Constraints on Types That a Container Can Hold

Almost any type can be used as the element type of a sequential container. In particular, we can define a container whose element type is itself another container. We define such containers exactly as we do any other container type: We specify the element type (which in this case is a container type) inside angle brackets:

vector<vector<string>> lines;      // vector of vectors

Here lines is a vector whose elements are vectors of strings.

Although we can store almost any type in a container, some container operations impose requirements of their own on the element type. We can define a container for a type that does not support an operation-specific requirement, but we can use an operation only if the element type meets that operation’s requirements.

As an example, the sequential container constructor that takes a size argument (§ 3.3.1, p. 98) uses the element type’s default constructor. Some classes do not have a default constructor. We can define a container that holds objects of such types, but we cannot construct such containers using only an element count:

// assume noDefault is a type without a default constructor
vector<noDefault> v1(10, init); // ok: element initializer supplied
vector<noDefault> v2(10);       // error: must supply an element initializer

As we describe the container operations, we’ll note the additional constraints, if any, that each container operation places on the element type.