C++primer 5 关于func参数

Question

Its question is" Give the second parameter of make_plural (§ 6.3.2, p. 224) a default argument of 's'. Test your program by printing singular and plural versions of the words success and failure" here is the make_plural.

string make_plural(size_t ctr, const string& word, const string& ending )
{
    return (ctr > 1) ? word + ending : word;
}

Does it mean that change the 'ending', but ending is the third parameter, isn't it? This question worries me a lot! Regards!

Answer 1

That must be a typo.

Looking at the code:

string make_plural(size_t ctr, const string& word, const string& ending )
{
    return (ctr > 1) ? word + ending : word;
}

the most reasonable thing would be to have "s" as default for ending , as this is how you make the plural by default (not always, but with "bee" -> "bees" eg it works).

A much stronger argument is that in C++ it is not possible (unless you find a magic workaround (*)) to have a default argument for the n-th parameter if the (n+1)-th has no default argument:

foo(int first = 0,int second) // not possible !!

With this example it is maybe not so clear why this isnt allowed, but consider having multiple default values. Lets say you would write:

foo(int first = 0,int second,int third = 0); // actually still not allowed

Then there would be no way to know if

foo(1,2);

is supposed to call

foo(0,1,2); 

or

foo(1,2,0);

To resolve this ambiguity some rule had to be invented and for C++ the rule is that default arguments have to be provided from right to left.

(*) If you can change the function and are willing to write some extra code, the workaround is rather trivial. You just have to encapsulate all parameters in a struct that provides creation of parameters with whatever combination of defaults you like.