The original intent of 13.3.3.1p4 is to describe how to apply the requirement in 12.3p4 that: 4 – At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value. Before defect 84, 13.3.3.1p4 was almost purely informative: 4 – In the context of an initialization by user-defined conversion (i.e., when … Read more
I assume, with your examples for std::vector<int> and std::string you meant to also cover the other containers, e.g., std::list<int>, std::deque<int>, etc. which have the same problem, obviously, as std::vector<int>. Likewise, the int isn’t the only type as it also applies to char, short, long and their unsigned version (possibly a few other integral types, too). … Read more
As pointed out by Nicol Bolas, the original version of this answer was incorrect: cppreference at the time of writing incorrectly documented the order in which constructors were considered in list-initialization. Below is an answer using the rules as they exist in the n4140 draft of the standard, which is very close to the official … Read more
There are two important reasons for templates not to do any deduction (the two that I remember in a discussion with the guy in charge) Concerns about future language extensions (there are multiple meanings you could invent – what about if we wanted to introduce perfect forwarding for braced init list function arguments?) The braces … Read more
Because they don’t do the exact same thing. As stated in 13.3.1.7 [over.match.list]: In copy-list-initialization, if an explicit constructor is chosen, the initialization is ill-formed. In short, you can only use implicit conversion in copy-list-initialization contexts. This was explicitly added to make uniform initialization not, um, uniform. Yeah, I know how stupid that sounds, but … Read more
When viewing things this way it is easy to say there is complete and utter chaos in the way an object is initialized. The big difference comes from the type of foo: if it is an aggregate type or not. It is an aggregate if it has: no user-provided constructors (a deleted or defaulted function … Read more
Answer for C++ standard versions before C++17: Your problem has to do with aggregate initialization: struct X is an aggregate while struct Y is not. Here is the standard quote about aggregates (8.5.1): An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no brace-or-equal-initializers for non-static data members (9.2), … Read more
std::initializer_list is not a container, don’t use it to pass values around and expect them to persist DR 1290 changed the wording, you should also be aware of 1565 and 1599 which aren’t ready yet. Then the return value’s array should also survive into the calling function, and it should be possible to preserve it … Read more
Basically copying and pasting from Bjarne Stroustrup’s “The C++ Programming Language 4th Edition”: List initialization does not allow narrowing (§iso.8.5.4). That is: An integer cannot be converted to another integer that cannot hold its value. For example, char to int is allowed, but not int to char. A floating-point value cannot be converted to another … Read more
Basically copying and pasting from Bjarne Stroustrup’s “The C++ Programming Language 4th Edition”: List initialization does not allow narrowing (§iso.8.5.4). That is: An integer cannot be converted to another integer that cannot hold its value. For example, char to int is allowed, but not int to char. A floating-point value cannot be converted to another … Read more