Why wasn't a double curly braces syntax preferred for constructors taking a std::initializer_list

问题

Uniform initialization is an important and useful C++11 feature. However, you can't just use {} everywhere since:

std::vector<int> a(10, 0);    // 10 elements of value zero
std::vector<int> b({10, 0});  // 2 elements of value 10 and 0 respectively
std::vector<int> c{10, 0};    // 2 elements of value 10 and 0 respectively
std::vector<int> d = {10, 0}; // 2 elements of value 10 and 0 respectively

auto e(0);    // deduced type is int
auto f = 0;   // deduced type is int
auto g{0};    // deduced type is std::initializer_list<int>
auto h = {0}; // deduced type is std::initializer_list<int>

Noting that aggregate initialization on e.g. std::arrays requires the use of {{}}, it seems to me that the whole problem with which vector constructor will be selected could have been avoided by requiring a {{}} to call constructors taking a std::initializer_list:

std::vector<int> i{10, 0};    // 10 elements of value zero
std::vector<int> j{{10, 0}};  // 2 elements of value 10 and 0 respectively
std::vector<int> k = {10, 0}; // 2 elements of value 10 and 0 respectively

auto l{0};    // deduced type is int
auto m{{0}};  // deduced type is std::initializer_list<int>
auto n = {0}; // deduced type is std::initializer_list<int>

I'm sure this was discussed, so what were the reasons against this? A quote/link from a standard proposal is preferred as answer.

Update. — There is a point in N2532 that states:

(3) The likely nasty ambiguity cases occur only for short initializer lists [...]

(5) Why should the language rules force programmers who wants terseness and ambiguity control (for perfectly good reasons) to write more to please programmers who prefer (for perfectly good reasons) to be more explicit – and can be?

[...]

Assume that a programmer expects f(X) to be called. How might a f(Y) “hijack” a call?

(4) Assume that X has no initializer-list constructor, but Y does. In this case, the priority given to initializer-list constructors favor the hijacker (remember we assumed that the programmer somehow expected f(X) to be called). This is analogous to someone expecting f(y) to invoke f(X) using a user-defined conversion and someone comes along with an f(Y) that matches exactly. I think it would be fair to expect that someone who uses {…} will remember the possibility of initializer-lists constructors. [emphasis mine]

I guess the key lies in the can be, which means you don't have to use uniform initialization. Using {} correctly is hard since:

• you not only have to check for the constructor you want to call but also for any constructor taking an initializer_list that might win (and probably will) over it;

• if you write code using {} and someone in the future adds an std::initializer_list constructor your code might break and do so silently.

Even if you have a class A with the constructors A(int, bool) and A(std::initializer_list<double>), the latter will be selected over the former for A a{0, false}; (which IMO is nuts), so I find it really hard to use uniform initialization on classes that have or might have (crystal ball superpowers required) initializer_list constructors.

The fact that your code can silently break worries me a lot.

回答1:

Here's what Stroustrup has said on the subject:

Uniform and universal was not designed to be just a 4th alternative. It was designed to be the initialization syntax,and was unfortunately [not] feasible to use with all legacy code, especially vector. Had I designed vector today, you would have had to say something like vector<int> {Count{9}}; to get a count.

And in response to the question "Is the problem vector or {}-init syntax?"

It's the vector design: Had I designed vector today, you would have had to say something like vector<int> {Count{9}}; to get a count.

The more general problem is to have several semantically different arguments of the same type eventually leads to confusion, especially if they can appear adjectly. For example:

vector<int> v(7,2);    // 7 (a count) element with the value 2

回答2:

(This isn't really an answer, just a discussion of what I've thought about on this issue.)

I think I'd like the compilers to give a warning in cases where there is ambiguity, advising the developer to use either ({ }) (if they do want the initializer_list), or ( ) if they do not. With an extra warning if the MostVexingParse is a risk! - perhaps recommend (( )) to avoid that?

(This following 'story' is probably not based on the correct historical chronology of how the feature was developed, but it's how I understand the current rules in the compiler.)

In the beginning we had constructors:

type t (...);

Then we had the idea of allowing { to give a literal collection for use in constructors (but also elsewhere).

type t ( {...} );

... along with a new initializer_list type in the constructors to match this.

Then, we were allowed to replace ( ) with { } in order to avoid the Most Vexing Parse:

type t { ... };
type t { {...} };

So far, so good. Pure extensions to the language.

Finally, the 'controversial' addition is that, when the compiler sees { ... } (as a constructor) it will first attempt to rewrite that as ({ ... }) (calling the initializer_list if it exists) before falling back on ( ... ). I think I'd prefer if both options were considered equally good and there to be a warning or error if both are possible.

来源：https://stackoverflow.com/questions/22501368/why-wasnt-a-double-curly-braces-syntax-preferred-for-constructors-taking-a-std