C++ default function parameter

Question

I want to achieve this:

- second parameter by default set to first argument

Something like:

int foo (int a, int b = a);

But how to do that?

Thanks a lot!

Answer 1

This is forbidden by:

8.3.6 Default arguments [dcl.fct.default]

9) Default arguments are evaluated each time the function is called. The order of evaluation of function arguments is unspecified. Consequently, parameters of a function shall not be used in default argument expressions, even if they are not evaluated. Parameters of a function declared before a default argument expression are in scope and can hide namespace and class member names. [ Example:

int a;

int f(int a , int b = a); / / error: parameter a

/ / used as default argument

typedef int I;

int g( float I , int b = I (2)); / / error: parameter I found

int h(int a , int b = sizeof (a )); / / error, parameter a used

/ / in default argument

—end example ]

An alternative is overloading:

int foo(int a, int b);

int foo(int a)
{
   return foo(a,a);
}

Answer 2

The reason this is disallowed has already been addressed, but another solution along the lines of @Vyktor's is to use boost::optional instead of magic numbers (This has pros and cons compared to creating an overload):

int foo(int a, boost::optional<int> b = boost::none)
{
    if(!b) b = a;
}