Question

I have been reading “C++ primer”. For the initialization of object, C++ supports 2 forms of initialization: direct and copy.
but the book does not refer the initialization of reference. And in the book I have never seen the direct initialize(if exists) of a reference. All is the copy one like:

int i;
int &j = i;//but not int &j(i);which also works in my experiment

I want to know that is it the same that are going on underneath for the initialization of a reference.
for the following codes:

string null_book = "9-999-99999-9";

the initialization progress is first create a temporary string object tmp(for instance) that will direct initialized with a c style string parameter, and then initialize the variable null_book with the copy Constructor. That make sense to me.
for this one:

int &j = i;

will ref j be initialized the same way? That will be a temp reference it &t(for example) initialized by i and then initialize j with t? that doesnt make sense??? Why the book never use the direct initialization for reference?
Thanks for your attention!

Answer 1

The main differences between direct-initialization and copy-initialization are covered in section 8.5, paragraph 17 of the standard. In general, the difference is that for class types in copy-initialization, explicit constructors are not considered (only converting constructors are considered) and a possibly elided copy is made; in direct-initialization explicit constructors are considered and the target is constructed directly. From section 8.5 of the standard:

14 – The form of initialization (using parentheses or =) is generally insignificant, but does matter when the initializer or the entity being initialized has a class type […]

For non-class types (including references), direct-initialization and copy-initialization have similar semantics; for references, a reference binding occurs in either case, as specified in 8.5.3 References [dcl.init.ref]. Direct-initialization and copy-initialization of a reference only have different semantics where a conversion function is involved (13.3.1.6 Initialization by conversion function for direct reference binding [over.match.ref]); again, direct-initialization is allowed to invoke explicit conversion functions where copy-initialization is not.

So, in

int &j = i;

8.5.3p5 applies and the reference j is bound directly to the lvalue i. No temporaries are invoked.

In terms of complexity, references are closer to fundamental (primitive) types than to class types. Primitives are initialized without a temporary being constructed (8.5p17, last bullet) and in general references are too. This is probably why the book only uses the = form for initialization of references; as with primitives, there is usually no difference and writing int i = x; is usually clearer than int i(x);.