Is initializing with “var{args}” a new feature of C++0x, or merely syntactic sugar?

Question

I was reading the C++0x faq and came across the section detailing initializer lists. The examples were mostly variations of:

vector vi = { 1, 2,          


        

Answer 1

The ({1, 2, 3}) form calls the constructors of vector directly, and passes as first argument a {1, 2, 3}. You could have passed more arguments

vector vk({1, 2, 3}, myAllocator);

If vector would not have a constructor whose first parameter is an initializer_list or of another type that could be initialized by {1, 2, 3} (like, another container class), it would not work. In your case it works because vector in fact has a constructor whose first parameter is a initializer_list. This is just like in normal function calls

void f(vector const& vk);
int main() { f({1, 2, 3}); }

If you omit the parentheses, as in vector vk{1, 2, 3}, the exact meaning depends on the class. A vector has an initializer list constructor, which is a constructor with a first parameter of type initializer_list (optionally a reference to it), and all other params with default arguments. If the class has such a constructor, then the initializer list is passed to that constructor. Alternatively the class could simply be an aggregate (like struct A { int a; int b; int c; };, the initializer list would then init the members) or have a constructor that accepts 3 separate int arguments.

Finally the = { 1, 2, 3 } form is almost identical to the version omitting the parentheses (i.e just removing =), except that it forbids to use explicit constructors (i.e had they declared it as explicit vector(initializer_list); or had they declared a explicit vector(int, int, int); instead, it would result in an error if you use = { 1, 2, 3 }).