CODEWITHSUNDEEP
c++
user2953119
user2953119

Reputation:

How can we use function arguments within a function declaration?

The standard N4296::3.3.4/1 [basic.scope.proto]:

In a function declaration, or in any function declarator except the declarator of a function definition (8.4), names of parameters (if supplied) have function prototype scope, which terminates at the end of the nearest enclosing function declarator.

I tried the following examples:

1.

template<const int a>
class A{ };

const int a = 4;

A<a> b; // OK

void foo(const int a = 4, A<a>); //non-type template argument is 
                                 //not a constant expression

DEMO

2.

void foo(const int a = 4, int b = a); //default argument references parameter 'a'

DEMO

How can we use this scope's feature? What is it introduced for?

Upvotes: 1

Views: 83

Answers (3)

T.C.
T.C.

Reputation: 137320

To add to user657267's example, trailing-return-types often depend on this feature:

template<class A, class F>
auto foo(const A& a, const F& f) -> decltype(f(a));

Upvotes: 1

user657267
user657267

Reputation: 21000

Here's a contrived example:

void foo(overly::long_type::which_should_be_a_typedef_anyway a, decltype(a) b); // fine

decltype(a) c; // oops, a not declared

Upvotes: 4

Jonathan Leffler
Jonathan Leffler

Reputation: 753675

I think the point of this is to say that the following code is valid:

extern int somefunc(int a, char *b, int c);
int somefunc(int, char *, int);

int somefunc(int number, char *bytes, int ipv4_address)
{
    …
}

The name a, b and c lose their significance at the closing parenthesis of the extern declaration. Within the declarator, the names are significant to the extent that int somefunc(int a, char *a, int a); is invalid because the same name a is used where distinct identifiers are required.

The names number, bytes and ipv4_address do not lose their significance at the close parenthesis because that is the 'declarator of a function definition'; they become the names of variables within the function.

Note that Stroustrup explicitly declined to tie parameter names in function declarations to parameter names in function definitions. There's a section in Design and Evolution of C++ that discusses this.

Upvotes: 3

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