CODEWITHSUNDEEP
c++staticstatic-methods
Dr Deo
Dr Deo

Reputation: 4848

why is static needed here

I am using curl library which returns data to me through a callback function with the prototype below

size_t write_data(void * data, size_t size, size_t nmemb, void * userpointer);

I noticed that if i declare a function fitting this prototype in my class

//file Dialog.h
class Dialog : public QDialog
{
    private:
    int new_data_callback(void * newdata, size_t size, size_t nmemb, QByteArray * buffer); 
}

If i try to use it in my Dialog.cpp

curl_easy_setopt(curl_handle,CURLOPT_WRITEFUNCTIION, new_data_callback);

I get an error

Invaid use of member (did you forget the '&'?)

If i add static to my function declaration, it compiles. 

static int new_data_callback(void * newdata, size_t size, size_t nmemb, QByteArray * buffer); //ok

Question

Why is static needed in this case?

PS: the classes beginning with Q eg QDialog are part of QT and dont affect the question.

Upvotes: 1

Views: 114

Answers (2)

Charlie
Charlie

Reputation: 1031

It is because of how functions get invoked. non-static class functions require additional information to be invoked. Namely the this pointer. When you pass new_data_callback to curl_easy_setopt this instance specific information is not provided. Thus, curl does not have enough information to invoke the function.

If a class function is defined static it, by definition, cannot access non-static members of a class. Therefore it does not need the additional instance information as above and can be passed to set_easy_setopt

Upvotes: 1

Luchian Grigore
Luchian Grigore

Reputation: 258648

Because a non-static method cannot be called without an instance. Since new_data_callback is a callback, the only way to attach an instance to it is through a parameter. Making it static removes the instance restriction.

Upvotes: 3

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