CODEWITHSUNDEEP
c++typedef
gln
gln

Reputation: 1031

C++ typedef declaration

Can you please explain what does the following line means?

typedef int (*Callback)(void * const param,int s)

Upvotes: 3

Views: 1708

Answers (3)

icecrime
icecrime

Reputation: 76755

It means that Callback is a new name for the type : pointer to a function returning an int and taking two parameters of type 'const pointer to void' and 'int'.

Given a function f :

int f(void * const param, int s)
{
    /* ... */
}

The Callback can be used to store a pointer to f :

Callback c = &f;

The function f can be later invoked through the pointer without directly referring to its name :

int result = c(NULL, 0);

At the point of the call, the name f does not appear.

Upvotes: 6

Loki Astari
Loki Astari

Reputation: 264381

It declares a function type:

// Set up Callback as a type that represents a function pointer
typedef int (*Callback)(void * const param,int s);

// A function that matches the Callback type
int myFunction(void* const param,int s)
{
    // STUFF
    return 1;
}

int main()
{
    // declare a variable and assign to it.
    Callback   funcPtr = &myFunction;
}

Upvotes: 2

Tony Delroy
Tony Delroy

Reputation: 106096

It creates a new "alias" or name by which you can refer to pointers to functions that return int and take two parameters: a void* const and an int. You can then create variables of that type, assign to them, invoke the function through them etc as in:

int fn(void * const param,int s) { ... }

Callback p;
p = fn;
int x = p(NULL, 38);

Note that typedefs do not really create new types... every equivalent typedef is resolved to the single real type for the purposes of overload resolution, template instantiation etc..

Upvotes: 2

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