c++c++11list-initializationaggregate-initializationctor-initializer

How to initialize an array member in a member initializer list


class C 
{
public:
 C() : arr({1,2,3}) //doesn't compile
{}
    /*
    C() : arr{1,2,3} //doesn't compile either
{}
    */
private:
 int arr[3];
};

I believe the reason is that arrays can be initialized only with = syntax, that is:

int arr[3] = {1,3,4};

Questions

  1. How can I do what I want to do (that is, initialize an array in a constructor (not assigning elements in the body)). Is it even possible?
  2. Does the C++03 standard say anything special about initializing aggregates (including arrays) in ctor initializers? Or the invalidness of the above code is a corollary of some other rules?
  3. Does C++11 list initialization solve the problem?

Note that I do not want to use std::array or another container to solve this problem.


Solution

    1. How can I do what I want to do (that is, initialize an array in a constructor (not assigning elements in the body)). Is it even possible?

    Yes. It's using a struct that contains an array. You say you already know about that, but then I don't understand the question. That way, you do initialize an array in the constructor, without assignments in the body. This is what boost::array does.

    Does the C++03 standard say anything special about initializing aggregates (including arrays) in ctor initializers? Or the invalidness of the above code is a corollary of some other rules?

    A mem-initializer uses direct initialization. And the rules of clause 8 forbid this kind of thing. I'm not exactly sure about the following case, but some compilers do allow it.

    struct A {
      char foo[6];
      A():foo("hello") { } /* valid? */
    };
    

    See this GCC PR for further details.

    Do C++0x initializer lists solve the problem?

    Yes, they do. However your syntax is invalid, I think. You have to use braces directly to fire off list initialization

    struct A {
      int foo[3];
      A():foo{1, 2, 3} { }
      A():foo({1, 2, 3}) { } /* invalid */
    };