WARNING: "implicit declaration of function '...' is invalid in C99"

Question

I'm getting this warning when I'm trying to compare RGB components of two UIColors

In .h file, I declared this

 -(int) ColorDiff:(UIColor *) color1 :(UIColor *)color2;

In .m file

 - (int) ColorDiff:(UIColor *) color1 :(UIColor *)color2{
   ... //get RGB components from color1& color2
   // compute differences of red, green, and blue values
   CGFloat red   = red1   - red2;
   CGFloat green = green1 - green2;
   CGFloat blue  = blue1  - blue2;

  // return sum of squared differences
  return (abs(red) + abs(green) + abs(blue));
  }

And then in same .m file, I compare 2 UIColors like this

 int d= ColorDiff(C1,C2);// I got the warning right here.

I did research and people say I must include the header file. I did this but didn't help in my case. Why am I getting this error?

Answer 1

It's because you defined your function as a instance method, not a function. There are two solutions.

One of which is this to change your method declaration to this:

int ColorDiff(UIColor *color1, UIColor *color2) {     // colorDiff's implementation } 

Or, you can change your call to this:

int d = [self ColorDiff:C1:C2]; 

Answer 2

The declaration in your .h file doesn't match your implementation in your .m file.

if the implementation of your method in your .m looks like this:

 - (int) ColorDiffBetweenColorOne:(UIColor *) color1 AndColorTwo:(UIColor *)color2
{
    ... //get RGB components from color1& color2
    // compute differences of red, green, and blue values
    CGFloat red   = red1   - red2;
    CGFloat green = green1 - green2;
    CGFloat blue  = blue1  - blue2;

    // return sum of squared differences
    return (abs(red) + abs(green) + abs(blue));
}

than you should declare it like this in .h:

- (int) ColorDiffBetweenColorOne:(UIColor *) color1 AndColorTwo:(UIColor *)color2; 

and to call it from that same .m file, use:

int d = [self ColorDiffBetweenColorOne:C1 AndColorTwo:C2];