[Solved] What is the output of this code? Am i missing something here?

A correct program can look for example the following way

#include <iostream>

int main() 
{
    int num, a = 15; 
    num = ( ----a )--;

    std::cout << num << std::endl;
    std::cout << a << std::endl;

    return 0;
}

Its output is

13
12

The post-decrement operator has a higher priority than the pre-decrement operator and its result is rvalue that may not be changed.

This expression

----a--

is equivalent to

----( a-- )

and will not compile.

So you need to use parentheses to make the program to compile.

Take into account that a corresponding program written in C as for example

#include <stdio.h>

int main( void ) 
{
    int num, a = 15; 
    num = ( ----a )--;

    printf( "%d\n", num );
    printf( "%d\n", a );

    return 0;
}

will not compile because the pre-decrement operator in C also returns rvalue and you may not apply the operator to rvalue.

Only the C++ program is valid.