Classes make interface funtion know

So as you can see I would like to make the read function known to the class, as I`m using it in a constructor in the class, it`s an exercise so I would not like to define it outside the class, does anyone how I can achieve this? because read uses a Sales_data type before it is defined

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#ifndef SALES_DATA_H
#define SALES_DATA_H

#include <iostream>
#include <string>

std::istream &read(std::istream &, Sales_data &);

struct Sales_data {

  Sales_data() = default;

  Sales_data(const std::string &s) : bookNo(s) {}

  Sales_data(const std::string &s, unsigned n, double p) : bookNo(s), units_sold(n), revenue (p*n) {}

  Sales_data(std::istream &is)
  {
    read(is, *this);
  }


  std::string isbn() const // funtion to call for the ISBN of the current book
  {
   	 return bookNo; // return the book numbers
  }

  Sales_data& combine (const Sales_data&rhs) // to combine two Objects of Sales_data
  {
  	units_sold += rhs.units_sold; // add the amount of books sold
  	revenue += rhs.revenue;       // for adding the revenue
  	return *this;                 // return the object that the funtion was called on
  }

	double avg_price() const // for calculating the average price
  {
		if (units_sold)
      return revenue/units_sold; // return: devide revenue by the amount of units sold
  	else
  	 return 0;                   // else return nothing
  }

  std::string bookNo;      // for holding our book number/ISBN
  unsigned units_sold = 0; // for holding the amount of books sold
  double revenue = 0.0;    // for holding the price they were sold at
};


std::istream &read(std::istream &is, Sales_data &item) // funtion for getting input into a Sales_data object
{
  double price = 0;                              // for getting our price that the objects were sold at
  is >> item.bookNo >> item.units_sold >> price; // get input
  item.revenue = price * item.units_sold;        // calculate the revenue
  return is;
}

std::ostream &print(std::ostream &os, const Sales_data &item) // funtion for output
{
  os << item.isbn() << " " << item.units_sold << " " << item.revenue << " " << item.avg_price(); // print out values
  return os;        // return the output stream
}

Sales_data add(const Sales_data &lhs, const Sales_data &rhs) // for adding to Sales_data objects
{
  Sales_data sum = lhs; // sum for returning
  sum.combine(rhs);     // combine sum with rhs
  return sum;           // return the combined object
}

#endif // SALES_DATA_H
ok guys I was really stupid. I declared the class before read and it worked. REALLY STUPID. Sorry
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