C++ Primer, Fifth Edition

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.

Note

Older compilers may require a space between the angle ...