Function argument taking other argument as default value

Question

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)
{

}

This link will help you http://stackoverflow.com/questions/4989483/where-to-put-default-parameter-value-in-c — Shankar Shastri, Jul 20 '16 at 03:22

R Sahu · Answer 1 · 2016-07-20T03:30:29.823

Using

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

is an error as per the standard.

From the C++11 Standard:

8.3.6 Default arguments

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 a default argument, even if they are not evaluated. Parameters of a function declared before a default argument 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

...
— end example ]

songyuanyao · Accepted Answer · 2016-07-21T01:24:25.390

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

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

Using function overloading would be simple and fine.

Function argument taking other argument as default value

2 Answers2