CODEWITHSUNDEEP
c++pointersfunction-pointers
user2836797
user2836797

Reputation:

Two styles of syntax for parameter taking a function?

I was reading Alex Allain's tutorial on function pointers (http://www.cprogramming.com/tutorial/function-pointers.html) in which he describes the syntax of receiving a function argument as such:

void doSomething(int (*GetNumber)(Player p) = myFunct) {

}

Then I was reading Accelerated C++, which shows the following syntax:

map<string, vector<int> >
xref(istream& in, vector<string> find_words(const string&) = split) {

}

Is there a difference between the two approaches? That is, A versus B:

A:

void doSomething(int (*GetNumber)(Player p) = myFunc) {

}

B:

void doSomething(int GetNumber(Player p) = myFunc) {

}

Upvotes: 1

Views: 58

Answers (2)

Keith Thompson
Keith Thompson

Reputation: 263257

There are two distinct language rules involved here.

One is that a parameter that appears to be of function type is adjusted at compile time to a pointer to the function:

After determining the type of each parameter, any parameter of type “array of T” or “function returning T” is adjusted to be “pointer to T” or “pointer to function returning T,” respectively.

That's the rule that makes these function declarations:

void foo(void (*p)());

and

void foo(void (p)());

equivalent.

The other is that an expression of function type is, in most contexts, implicitly converted to a pointer to the function:

An lvalue of function type T can be converted to a prvalue of type “pointer to T.” The result is a pointer to the function.

Given a function declaration:

void func();

this rule is what makes all these calls:

foo(&func);
foo(func);
foo(*func);
foo(**func);

equivalent. On the last one, the expression func is of function type, so it's converted to a pointer. The inner * dereferences that pointer yielding an expression of function type, which is then implicity converted to a pointer again. And the outer * does the same thing.

In fact, the expression foo is also converted to a pointer -- which is just what the function-call operator requires as the operand preceding the (.

Upvotes: 2

Vlad from Moscow
Vlad from Moscow

Reputation: 310980

There is no difference because functions passed by values as arguments are implicitly converted to pointers. So these two declarations

void doSomething(int (*GetNumber)(Player p) );

void doSomething(int GetNumber(Player p) );

declare the same function.

You could write the declarations also as

void doSomething(int (*)(Player) );

void doSomething(int( Player ) );

It is the same as the equivalence of the following declarations

void f( int a[] );

and

void f( int *a );

and

void f( int a[10] );

and even

void f( int a[100] );

They all declare the same function.

Upvotes: 4

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