Some Interesting facts about default arguments in C++

Predict the output of following C++ programs.

1)

#include <iostream>
void init(int a=1, int b=2, int c=3);

int main()
{
    init();
    return 0;
}
void init(int a=1, int b=2, int c=3)
{
    std::cout << a << ' ' << b << ' ' << c;
}

The above program looks correct at first glance but will fail in compilation. If function uses default arguments then default arguments can’t be written in both function declaration & definition. It should only be in declaration, not in definition.

The following program is now correct.

#include <iostream>
void init(int a=1, int b=2, int c=3);
int main()
{
    init();    // It is fine
    return 0;
}
void init(int a,int b,int c)
{
    std::cout << a << ' ' << b << ' ' << c;
}

2)

#include <iostream>

// something looks missing
void init(int =1, int =2, int =3);

int main()
{
    init();
    return 0;
}

void init(int a, int b, int c)
{
    std::cout << a << ' ' << b << ' ' << c;
}

If you closely observe function prototype then it looks like an error but it isn’t actually. Variable names can be omitted in default arguments.