operator+ must take zero or one argument

struct deathNote{

uint32_t apples;
uint32_t chocolate;
uint32_t pages;

deathNote operator+(const deathNote &a, const deathNote &b){
   deathNote ret_val;
   ret_val.apples = a.apples + b.apples;
   ret_val.chocolate = a.chocolate + b.chocolate;
   ret_val.pages = a.pages + b.pages;
   return ret_val;
}

};//struct

//ERROR: operator+ must take zero or one argument.
//The fix? One parameter is implied via the "this" pointer.

//WORKING OVERLOADED OPERATOR:
deathNote operator+(const deathNote &a){
   deathNote ret_val;
   ret_val.apples = a.apples +  this -> apples;
   ret_val.chocolate = a.chocolate +   this -> chocolate;
   ret_val.pages = a.pages +   this -> pages;
   return ret_val;
}