How to operate overload “>>”?

问题

Product **products;

int numProducts = 0;

void setup()
{
    ifstream finput("products.txt");
    //get # of products first.
    finput >> numProducts;
    products = new Product* [numProducts];

    //get product codes, names & prices.
    for(int i=0; i<numProducts; i++) {
        products[i] = new Product;
        finput >> products[i]->getCode() >> products[i]->getName() >> products[i]->getPrice();
    }
}

I am getting an "invalid operands to binary expression" error for this line:

finput >> products[i]->getCode() >> products[i]->getName() >> products[i]->getPrice();

Do I need to operator overload >> and how would I do it?

回答1:

Let's take a very simple example, assuming a basic definition for Product as:

class Product
{
   int code;
   string name;
   double price;

public:
   Product(int code, const std::string& name, double price)
      : code{code}, name{name}, price{price}
   {}

   int getCode() const { return code; }
   const std::string& getName() const { return name; }
   double getPrice() const { return price; }
};

You can't read in using operator>> directly into the return values from getCode(), getName() or getPrice(). Those are for accessing those values.

Instead, you need to read in the values and construct products from those values like this:

for(int x = 0; x < numProducts; ++x)
{
   int code = 0;
   string name;
   double price = 0;

   finput >> code >> name >> price;
   products[i] = new Product{code,name,price};
}

Now, you could refactor this into operator>>:

std::istream& operator>>(std::istream& in, Product& p)
{
   int code = 0;
   string name;
   double price = 0;

   in >> code >> name >> price;
   p = Product{code,name,price};
   return in;
}

There are a bunch of other things to consider about this code:

• Use std::vector<Product> instead of your own array
• The examples below won't work if name has spaces
• There's no error checking and operator>> can fail

回答2:

In your Class, write down this function

friend ifstream& operator >> (ifstream& in, Product& p1)
{
    in >> p1.code >> p1.name /* ..etc */;

    return in;
}

来源：https://stackoverflow.com/questions/61069078/how-to-operate-overload