std::relational operators (map)

(1)	template <class Key, class T, class Compare, class Alloc> bool operator== ( const map<Key,T,Compare,Alloc>& lhs, const map<Key,T,Compare,Alloc>& rhs );
(2)	template <class Key, class T, class Compare, class Alloc> bool operator!= ( const map<Key,T,Compare,Alloc>& lhs, const map<Key,T,Compare,Alloc>& rhs );
(3)	template <class Key, class T, class Compare, class Alloc> bool operator< ( const map<Key,T,Compare,Alloc>& lhs, const map<Key,T,Compare,Alloc>& rhs );
(4)	template <class Key, class T, class Compare, class Alloc> bool operator<= ( const map<Key,T,Compare,Alloc>& lhs, const map<Key,T,Compare,Alloc>& rhs );
(5)	template <class Key, class T, class Compare, class Alloc> bool operator> ( const map<Key,T,Compare,Alloc>& lhs, const map<Key,T,Compare,Alloc>& rhs );
(6)	template <class Key, class T, class Compare, class Alloc> bool operator>= ( const map<Key,T,Compare,Alloc>& lhs, const map<Key,T,Compare,Alloc>& rhs );

Relational operators for map

Performs the appropriate comparison operation between the map containers lhs and rhs.

The equality comparison (operator==) is performed by first comparing sizes, and if they match, the elements are compared sequentially using operator==, stopping at the first mismatch (as if using algorithm equal).

The less-than comparison (operator<) behaves as if using algorithm lexicographical_compare, which compares the elements sequentially using operator< in a reciprocal manner (i.e., checking both a<b and b<a) and stopping at the first occurrence.

The other operations also use the operators == and < internally to compare the elements, behaving as if the following equivalent operations were performed:

operation	equivalent operation
`a!=b`	`!(a==b)`
`a>b`	`b<a`
`a<=b`	`!(b<a)`
`a>=b`	`!(a<b)`

Notice that none of these operations take into consideration the internal comparison object of either container, but compare the elements (of type value_type) directly.

value_type is a pair type, and as such, by default, two elements will compare equal only if both their key and mapped value compare equal, and one compare lower than the other only if the first key is lower, or if the keys are equivalent and the mapped value is lower.

These operators are overloaded in header <map>.

Parameters

lhs, rhs: map containers (to the left- and right-hand side of the operator, respectively), having both the same template parameters (Key, T, Compare and Alloc).

Example

// map comparisons
#include <iostream>
#include <map>

int main ()
{
  std::map<char,int> foo,bar;
  foo['a']=100;
  foo['b']=200;
  bar['a']=10;
  bar['z']=1000;

  // foo ({{a,100},{b,200}}) vs bar ({a,10},{z,1000}}):
  if (foo==bar) std::cout << "foo and bar are equal\n";
  if (foo!=bar) std::cout << "foo and bar are not equal\n";
  if (foo< bar) std::cout << "foo is less than bar\n";
  if (foo> bar) std::cout << "foo is greater than bar\n";
  if (foo<=bar) std::cout << "foo is less than or equal to bar\n";
  if (foo>=bar) std::cout << "foo is greater than or equal to bar\n";

  return 0;
}

Output:

foo and bar are not equal
foo is greater than bar
foo is greater than or equal to bar

Return Value

true if the condition holds, and false otherwise.

Complexity

Up to linear in the size of lhs and rhs.

For (1) and (2), constant if the sizes of lhs and rhs differ, and up to linear in that size (equality comparisons) otherwise.
For the others, up to linear in the smaller size (each representing two comparisons with operator<).

Iterator validity

No changes.

Data races

Both containers, lhs and rhs, are accessed.
Up to all of their contained elements may be accessed.

Exception safety

If the type of the elements supports the appropriate operation with no-throw guarantee, the function never throws exceptions (no-throw guarantee).
In any case, the function cannot modify its arguments.

member functions

non-member overloads