C++ constructor with one default parameter and one variable parameter

Question

I'm writing this class constructor:

element(int f=0, int a)
{
    first = f;
    inc = a;
    current = first - inc;
}

The parameters are assigned to member variables in the body of the constructor. I've been asked to get the following calls in main() to work:

prog = new element(3,5);
prog = new element(5);

I cannot change the order of (3,5). As in within the constructor, f needs to be passed first, and a second. However, f needs to be initialized to 0 if no value is passed in, that way the second call keeps f at 0 and instead only initializes a to 5.

The issue with this is that I get an error if I place the parameters in this order within the constructor signature.

How do I solve this?

Remy Lebeau · Accepted Answer · 2020-04-02 19:28:41Z

6

This declaration of the constructor is invalid:

element(int f=0, int a)
{
    first = f;
    inc = a;
    current = first - inc;
}

If a parameter has a default argument, all subsequent parameters are also required to have a default argument.

What you need is to declare two constructors, like for example:

element(int f, int a) : first( f ), inc( a )
{
    current = first - inc;
}

element(int a) : element( 0, a )
{
}

It is desirable to declare the second constructor as explicit to prevent implicit conversions from a single integer to the element type:

explicit element(int a) : element( 0, a )
{
}

edited Apr 2, 2020 at 19:28

Remy Lebeau

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answered Apr 2, 2020 at 18:51

Vlad from Moscow

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3 Comments

Jad Over a year ago

This is what I initially wanted to do, however the task implies I only should have one constructor. Do you think that's just a misstep on their part?

Vlad from Moscow Over a year ago

@Jad It is impossible with one constructor.

Jad Over a year ago

Thanks for the help.

Remy Lebeau · Accepted Answer · 2020-04-02 19:29:49Z

3

You cannot have parameters with default values precede normal parameters without default values. So, you need to reorder the arguments in your constructor prototype:

element(int a, int f=0)
{
    first = f;
    inc = a;
    current = first - inc;
}

Another alternative is to define an overloaded constructor:

element(int f, int a)
{
    first = f;
    inc = a;
    current = first - inc;
}

element(int a)
{
    first = 0;
    inc = a;
    current = first - inc;
}

edited Apr 2, 2020 at 19:29

Remy Lebeau

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answered Apr 2, 2020 at 18:54

Abhishek Jaisingh

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Comments

Remy Lebeau · Accepted Answer · 2020-04-02 19:31:01Z

2

First of all, you cannot have a non-default argument after a default argument. Default arguments must be last in the function argument list. See Default arguments.

You can create an overload for the constructor:

element(int f, int a)
{
    first = f;
    inc = a;
    current = first - inc;
}

element(int a) : element(0,a)
{
}

edited Apr 2, 2020 at 19:31

Remy Lebeau

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answered Apr 2, 2020 at 18:52

Muhammet Ali Asan

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