function inside objects oop - c++

Question

I don't understand what is the point of having a function inside an object if you can even modify data outside the object for example (I'm using struct for this example):
int lamborghini_horsepower1 = 350;
struct car {
string model;
string color;
int horsepower;
void check(int horsepower) {
if (horsepower > 349) {
cout << "this car has many horsepowers";
}
}
};
car ford;
ford.check(lamborghini_horsepower1);
in this case it would run anyways.
it would make sense if the function could only operate inside data of the object because as it say they are the data of the object and why can the function access to others variables? for example what if you pass another variable in this case lamborghini_horsepower is not part of the the object ford. and still can be cecked by the ford function even if they are different veichle.
are for class different?

Answer 1

In structs, functions don't have too much of an impact because data can be modified from the outside. However, they still can act as a convenience feature, especially when properly written. Your function takes horsepower as an argument, which is unnecessary because horsepower is already a field in the struct. If you change your function to:
void check() {
if (horsepower > 349) {
cout << "this car has many horsepowers" << endl;
}
}
you can call it with ford.check() and it'll automatically retrieve the horsepower from the ford variable. In classes, fields can be declared as private, which means that they can't be accessed from the outside and must be modified purely with functions defined inside the class.

Answer 2

are for class different?
Classes are not accessible from the outside by default, so if you did
class car{
string model;
string color;
int horsepower;
void check(int horsepower) {
if(horsepower>349)
{
cout << "this car has many horsepowers";
}
}
};
Then nothing in that struct would be accessible from outside the class. But there is a way to modify the what variables and functions can be accessed from the outside with access modifiers.
The purpose of making functions and variables inaccessible outside the object is to protect them from being unintentionally called or altered
The short answer is that your example wouldn't be very efficient for objects. I would just keep the function outside the struct in that example.
The purpose of classes and structs is to put variables and functions into one package. The example you have doesn't require an object. The basis behind object design is to mimic real world objects by giving them abilities(functions) and properties (variables).
The check() function here isn't an ability we normally find in cars. Instead, a car has certain properties like color, speed, and direction. A car also has abilities like accelerate, reverse, turn left and turn right. With this we can create a car object with these properties (variables) and abilities (functions)
struct Car
{
int speed;
std::string color;
std::string direction;
Car() // As soon as the object is created, this function will be called and set speed to 0.
{
speed = 0;
}
void speedUp()
{
speed += 5;
}
void slowDown()
{
speed -= 5;
}
void stop()
{
speed = 0;
}
void reverse()
{
stop() // This will stop the car first
speed -= 2; // Move the car backwords
}
void turnLeft()
{
direction = "left";
}
void turnRight()
{
direction = "right";
}
};
So now you can create different objects of the same class\struct with different speeds, direction, and color.