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.

This article is contributed by Pravasi Meet. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.