I know that for scalar types you can assign values with braces like int a { 0 };.
This helps with cast, type conversion ecc.
But what for udt? Is
shared_ptr<int> myIntSmartPtr { my_alloc(42), my_free };
the same as
shared_ptr<int> myIntSmartPtr = shared_ptr<int>(my_alloc(42), my_free);
The braces should call the constructor, right?
Is it like an initializer list?
I know what an std::initializer_list is, but it must be the same type T, while in { my_alloc(42), my_free } the types diverge.
CodePudding user response：
This is direct list initialization.
shared_ptr<int> myIntSmartPtr { my_alloc(42), my_free };
This is an example of the first syntax:
T object { arg1, arg2, ... }; (1)
The exact effect it has is therefore
List initialization is performed in the following situations:
- direct-list-initialization (both explicit and non-explicit constructors are considered)
- initialization of a named variable with a braced-init-list (that is, a possibly empty brace-enclosed list of expressions or nested braced-init-lists)
And for more detail about what that actually means:
The effects of list-initialization of an object of type T are:
... [A bunch of cases that don't apply]
Otherwise, the constructors of T are considered, in two phases:
- All constructors that take std::initializer_list as the only argument, or as the first argument if the remaining arguments have default values, are examined, and matched by overload resolution against a single argument of type std::initializer_list
- If the previous stage does not produce a match, all constructors of T participate in overload resolution against the set of arguments that consists of the elements of the braced-init-list, with the restriction that only non-narrowing conversions are allowed. If this stage produces an explicit constructor as the best match for a copy-list-initialization, compilation fails (note, in simple copy-initialization, explicit constructors are not considered at all).
std::shared_ptr does not have a constructor that takes an std::initializer_list, so the second bullet point applies and it's constructed from the arguments therein.