9.2.7. Relational Operators

Every container type supports the equality operators (== and !=); all the containers except the unordered associative containers also support the relational operators (>, >=, <, <=). The right- and left-hand operands must be the same kind of container and must hold elements of the same type. That is, we can compare a vector<int> only with another vector<int>. We cannot compare a vector<int> with a list<int> or a vector<double>.

Comparing two containers performs a pairwise comparison of the elements. These operators work similarly to the string relationals (§ 3.2.2, p. 88):

• If both containers are the same size and all the elements are equal, then the two containers are equal; otherwise, they are unequal.

• If the containers have different sizes but every element of the smaller one is equal to the corresponding element of the larger one, then the smaller one is less than the other.

• If neither container is an initial subsequence of the other, then the comparison depends on comparing the first unequal elements.

The following examples illustrate how these operators work:

vector<int> v1 = { 1, 3, 5, 7, 9, 12 };
vector<int> v2 = { 1, 3, 9 };
vector<int> v3 = { 1, 3, 5, 7 };
vector<int> v4 = { 1, 3, 5, 7, 9, 12 };
v1 < v2  // true; v1 and v2 differ at element [2]: v1[2] is less than v2[2]
v1 < v3  // false; all elements are equal, but v3 has fewer of them;
v1 == v4 // true; each element is equal and v1 and v4 have the same size()
v1 == v2 // false; v2 has fewer elements than v1

Relational Operators Use Their Element’s Relational Operator

The container equality operators use the element’s == operator, and the relational operators use the element’s < operator. If the element type doesn’t support the required operator, then we cannot use the corresponding operations on containers holding that type. For example, the Sales_data type that we defined in Chapter 7 does not define either the == or the < operation. Therefore, we cannot compare two containers that hold Sales_data elements:

vector<Sales_data> storeA, storeB;
if (storeA < storeB) // error: Sales_data has no less-than operator