A simple question on passing by pointer to a function in VC++

This gets complicated, but ... it's to do with evaluation order.
Once I fix the syntax error - you are missing a semicolon from this line:

int* pnum = &num

Rejig your code a little, you can see the problem more clearly:

#include <iostream>

int incr10(int* num);

using namespace std;

int main(void)
{
	int num;
	int* pnum = #

    num = 3;
	cout << "incr10(pnum) = " << incr10(pnum) << endl
	     << "num= " << num  
	     << endl;
	     
    num = 3;
	cout << "incr10(pnum) = " << incr10(pnum) << endl;
	cout << "num= " << num << endl;
	
    num = 3;
    incr10(pnum);
	cout << "num = " << num << endl;
	return 0;
}

//function to increment a variable by 10
int incr10(int* num)
{
	*num += 10;
	return *num;
}

Gives the results:

incr10(pnum) = 13
num= 3
incr10(pnum) = 13
num= 13
num = 13

Which shows the problem. It's even clearer in a smaller example:

#include <iostream>

using namespace std;
int A() { cout << "A" << endl; return 1; }
int B() { cout << "B" << endl; return 2; }
int main(void)
{
    cout << A() << B();
	return 0;
}

Which gives you:

B
A
12

Showing that the result of B is calculated before the result of A.

Evaluation order is unspecified, so this problem way only occur in some compilers: I used the gdb compiler, which evaluates right to left, but others may work the another way round!
Order of evaluation - cppreference.com[^]