How to initialize array of objects with the same value in constructor

Question

I am trying to initialize array of objects with the same value

class A
{
   A(int a1)
   {
      var1 = a1;
   }

   var1;
}

int main(void)
{
    // I want to initialize all the objects with 5 without using an array 
    A *arr = new A[10](5); // How to use the constructor to do something like this
    return 0;
}

As i want to pass the same value to all the objects, is there a way to avoid using an array. i.e. I want to avoid doing the following:

A *arr = new A[10]{5, 5, 5, 5, 5, 5, 5, 5, 5, 5}

The easy answer is to use a vector, but I assume that's out of the question for some reason. — Qaz
– Qaz, Commented Sep 3, 2015 at 22:59
Since it is an int array, how about a simple memset() call? — syntagma
– syntagma, Commented Sep 3, 2015 at 23:00
@REACHUS the int is just for example. The actual array can be pretty large with some other params in constructor so I was trying to see if the unnecessary memory can be avoided. — L Lawliet
– L Lawliet, Commented Sep 3, 2015 at 23:02

paddy · Accepted Answer · 2015-09-03 23:17:40Z

4

Normally we avoid raw pointers, especially when not encapsulated in std::unique_ptr or other automatic (RAII) pointer types.

Your first choice should be std::array if size is known at compile time, or std::vector if you want dynamic size or a very large array.

The second choice, if you insist on managing the memory yourself, is to use std::fill or std::fill_n from <algorithm>.

In this case, std::fill_n is probably cleaner:

A *arr = new A[10];
std::fill_n( &arr[0], 10, A(5) );

Please at least consider automatic pointer management though:

std::unique_ptr<A[]> arr( new A[10] );
std::fill_n( &arr[0], 10, A(5) );

edited Sep 3, 2015 at 23:17

answered Sep 3, 2015 at 23:04

paddy

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