How to bind a constructor in C++?

Question

Needless to over explain. The following code is self-evident:

struct X
{
    X(int n){}
};

int main()
{
    std::vector<int> src;
    std::vector<X>   dest;

    // Below is not valid in current C++, but that is just what I want.
    transform(src.begin(), src.end(), back_insert(dest), std::bind(&X::X, _1)); 
}

A constructor takes some arguments and returns an object of the class of the constructor.

A constructor looks like a function, acts like a function, and is exactly a function.

So, I think std::bind should uniformly treat constructors and other callable objects.

However, how can I extend the function template "bind" to implement that?

Answer 1

Build a factory function. These are static member functions or stand-alone functions that also construct objects, usually after validating inputs. They're useful whenever you want to ensure your objects cannot be built with invalid input (after all, you cannot abort construction within a ctor).

Since a factory is an ordinary function, you can easily bind to it to get the same effect you appear to want- a bound function that builds objects.

class X
{
public:
   X(OtherObject a, OtherData data);
   virtual ~X() {}
};

// Factory- return "X" instead of "X*" if not using the heap
X* CreateX(OtherObject a, OtherData data) {
    /*
        Logic that checks a, data for validity...
    */
    if(invalid) {
       return 0;   // You get no object
    }
    return new X();
}

Now you just bind to "CreateX" to build objects. It is a normal function, however, not a constructor so all the normal rules for functions apply- especially those for copy and move construction of objects.