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suppose I have a structure as follwos -
[cpp]
struct test_
{
int id,val1,val2;
};
[/cpp]

Then a STL priority queue with type test_ is created. Now if I want to set val1 as the key of the priority queue, I guess I need to write my own comparison function overriding the default COMP. Now can someone please give me an example of such a custom compare function & how the priority_queue is to be declared then? Thanks in advance.

No, you just have to add operator< to test_ class. For instance:
[cpp]struct test_{
int id, val1, val2;
bool operator<(const test_ &t)const{
if(val1 != t.val1)return (val1 < t.val1);
else return (val2 < t.val2);
}
}

int main(){
priority_queue<test_> q;
}

[/cpp]Note that priority_queue will return by default the biggest element when top() is called.
If you want to use a custom binary bool function object, then you should do it this way:
[cpp]
priority_queue<test_, vector<test_>, cmp_object> q;
[/cpp]
For instance, to get the smallest element in queue, use greater<type_> object:
[cpp]
priority_queue<test_, vector<test_>, greater<test_> > q;
[/cpp]
In this case you have to define operator> in class test_.

[cpp]struct Foo
{
int key;
int value;

bool operator<(const Foo& foo) const
{
return key < foo.key;
}
};

priority_queue<Foo> myPriorityQueue;[/cpp]

There are many other ones to do it, but usually it is just best to override the less than operator, which is the default for any ordered STL container.

Many thanks to both of you for help. I have finally been made it to work.

I wonder if someone could help me:
I have priority_queue of pointers to struct:

struct type{
int val;
};

priority_queue<type*>Q;

How can I overload operator?

It's quite straightforward. You must provide a function or a function object that compares type*. An example:
You use it this way: