Why is my program giving weird results? - c++

Question

The following snippets are the header file and the actual main() function. I am using Visual Studio 2017 with Windows 10.
.h
#ifndef SALES_DATA_H
#define SALES_DATA_H
#include <iostream>
using namespace std;
struct Sales_data
{
int amount;
int rate;
int price = amount * rate;
};
#endif
.cpp
#include <iostream>
#include "Sales_data.h"
using namespace std;
int main()
{
Sales_data item1;
cout << "Enter rate and amount" << endl;
cin >> item1.rate >> item1.amount;
cout << item1.price << endl;
cin.get();
cin.get();
return 0;
}
It keeps showing this as the output: "687194768".
I also tried initializing the variables but it does not seem to work.

Answer 1

What you probably want is:
struct Sales_data
{
int amount = 0;
int rate = 0;
int price() const { return amount * rate; }
};
And then
std::in >> item1.rate >> item1.amount;
std::cout << item1.price() << std::endl;

Answer 2

Price here is being calculated only at initialisation time to get its initial value, however since amount and rate have not been initialised yet, the result is undefined.
It is not a function.
struct Sales_data
{
int amount;
int rate;
int price = amount * rate;
};
You most likely wanted a function, e.g.:
struct Sales_data
{
int amount;
int rate;
int calcPrice()
{
return = amount * rate;
}
};
std::cout << item1.calcPrice() << std::endl;
Or you would have to refactor to initialise amount and rate to make use of such syntax, e.g. with a constructor.
struct Sales_data
{
int amount;
int rate;
int price = amount * rate;
Sales_data(int amount, int rate) : amount(amount), rate(rate) {}
};
Sales_data x(10, 5);
//x.price == 50

Answer 3

the reason the code is printing that is because you are calculating that price with uninitialized variables...
define a function in the struct and call it after the input is given
void calculatePrice() {
price = amount * rate;
}
cin >> item1.rate >> item1.amount;
item1.calculatePrice();
cout << item1.price << endl;

Answer 4

As others have commented, your definition for Sales_data uses amount or rate before they were ever initialised. This is undefined behaviour, and your compiler is more or less free to do whatever it pleases with this.
Many compilers will initialise variables with some sort of recognisable guard value (a popular choice is 0xDEADBEEF) as a way to make it quite obvious when a value is uninitialised when looking at it with a debugger. In this case, it looks like your compiler uses 0xCCCCCCCC as that guard value:
(lldb) p (int) 0xcccccccc * (int) 0xcccccccc
(int) $2 = 687194768