C++ allocate and construct objects in array with parameters

Question

From here http://www.cplusplus.com/reference/new/operator%20new[]/, it is unclear to me is it possible to allocate and construct objects with parameters. Like this:

struct MyClass {
  int data;
  MyClass(int k) {} 
};

int main () {
  // allocates and constructs five objects:
  MyClass * p1 = new MyClass[5](1);               // allocate 5 objects, and init all with '1'
}

Which is not work...

Similar: http://stackoverflow.com/questions/24500164/is-direct-initialization-forbidden-for-array — chris, Jul 04 '14 at 18:37

score 2 · Answer 1 · answered Jul 04 '14 at 18:24

2

That wouldn't work, but you can use an std::vector<MyClass> for that:

std::vector<MyClass> p1(5, 1);

This will create a vector containing 5 MyClass objects. Note that this works because your single parameter constructor is not explicit. If it were, you would need

std::vector<MyClass> p1(5, MyClass(1));

answered Jul 04 '14 at 18:24

juanchopanza

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But it construct it one, by one? I don't need vector. – tower120 Jul 04 '14 at 18:27
@tower120 Yes, just like when you instantiate an array. – juanchopanza Jul 04 '14 at 18:28
I have to do this `MyClass * p3 = static_cast (::operator new (sizeof(MyClass[5])));` And than call constructor explicitly for each object? – tower120 Jul 04 '14 at 18:28
1

@tower120 No, you do what I told you to do in my answer. – juanchopanza Jul 04 '14 at 18:29
I have pool object implementation that use C native arrays. I create bunch of objects at once. Now I want to create initialized objects. – tower120 Jul 04 '14 at 18:33
@tower120 I am not sure how you create a bunch of objects "at once", but you may want to look at [*placement new*](http://en.wikipedia.org/wiki/Placement_syntax). – juanchopanza Jul 04 '14 at 18:39

Deduplicator · Answer 2 · 2014-07-04T18:42:24.553

2

What you wrote does not work, but you could do this since C++11.
Still, better avoid dynamic allocation and go for automatic object.
Also, most times you really should use a standard container like std::array or std::vector.

MyClass * a = new MyClass[5]{1,1,1,1,1};
MyClass b[5] = {1,1,1,1,1};
std::vector<MyClass> c(5, 1);
std::vector<MyClass> d(5, MyClass(1));
std::array<MyClass,5> e{1,1,1,1,1};

edited Jul 04 '14 at 18:42

answered Jul 04 '14 at 18:30

Deduplicator

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Looks good, but what about variable number of instances and per object arguments? :) – tower120 Jul 04 '14 at 18:31
@tower: In that case, hope you can default-construct them. Or it won't work at all. – Deduplicator Jul 04 '14 at 18:32
Well, lets consider we don't have default constructor. Than I have to iterate each constructor. Right? – tower120 Jul 04 '14 at 18:34

C++ allocate and construct objects in array with parameters

2 Answers2