Example to print all prime numbers between two numbers (entered by the user) by making a user-defined function.

To understand this example, you should have the knowledge of the following C++ programming topics:

```
#include <iostream>
using namespace std;
int checkPrimeNumber(int);
int main()
{
int n1, n2;
bool flag;
cout << "Enter two positive integers: ";
cin >> n1 >> n2;
cout << "Prime numbers between " << n1 << " and " << n2 << " are: ";
for(int i = n1+1; i < n2; ++i)
{
// If i is a prime number, flag will be equal to 1
flag = checkPrimeNumber(i);
if(flag)
cout << i << " ";
}
return 0;
}
// user-defined function to check prime number
int checkPrimeNumber(int n)
{
bool flag = true;
for(int j = 2; j <= n/2; ++j)
{
if (n%j == 0)
{
flag = false;
break;
}
}
return flag;
}
```

**Output**

Enter two positive integers: 12 55 Prime numbers between 12 and 55 are: 13 17 19 23 29 31 37 41 43 47 53

To print all prime numbers between two integers, `checkPrimeNumber()`

function is created. This function checks whether a number is prime or not.

All integers between `n1` and `n2` are passed to this function.

If a number passed to `checkPrimeNumber()`

is a prime number, this function returns true, if not the function returns false.

If the user enters larger number first, this program will not work as intended. To solve this issue, you need to swap numbers first.