CODEWITHSUNDEEP
c++functionarguments
bpw1621
bpw1621

Reputation: 3062

Can I set a default argument from a previous argument?

Is it possible to use previous arguments in a functions parameter list as the default value for later arguments in the parameter list? For instance,

void f( int a, int b = a, int c = b );

If this is possible, are there any rules of use?

Upvotes: 61

Views: 17479

Answers (7)

t0mm13b
t0mm13b

Reputation: 34600

I do not think you can do that as that is an illegal syntax. But however, consult the C99 standard in pdf format (n1136.pdf).

However, you may get around this by using static as in declaring the variables static and using them within the function f

static int global_a;

/* In some other spot where you are calling f(), do this beforehand */
/* global_a = 4; f(); */

void f(void){
   int a = global_a;
   b = c = a;
   /* ..... */
}

Kudos to Michael Burr for pointing out my error! :)

It sounds like you need to rethink your code and change it around for something like that.

Upvotes: 0

anon
anon

Reputation:

No, that is not legal C++. This is specified in section 8.3.6/9 of the C++ Standard:

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.

and:

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

And C89 at least does not support default parameter values.

Upvotes: 39

Domino
Domino

Reputation: 6778

Your first idea might be to do something like this :

void something(int a, int b=-1, int c=-1){
    if(b == -1)
        b = a;
    if(c == -1)
        c = b;
}

I used -1 because this function only works with positive values. But what if someone uses my class and makes a mistake which ends up sending -1 to the method? It would still compile and execute, but the result would be unpredictable for the user. So the smart thing to do would be to remove any default argument and instead make a bunch of methods with the same name like this:

void something(int a, int b, int c){
    /* Do something with a, b and c */
}

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

void something(int a, int b){
    something(a, b, b);
}

It doesn't really take much longer to code, and if someone uses it in a programming interface with auto-complete features, it will show the 3 possible prototypes.

Upvotes: 1

Mike Seymour
Mike Seymour

Reputation: 254751

The answer is no, you can't. You could get the behaviour you want using overloads:

void f(int a, int b, int c);
inline void f(int a, int b) { f(a,b,b); }
inline void f(int a)        { f(a,a,a); }

As for the last question, C doesn't allow default parameters at all.

Upvotes: 88

James Eichele
James Eichele

Reputation: 119194

As a potential workaround, you could do:

const int defaultValue = -999; // or something similar

void f( int a, int b = defaultValue, int c = defaultValue )
{
    if (b == defaultValue) { b = a; }
    if (c == defaultValue) { c = b; }

    //...
}

Upvotes: 7

Prasoon Saurav
Prasoon Saurav

Reputation: 92922

No, you cannot do that.
You will surely get an error "Local variable may not appear in this context".

Upvotes: 3

Andreas Brinck
Andreas Brinck

Reputation: 52549

This is not possible

Upvotes: 3

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