如何修改引用传递的结构？

Question

I'm passing a struct to a function by reference since I want to modify the struct inside the function. However, the compiler keeps giving me "Error C1421: Undefined class/struct/union" at "++kpad.pin_chars;". What am I missing? Here's my test code:

struct Keypad{
   int pin_chars;
};


void check_keypad(struct Keypad *kpad);


void main(void){
    struct Keypad kpad; 
    kpad.pin_chars = 0;
    check_keypad(&kpad);
}


void check_keypad(struct Keypad *kpad){
    ++kpad.pin_chars;
}

Answer 1

You are passing a pointer, using the struct Keypad *kpad syntax. That's the * part.

When you are passing a pointer, you cannot use "dot" to access a member without first dereferencing the pointer. You must dereference or use "arrow" ( -> ) as your operator:

++ *kpad.pin_chars

or

++kpad->pin_chars

In general, when using pointers the arrow syntax is the preferred approach.

Edit:

It's worth pointing out the precedence of the various operators involved. Postfix ++ (and -- ) have the same precedence as . and -> , but would bind to the member in ptr->member++ due to location, whereas prefix ++ (and -- ) have a lower precedence than . and -> , and so bind to the result of the member-access expression as a result of precedence, not position.

You get the result in two slightly different ways.

Answer 2

Since you passed a parameter by address, you need to de-reference it to get to its value. The following code works:

void check_keypad(struct Keypad *kpad){
 ++((*kpad).pin_chars);
}

C provides another format for the same purpose:

void check_keypad(struct Keypad *kpad){
 ++(kpad->pin_chars);
}