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Can a std::initializer_list contain reference types (both rvalue and lvalue)? Or does one have to use pointers or a reference wrapper (such as std::ref)?

EDIT:

Perhaps more clarification is due:

I have a member variable, ::std::vector<std::function<void()> >, into which I would like to forward a lambda object. This would usually be accomplished with emplace_back, but I wanted to do it in the constructor's initialization list. Alas, as I read, this would make forwarding impossible.

user1095108
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2 Answers2

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Can a std::initializer_list contain reference types (both rvalue and lvalue)?

std::initializer_list<T> doesn't hold references to its elements. It uses copy-semantics by holding its values as const objects:

18.9 Initializer List [support.initlist]

An object of type initializer_list<E> provides access to an array of objects of type const E.

An initializer_list of references will cause a compilation error because iternally pointers are used for iterators:

#include <initializer_list>
int main()
{
    int x;
    std::initializer_list<int&> l = {x};

    // In instantiation of 'class std::initializer_list<int&>':
    // error: forming pointer to reference type 'int&'

    // typedef const _E*  iterator;
}

An initializer_list also doesn't support move-semantics as const objects cannot be moved from. Holding your objects in a std::reference_wrapper<T> is the most viable solution if you wish to maintain reference-semantics.

David G
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  • I believe they do support a kind of move semantics, as you technically _can_ "move" a `const &&`. – user1095108 Jun 08 '14 at 19:08
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    @user1095108 No they don't. A move doesn't happen just from calling `std::move()`, it's simply a cast to an rvalue. A constructor taking a non-const rvalue reference `(X&&)` is where the movement actually happens, and "moving" a `const` object will actually create a copy in some cases, and in others a compilation error. – David G Jun 08 '14 at 19:14
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    I know, but nothing prevents a move constructor from copying, instead of moving, either. I'm just nitpicking. – user1095108 Jun 08 '14 at 19:16
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From http://www.cplusplus.com/reference/initializer_list/initializer_list/

initializer_list objects are automatically constructed as if an array of elements of type T was allocated

thus they can't be used with something like std::initializer_list<int&>. The reason is the same for which the following gives a compiler error

int& arr[20];

error: declaration of ‘arr’ as array of references

and that is dictated by the C++ standard: https://stackoverflow.com/a/1164306/1938163

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Marco A.
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