What does vectoralphabet(26) do in C plus plus?

Answer 1

It doesn't do anything other than to create a compiler error. A vector is a class template thus you must specify the element type in the type declaration. For example, a std::vector<T> is a vector of type T elements.

It is assumed you really meant the following:

std::vector<char> alphabet (26);

This declaration constructs a vector of type char with a length of 26 elements. The elements are default initialised, thus the vector will contain 26 NULL characters (ASCII character code 0).