CODEWITHSUNDEEP
c++c++11list-initializationaggregate-initializationctor-initializer
Armen Tsirunyan
Armen Tsirunyan

Reputation: 133112

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.

Upvotes: 125

Views: 229207

Answers (7)

Ed Graham
Ed Graham

Reputation: 4725

This worked for me using C++17 on Windows:

File A.h:

class A
{
  private:
    float arrayVar[3];
}

File A.cpp:

A::A() : arrayVar{ 0.0f, 0.0f, 0.0f } { }

Upvotes: 1

no one special
no one special

Reputation: 1762

I found this question very useful, but found no example for the case when the member array element is an object without default constructor and with deleted copy/move constructors. In other words the example that initializes the member array without unnecessary object copy.

For example, with a following class A:

class A {
  public:
    int v;
    A(int v) : v(v) { }
    A() = delete;
    A(A &&) = delete;
    A(const A &) = delete;
    A &operator =(A &&) = delete;
    A &operator =(const A &) = delete;
};

In-place initialization with non-default constructor would look like:

class B {
  public:
    A a[3];
    B() : a { {1}, {2}, {3} } {}
};

Upvotes: 3

eold
eold

Reputation: 6052

How about

...
  C() : arr{ {1,2,3} }
{}
...

?

Compiles fine on g++ 4.8

Upvotes: 1

Cheers and hth. - Alf
Cheers and hth. - Alf

Reputation: 145429

C++98 doesn't provide a direct syntax for anything but zeroing (or for non-POD elements, value-initializing) the array. For that you just write C(): arr() {}.

I thing Roger Pate is wrong about the alleged limitations of C++0x aggregate initialization, but I'm too lazy to look it up or check it out, and it doesn't matter, does it? EDIT: Roger was talking about "C++03", I misread it as "C++0x". Sorry, Roger. ☺

A C++98 workaround for your current code is to wrap the array in a struct and initialize it from a static constant of that type. The data has to reside somewhere anyway. Off the cuff it can look like this:

class C 
{
public:
    C() : arr( arrData ) {}

private:
     struct Arr{ int elem[3]; };
     Arr arr;
     static Arr const arrData;
};

C::Arr const C::arrData = {{1, 2, 3}};

Upvotes: 39

Johannes Schaub - litb
Johannes Schaub - litb

Reputation: 507343

  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 */
};

Upvotes: 68

Alexey Malistov
Alexey Malistov

Reputation: 27023

Workaround:

template<class T, size_t N>
struct simple_array { // like std::array in C++0x
   T arr[N];
};


class C : private simple_array<int, 3> 
{
      static simple_array<int, 3> myarr() {
           simple_array<int, 3> arr = {1,2,3};
           return arr;
      }
public:
      C() : simple_array<int, 3>(myarr()) {}
};

Upvotes: 9

Roger Pate
Roger Pate

Reputation:

  1. No, unfortunately.
  2. You just can't in the way you want, as it's not allowed by the grammar (more below). You can only use ctor-like initialization, and, as you know, that's not available for initializing each item in arrays.
  3. I believe so, as they generalize initialization across the board in many useful ways. But I'm not sure on the details.

In C++03, aggregate initialization only applies with syntax similar as below, which must be a separate statement and doesn't fit in a ctor initializer.

T var = {...};

Upvotes: 4

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