How to set default parameter as class object in c++?

Question

I want to set my function with class object parameter set as default. But when I try to do that it fails in compilation.

class base {
 // ...
};

int myfunc(int a, base b = NULL) {
    if (NULL = b) {
        // DO SOMETHING
    } else {
    // DO SOMETHING
    }
}

Here when i am trying to compile it, this gives me error that "Default Argument base b have int type"

You need to make `b` a pointer to `base` for this to work: `base* b = NULL` — jrok, Aug 25 '12 at 11:47

score 27 · Answer 1 · answered Aug 25 '12 at 11:48

27

Objects can't be NULL in C++.

To set the parameter to default, just use:

int myfunc(int a, base b = base())

answered Aug 25 '12 at 11:48

Luchian Grigore

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Is `base()` calling `base`'s constructor? – David G Aug 25 '12 at 11:50
@David yes, `base()` default-constructs an object of type `base`. – Luchian Grigore Aug 25 '12 at 11:52
4

In this case how would one check if an argument was supplied since `base` can't be compared to `NULL`? – A__ Mar 22 '19 at 21:48

score 19 · Accepted Answer · answered Aug 25 '12 at 12:02

You have three obvious options here.

First, use overloads so the caller can choose to pass b or not.

int myfunc(int a) { ... }
int myfunc(int a, base& b) { ... }

This way you can pass b without having to use a pointer. Note that you should make b a reference or pointer type to avoid slicing the object.

Secondly, if you don't want 2 separate implementations, make b a pointer, which can be set to NULL.

int myfunc(int a, base* b = NULL) { ... }

Third, you could use something to encapsulate the concept of nullable, such as boost::optional.

int myfunc(int a, boost::optional<base&> b = boost::optional<base&>()) { ... }

Evandro Coan · Answer 3 · 2019-01-01T11:23:14.160

@tenfour answer forgot to mention another possible way. You also define a global variable object which you can construct as you like, then set it as the default value:

#include <iostream>

class MyCustomClassType
{
  int var;

  friend std::ostream &operator<<( 
        std::ostream &output, const MyCustomClassType &my_custom_class_type )
  {
    output << my_custom_class_type.var;
    return output;
  }
};

// C++11 syntax initialization call to the default constructor
MyCustomClassType _my_custom_class_type{};

void function(MyCustomClassType my_custom_class_type = _my_custom_class_type) {
  std::cout << my_custom_class_type << std::endl;
}

/**
 * To build it use:
 *     g++ -std=c++11 main.cpp -o main
 */
int main (int argc, char *argv[]) {
  function();
}

score 0 · Answer 4 · answered Jan 01 '19 at 11:21

A hack or ugly solution would be do a static cast from null:

#include <iostream>

class MyCustomClassType {
  int var;

  friend std::ostream &operator<<( 
         std::ostream &output, const MyCustomClassType &my_custom_class_type )
  {
    output << my_custom_class_type.var;
    return output;
  }
};

void function(
       MyCustomClassType my_custom_class_type = *static_cast<MyCustomClassType*>( nullptr )
    ) 
{
  std::cout << my_custom_class_type << std::endl;
}

/**
 * To build it use:
 *     g++ -std=c++11 main.cpp -o main
 */
int main (int argc, char *argv[]) {
  function();
}

But, running this gives you directly a segmentation fault due dereferencing a null pointer. I am not sure when this would be useful.

How to set default parameter as class object in c++?

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