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Initializing from an initializer list, but without {{{{{{{{ ... }}}}}}}}?

Asked 2011-04-20T16:50:13.353
25

I recently stumbles across some problem with initializer lists. Consider a program that stores map-like data

struct MyMapLike {
  MyMapLike(std::map<std::string, int> data)
    :data(std::move(data))
  { }

private:
  std::map<std::string, int> data;
};

That looks straight forward. But when initializing it, it becomes ugly. I want to let it look like

MyMapLike maps = { { "One", 1 }, { "Two", 2 } };

But the compiler doesn't want to accept this, because the above means that it should look for a two-parameter constructor that can accept { "One", 1 } and { "Two", 2 } respectively. I need to add extra braces, to make it look like a single-parameter constructor accepting the { { ... }, { ... } }

MyMapLike maps = { { { "One", 1 }, { "Two", 2 } } };

I would not like to write it like that. Since I have a map-like class, and the initializer has the abstract value of a mapping-list, I would like to use the former version, and be independent of any such implementation details like level of nesting of constructors.

One work around is to declare an initializer-list constructor

struct MyMapLike {
  MyMapLike(std::initializer_list< 
    std::map<std::string, int>::value_type
    > vals)
    :data(vals.begin(), vals.end())
  { }

  MyMapLike(std::map<std::string, int> data)
    :data(std::move(data))
  { }

private:
  std::map<std::string, int> data;
};

Now I can use the former, because when I have an initializer-list constructor, the whole initializer list is treated as one element instead of being splitted into elements. But I think this separate need of the constructor is dead ugly.

I'm looking for guidance:

  • What do you think about the former and latter form of initialization? Does it make sense to be required to have extra braces in this
c++ c++11 initializer-list
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2 Answers

10

Since I have a map-like class, and the initializer has the abstract value of a mapping-list, I would like to use the former version

And herin lies the problem: it's up to you to supply the constructors that allow your class to be treated like a map. You called your solution a work-around, but there's nothing to work around. :)

But I think this separate need of the constructor is dead ugly.

It is, but unfortunately since it's your class, you have to specify how the initializer lists work.

answered 2011-04-21T15:51:48.630
6

What do you think about the former and latter form of initialization? Does it make sense to be required to have extra braces in this case?

I think so. I think it would permit too much ambiguity to allow a constructor be called not just when the syntax matches that constructor, but when the syntax matches some constructor of the single argument to the constructor, and so on recursively.

struct A { int i; };
struct B { B(A) {}  B(int) {} };
struct C { C(B) {} };

C c{1};

Do you consider the requirement for addition of an initializer list constructor in this case bad?

No. It lets you get the syntax you want but without creating the problems that arise if we make the compiler search harder for a constructor to use.

answered 2012-04-02T22:31:31.267

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