Question

For example, I have class A:

 class A{
           int value_;
           public:
           A(A& a){
                value_ = a.value_;
           }
           A(int value){
                value_ = value;
           }
 };

I want a vector of class A but I’d like to pass a value to A(int value) for all of them.

 std::vector<A,allocator<A>> my_vector;

• What is the best way to do it?
• Is there a way by using allocator?

Answer 1

c++11

With the new standard added functionality was granted to objects of Allocator type.

One of the features added was that Allocators now allows emplacement construction, aka. construction of objects using a constructor other than copy/move.

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template< class U, class... Args > void construct( U* p, Args&&... args );

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The standard does guarantee that STL containers must use this new feature, and with that said you could implement your own allocator just for the purpose of default initializing a non-default-initializable object.

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^{It’s not the prettiest solution, but whatever floats your boat..}

c++03

The allocator has nothing to do with that part of object initialization, it’s only purpose is to allocate/deallocate memory, the type of initialization you are referring to is done elsewhere.

The only constructor an allocator will call is the copy-constructor when someone asks it to perform a placement-new, and the value passed to that copy-ctor has already been established somewhere else.

To sum things up; No, you cannot use an allocator so solve this particular problem.

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when does std::vector require the use of a default-ctor?

std::vector only uses the default-constructor of the type it holds in two situations:

1. You specify the number of elements of your std::vector in the appropriate constructor overload but doesn’t supply a default value

2. You use std::vector<T>::resize (n) and increase the number of objects in the container (note the lack of specifying the 2nd argument to the member-function)

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With the above in mind we can do plenty of things using the container without supplying a default constructor in our object, like initializing it to contain N elements of value X.

struct A{
  A (A const& a)
   : value_ (a.value_) 
  { } 

  A (int value)
    : value_ (value)
  {}  

  int value_;
};

int
main (int argc, char *argv[])
{
  std::vector<A> vec (5, A(1)); // initialize vector with 5 elements of A(1)

  vec.push_back (A(3));         // add another element
}

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But I really want to be able to use vec.resize ()!?

Then you have two, three, four options:

1. Go with the C++11 approach of using Allocators

2. make your object have a default constructor

3. wrap your object with a very thin wrapper who’s only purpose is to default initialize the containing object (this might be easier said then done in some circumstances)

4. "Wrapping [the object] in boost::optional practically gives any type a default ctor" – @Xeo