Default values for arrays members of struct

Question

Possible Duplicate:
Intitialzing an array in a C++ class and modifiable lvalue problem

As seen in this question, it's possible to give a ctor to a struct to make it members get default values. How would you proceed to give a default value to every element of an array inside a struct.

struct foo
{
   int array[ 10 ];
   int simpleInt;
   foo() : simpleInt(0) {}; // only initialize the int...
}

Is there some way to make this in one line similar to how you would do to initialize an int?

@Francis: As written right now, this code doesn't work in C, since it uses a constructor. Constructors are a C++ feature. — unwind, Jul 11 '11 at 15:04

Cubbi · Accepted Answer · 2011-07-11T16:03:57.003

7

Thew new C++ standard has a way to do this:

struct foo
{
   int array[ 10 ];
   int simpleInt;
   foo() : array{1,2,3,4,5,6,7,8,9,10}, simpleInt(0) {};
};

test: https://ideone.com/enBUu

If your compiler does not support this syntax yet, you can always assign to each element of the array:

struct foo
{
   int array[ 10 ];
   int simpleInt;
   foo() : simpleInt(0)
   {
        for(int i=0; i<10; ++i)
            array[i] = i;
   }
};

EDIT: one-liner solutions in pre-2011 C++ require different container types, such as C++ vector (which is preferred anyway) or boost array, which can be boost.assign'ed

#include <boost/assign/list_of.hpp>
#include <boost/array.hpp>
struct foo
{
    boost::array<int, 10> array;
    int simpleInt;
    foo() : array(boost::assign::list_of(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)),
            simpleInt(0) {};
};

edited Jul 11 '11 at 16:03

answered Jul 11 '11 at 15:13

Cubbi

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So there is no _one liner_ for this type of operation? – ALOToverflow Jul 11 '11 at 15:43
@Francis - by "one liner" do you mean "in the ctor-initializer"? If it's a "one liner" you're seeking, just hoist the ctor body onto the same line (I'm not being snarky, just trying to understand your comment a little better) – Dan Jul 11 '11 at 16:51
@Dan - I was referring to ctor-initializer – ALOToverflow Jul 11 '11 at 17:45
What do you mean by "which is preferred anyway"? – Moberg Sep 20 '17 at 12:22

score 4 · Answer 2 · answered Jul 11 '11 at 15:15

4

Changing the array to a std::vector will allow you to do simple initialization and you'll gain the other benefits of using a vector.

#include <vector>
struct foo
{
  std::vector<int> array;
  int simpleInt;
  foo() : array(10, 0), simpleInt(0) {}; // initialize both
};

answered Jul 11 '11 at 15:15

Nathan Buckles

221
1
5

You will also get the negatives of using a vector. – Moberg Sep 20 '17 at 12:23
@Moberg which are? – user3085931 Nov 02 '18 at 09:15

score 2 · Answer 3 · answered Jul 11 '11 at 15:43

2

If you just want to default-initialize the array (setting built-in types to 0), you can do it like this:

struct foo
{
   int array[ 10 ];
   int simpleInt;
   foo() : array(), simpleInt(0) { }
};

answered Jul 11 '11 at 15:43

Nemo

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There is no way to assign a default value other than 0? – ALOToverflow Jul 11 '11 at 15:44
@Francis: Not prior to C++0x, no. For alternatives, see the other answers. – Nemo Jul 11 '11 at 16:20

score 1 · Answer 4 · answered Jul 11 '11 at 16:51

1

#include <algorithm>

struct foo
{
  int array[ 10 ];
  int simpleInt;
  foo() : simpleInt(0) { std::fill(array, array+10, 42); }
};

or use std::generate(begin, end, generator); where the generator is up to you.

answered Jul 11 '11 at 16:51

René Richter

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Default values for arrays members of struct

4 Answers4

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