9.12.txt

Exercise 9.12: Explain the differences between the constructor that takes a container
to copy and the constructor that takes two iterators.

by Faisal Saadatmand

From table 9.3 (p.335)

	C c1(c2) // constructor that takes a container
	C c(b, e) // constructor that takes two iterators

Both constructor copy elements of one container to another. The former copies
all elements, while the latter copies a range of elements between two iterators
from given container to another. There some requirements that we must observe
when we use either constructor.

When we copy using the constructor that takes a container as a parameter, the
types of the containers and their elements must match exactly (in the case of
the library array the size too must match, as it is part of the container's
type). Note, the constructor is explicit, so no implicit conversion between
elements of convertible types is allowed either.

	vector<int> v1{93, 5, 26, 65, 7};
	vector<int> v2(v1); // O.K.: types match
	list<int> l(v1); // erro: container types mismatch
	vector<string> v3(v1); // error: element types mismatch
	vector<double> v4(v1); // error: no implicit conversion types is allowed

Such a requirement is not imposed when we use the constructor that takes
iterators to copy elements from one container to another. With the exception of
the library array, we can copy from one container to another even if they
differ in type. Moreover, their elements can differ too, provided that it is
possible to convert the element type of the container we are initializing.

	list<int> l1(v1.cbegin(), v1.cend());  // O.K.: container types can differ
	vector<double> v5(v1.cbegin(), v1.cend()); // O.K.: implicit conversion