The comparison between set objects is based on a pairwise comparison of their elements. The less than or equal to relationship between two objects is based on a comparison of the first pair of unequal elements.

// set_op_le.cpp
// compile with: /EHsc
#include <set>
#include <iostream>
int main( )
{
using namespace std;
set <int> s1, s2, s3, s4;
int i;
for ( i = 0 ; i < 3 ; i++ )
{
s1.insert ( i );
s2.insert ( i * i );
s3.insert ( i - 1 );
s4.insert ( i );
}
if ( s1 <= s2 )
cout << "Set s1 is less than or equal to the set s2." << endl;
else
cout << "The set s1 is greater than the set s2." << endl;
if ( s1 <= s3 )
cout << "Set s1 is less than or equal to the set s3." << endl;
else
cout << "The set s1 is greater than the set s3." << endl;
if ( s1 <= s4 )
cout << "Set s1 is less than or equal to the set s4." << endl;
else
cout << "The set s1 is greater than the set s4." << endl;
}

Set s1 is less than or equal to the set s2.
The set s1 is greater than the set s3.
Set s1 is less than or equal to the set s4.