Is there a way to ensure two arguments passed to a function are treated as the first and third argument in C++?

Question

Suppose there is a function with the following prototype:

void fun (int = 10, int = 20, int = 30, int = 40);

If this function is called by passing 2 arguments to it, how can we make sure that these arguments are treated as first and third, whereas, the second and the fourth are taken as defaults.

Answer 1

// three and four argument version
void fun (int a, int b, int c, int d = 40)
{
    ...
}

// two argument version
void fun (int a, int c)
{
     fun(a, 20, c, 40);
}

// one and zero argument version
void fun(int a = 10)
{
     fun(a, 20, 30, 40);
}

But really my advice would be don't.

Answer 2

You can define the Args structure like:

struct Args {
   int a = 10;
   int b = 20;
   int c = 30;
   int d = 40;
};

and then you would have the following:

void fun(Args);

fun({.a=60, .c=70}); // a=60, b=20, c=70, d=40

Besides this approach, you can use NamedType library that implements named arguments in C++. For more usage info, check here.

UPDATE

Designated initializers feature is available by GCC and CLANG extensions and, from C++20, it is available by C++ standard.