Default argument for structure reference in C++

Question

I want to give a default value to a function parameter, which is reference to a structure . What can I give as the default value ?

Answer 1

First solution:

If it is a reference to a struct, then you've to make it const reference, and do this:

struct A
{
    //etc
    A(int, int);
};  

void f(int a, const A & = A(10,20) ) //const is necessary
{
    //etc
}

Its not that good for the obvious reasons: it makes the parameter const (you may not want it), and your struct needs to have constructor (you may not have it).

Second solution:

So if you don't want to make it const, or if you don't want to have a constructor in the struct, then you can do this:

struct Point
{
     int x, y, z;
};  

Point g_default_point = {10,20,30};
void g(int a, Point & p = g_default_point )
{
    //etc
}

Still not good. Defining a global variable is not a great idea.

---

Best solution : define an overload function

void g(int a, Point & p)
{
    //your code
}
void g(int a) //this function would behave as if you opt for default value!
{
     Point default_value = {10,20,30};
     g(a, default_value);
}

Now it doesn't require you to make the parameter const, neither does it force you to have constructor in your struct.

Answer 2

This works in C++0x:

#include <iostream>

struct structure { int x, y ; } ;

int f(int a, structure &s = structure{10,20}) {
  return s.x + s.y ;
}

int main() {
  std::cout << f (99) << std::endl ;
}

Note that s does not have to be const. See http://ideone.com/sikjf.