cin>>bd1;
cout<<"Enter corresponding month "<<endl;
cin>>bm1;
findweek(c,count,bd1,bm1);
return 0;
}
//to swap the structure student arrays for sorting
void swap(struct student a,struct student b)
{
struct student t;
t=a;
a=b;
b=t;
}
void findweek(struct student ar[10],int l,int bd1,int bm1)
{
int count=0;
for(int i=0;i<l;i++)
{
int month_end=30;
int next_month=bm1+1;
//if(bd1>=23)
int end_date=bd1+7-month_end;
// else
int endofweek=bd1+7;
//l is length of ar , ar=copy of merged array, bd1&bm1 are date and month to search for birthday in that week
if((ar[i].m==bm1&&ar[i].bd>=bd1&&ar[i].bd<=endofweek)||ar[i].m==bm1+1&&ar[i].bd<=end_date)
{
if(month_end-bd1>7)
cout <<ar[i].name<<"\t"<<ar[i].prn<<"\t"<<ar[i].bd<<"|"<<ar[i].m<<endl;
else
{
if((ar[i].m==bm1&&ar[i].bd>=bd1)||(ar[i].m==next_month&&ar[i].bd<=end_date))
{
cout <<ar[i].name<<"\t"<<ar[i].prn<<"\t"<<ar[i].bd<<"|"<<ar[i].m<<endl;
}
}
count++;
if(count>7)
break;
}
}
}

Answer 1

It compiles even if you remove this line:
void swap(struct a,struct b);
Also, it compiles if you remove the whole swap function.
How is it?
Quite simple.
You are defining a function that takes two arguments: an incomplete type struct a and an incomplete type struct b.
That function is simply discarded from the overload set while searching the one to be used at function call.
Your main is not using your swap function. Instead, it's using the one from std:: namespace.
It is probably introduced by iostream or string, it's implementation defined.
Try changing the name of the function or putting a throw in your implementation of swap. In the second case, your runtime won't be affected.
Minimal, (not-)working example to reproduce the issue:
void f(struct s);
struct S {};
int main() { f(S{}); }
void f(S) {}
As you can see, the error is that you are referring to an incomplete type struct s.
swap is kinda of a somehow misleading example that compiles for the reasons above.
Reproducing an issue with a minimal example is often helpful.