Newb C++ question about object instantiation
Author
Hi,
Im working on a program in c++ that is just a basic group of classes that i need to work with. I am a decent java programmer but im having a bit of difficulty converting to c++. here is what i need to do:
i have 3 classes:
class car in file car.cpp
class track in file track.cpp
class ford in file ford.cpp
Now, i want to create a car object in the track class file. (in java this would be something like: car myCar; or car myCar = new car();)
in c++ i believe it is the same: car myCar(); but i get errors when i compile saying that the car class isnt an identifier, and it defaults it to int, or something like that. I thought maybe i have to do an #include of the car class but i dont think i should have to right???
the second question i have is that the class ford implements class car:
class ford : public car
{
}
is that correct? I can get that to work(when i only compile car and ford), however when i compile the whole project(car, track, and ford) i get an error that the methods in the car class are being redefined.
I can find a lot of examples on instantiation however they all have something where the car class declaration is in the same file as the ford class declaration. is there a way i can do it in separate files???
Thank you very much for your help,
cp51
For separate files, many people declare the class and members in a header file, and define the functions in a cpp file, which includes the class header file.
and then the cpp file for the ford class, etc.
Your inheritance for the ford class is correct also, just need a semi-colon after the second curly bracket.
//car.h//this prevents the header from being included multiple times#ifndef CAR_H#define CAR_Hclass car{ public: car(); ~car(); void drive(); float mpg;};#endif//car.cpp#include "car.h"car::car(){ cout << "car" << endl;}car::~car(){}void car::drive(){ cout << "vroom" << endl;}//ford.h#include "car.h"class ford : public car{ public: ford(); ~ford(); void breakDown();};and then the cpp file for the ford class, etc.
Your inheritance for the ford class is correct also, just need a semi-colon after the second curly bracket.
For most of your questions, you should probably read this article: Organization Code Files, and then ask if you still have any confusions.
However, it doesn't explain one thing, and this is one of C++'s ugly warts. This:
Doesn't create a car named myCar. It declares a function that return a car that takes no arguments called myCar. Basically anything that can be interpreted as a function declaration is interpreted as a function declaration in C++. It's something of a pain. To declare a car named myCar, leave off the parenthesis.
However, since you were talking about inheritance, you probably want a pointer.
However, it doesn't explain one thing, and this is one of C++'s ugly warts. This:
car myCar();Doesn't create a car named myCar. It declares a function that return a car that takes no arguments called myCar. Basically anything that can be interpreted as a function declaration is interpreted as a function declaration in C++. It's something of a pain. To declare a car named myCar, leave off the parenthesis.
car myCar;However, since you were talking about inheritance, you probably want a pointer.
car * myCar = new car(); // or new ford();
Are you using include guards? If one files includes another one twice, errors like this might occur. To prevent this, the following file template is usually used :
#ifndef FILENAME_H_INCLUDED
#define FILENAME_H_INCLUDED
#include "whateveryouneedtoinclude.h"
class YourClass
{
};
#endif
This prevents the file from being included twice
#ifndef FILENAME_H_INCLUDED
#define FILENAME_H_INCLUDED
#include "whateveryouneedtoinclude.h"
class YourClass
{
};
#endif
This prevents the file from being included twice
Something else to note is that public inheritance almost always denotes an "is-a" relationship in C++ i.e. you're designing a hierarchy for run-time polymorphism. If this is the case, you'll need to use virtual functions and make the destructor virtual, too.
Edd
Edd
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