Function argument taking other argument as default value

I want a function argument to take the value of an other argument as a default value.

My question is: Why am i not allowed to do that ?

void foo(int a, int b = a)
{

}

and is there an other way to do it than that ?

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

void foo(int a, int b)
{

}

1. Why am i not allowed to do that ?

Because the evaluation order of function arguments is unspecified, it's not guaranteed that b will be initialized with the value passed in as the argument of a.

From $8.3.6/9 Default arguments [dcl.fct.default]:

A default argument is evaluated each time the function is called with no argument for the corresponding parameter. A parameter shall not appear as a potentially-evaluated expression in a default argument.
[ Example:

int a;
int f(int a, int b = a);            // error: parameter a
                                    // used as default argument

and

2. and is there an other way to do it than that ?

Using function overloading would be simple and fine.