CODEWITHSUNDEEP
c++arrayscontainersnoncopyable
Ralph Tandetzky
Ralph Tandetzky

Reputation: 23660

C++ container with non-copyable non-movable element type

I need a container of elements that are neither copyable nor movable. These elements are not default constructible, but their constructors get identical arguments.

The size of the container does not change during it's lifetime. It should be as simple as a built-in array, but it's size is determined at run-time when the constructor is called.

Is there an easy way to implement that without the overhead of memory allocation and indirection incurred by using std::vector<std::unique_ptr<T>>?

Upvotes: 9

Views: 2789

Answers (3)

Gabriel C.
Gabriel C.

Reputation: 51

I was trying to do the same thing, and I'm using a simple workaround. What's wrong with this example?

class test{
  const int a;
public:
  test(int i): a(i) {} //no default constructor
};

//trick for static initializer
template <typename T> class defer_params: public T{
public:
  defer_params(): T(1) {} //fixed params
};

//static array
//test list1[5]; //this doesn't work
defer_params<test> list2[5];

Regards, Gabriel

Upvotes: 0

mindriot
mindriot

Reputation: 5678

Here's a simple, yet incomplete solution under the assumption that each element is constructed with the same arguments. It uses placement new to construct the elements in-place (see also this SO question):

#include <cstdlib>
#include <utility>
#include <new>

// sample structure, non-copyable, non-moveable, non-default-constructible
struct Foo
{
  Foo() = delete;
  Foo(const Foo&) = delete;
  Foo& operator = (const Foo&) = delete;
  Foo(Foo&&) = delete;
  Foo& operator = (Foo&&) = delete;

  Foo(int a, char b, double c) : m_a(a), m_b(b), m_c(c) { }

  int m_a;
  char m_b;
  double m_c;
};

template <typename T>
struct MyArray
{
  // Array ctor constructs all elements in-place using the
  // provided parameters
  template <typename... Args>
  MyArray(std::size_t sz, Args&&... args)
    : m_sz(sz),
      m_data(static_cast<T*>(malloc(sz * sizeof(T))))
  {
    for (std::size_t i=0; i<m_sz; ++i)
    {
      new (&m_data[i]) T(std::forward<Args>(args)...);
    }
  }

  ~MyArray()
  {
    for (std::size_t i=0; i<m_sz; ++i)
    {
      m_data[i].~T();
    }
    free(m_data);
  }

  std::size_t m_sz;
  T *m_data;
};

int main()
{
  Foo foo(1, '2', 3.0);
  std::size_t s = 5;
  MyArray<Foo> foo_arr(s, 1, '2', 3.0);
}

Note that a few things are missing:

  • This basic implementation will leak memory if an exception is thrown inside MyArray's constructor.
  • You will probably want an iterator implementation, begin()/end() operators etc., for more convenience and to get the same behaviour as provided by the standard containers.
  • For illustration's sake I also didn't bother with proper encapsulation. You should probably make m_sz and m_data private members.

Upvotes: 3

UKMonkey
UKMonkey

Reputation: 7013

"Is there an easy way to implement that without the overhead of memory allocation and indirection incurred by using std::vector>?"

The overhead is so minimal, why do you care? This is almost certainly premature optimisation, and you're just going to make yourself a maintenance headache.

Upvotes: -2

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