Modifying set elements - c++

Question

With the below code, I am trying to update the values in a set but its not compiling when i try to compile .
It's giving me the error at the bottom, can you please help ?
What wrong I am doing here?
#include < iostream >
#include < set >
using namespace std;
class A
{
public:
int a,b;
bool operator()(A a1,A a2)
{
return true;
}
A(int a ,int b)
{
this.a=a;
this.b=b;
}
};
void print_set(const std::set<A>&st) const
{
std::set<A>::iterator it;
std::cout<<"\nvalues in set";
for(it=st.begin();it!=st.end();it++)
{
std::cout<<"\na="<<it->a<<"b="<<it->b;
}
}
int main ()
{
std::set<A> s;
for ( int i=0;i<5;i++)
{
s.insert(A(i,i+1));
}
print_set(s);
std::set<A>::iterator it;
for(it=s.begin();it!=s.end();it++)
{
A tmp=*it;
s.erase(it);
tmp.a=10;
tmp.b=20;
s.insert(tmp);
std::cout<<"\ninserting tmp "<<tmp.a<<" "<<tmp.b;
}
print_set(s);
return 0;
}
I am getting an error like this :
/usr/include/c++/4.6/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = A]’:
/usr/include/c++/4.6/bits/stl_tree.h:1277:4: instantiated from ‘std::pair<std::_Rb_tree_iterator<_Val>, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = A, _Val = A, _KeyOfValue = std::_Identity<A>, _Compare = std::less<A>, _Alloc = std::allocator<A>]’

Answer 1

A few issues in your code, see below comments:
A(int a ,int b)
{
//this.a=a;
//this.b=b;
this->a = a; // this pointer should be accessed by this->
this->b = b;
}
// to store element in std::set, it must follow strict weak ordering rule.
// by default, operator< need to be defined
bool operator<(const A& lhs, const A& rhs)
{
return lhs.a < rhs.a;
}
// print_set is not a member function, can't have trailing const after function name
void print_set(const std::set<A>&st) // const