Question

This is a pretty basic C++ design question:

I have a class that contains some data which is read-only, once the object is constructed:

class Foo {
private:
  class Impl;
  Impl* impl_;  
public:
  int get(int i); // access internal data elements
};

Now, I’d like to implement several ways to construct a Foo object and fill it with data: from std::istream, from an iterator, a vector, etc. What’s the best way to implement that?

I could just add all those constructors directly in Foo, but I don’t really want the Foo user to have to include std::istream etc. I’m also worried about classes containing too much code.

What is the most idiomatic way to do this? I guess, add some private addElement function, and then define friend factory functions that create Foo objects by reading data, calling addElement and return the constructed object? Any other options?

Answer 1

If you want to construct something from a range, perhaps:

class X
{
public:
    template <class InputIterator>
    X(InputIterator first, InputIterator last);
};

Usage:

//from array
X a(array, array + array_size);

//from vector
X b(vec.begin(), vec.end());

//from stream
X c((std::istream_iterator<Y>(std::cin)), std::istream_iterator<Y>());