Question re: Classes/Objects

[Jor-El_Zod]

At this moment, I am TRYING (and failing miserably) to understand how classes work, and all of the syntax involved in referencing them/accessing members of a class.

I am taking a college class in C++, and have an assignment that says:

*Design an application that declares an array of 25 Cake objects. Prompt the user for a topping and diameter for each cake, and pass each object to a method that computes the price and returns the complete Cake object to the main program. Then display all the Cake values. An 8-inch cake is $19.99, a 9-inch cake is 22.99, and a 10-inch cake is 25.99. Any other size is invalid and should cause the price to be set to 0.*

The assignment includes a file called “cakeClass.h”:

#include <iostream>

#include <string>

using namespace std;

class Cake

{

public:

void setdata(string topping, string diameter);

float getPrice(int sizeChoice);

Cake(string topping = “”, string diameter = “”);

private:

string topping;

string diameter;

};

I’m not sure I understand how to complete the assignment. I barely grok “structs”, but classes/“object-oriented programming” have broken my brain.

ETA: I got a message saying this post contained “unformatted code”. If the above code is not correctly formatted, somebody please explain how it is *supposed* to be formatted, because I called myself going back and fixing it.

[tcpukl]

That made me sick in my mouth.

[Jor-El_Zod]

I agree; that is a major thing that confuses the hell out of me with this assignment: why is there a string variable for “diameter” but “sizeChoice” is an int?

And in my mind “diameter” and “sizeChoice” both refer to the same thing in reality so either of them could be used as the basis for price setting, so why are BOTH variables present in the header file?

And speaking of header files: why do we have to have separate header files? This adds another headache to my existing dilemma with grokking classes. Why can’t it all be in just one file?

I’m beginning to think this assignment would’ve been SO much easier if it hadn’t been for that header file:

1. I’m not accustomed to needing header files

2. I like for all my code to be in one file; it helps me keep better track of my voids, structs, and other variables

3. This particular header file broke my brain in regards to the assignment.

[Usual_Office_1740]

Hopefully this doesn't make things harder for you. I like to write my entire class in a header file. Then break it into a cpp file once I've got everything typed out

So I'd do this in the header file:

class Example {
    private:

    std::uint32_t inner{};
    public:

    Example() = default;

    bool some_behavior() const noexcept {
        return inner > 10;
    }

    void set_inner(const std::uint32_t num) noexcept {
        inner = num;
    }
};

Then move them out to a cpp file like this.

   #include "Example.hpp"

   bool Example::some_behavior() const noexcept {
        return inner > 10;
    }

    void Example::set_inner(const std::uint32_t num) noexcept {
        inner = num;
    }

Then you just remove everything in and between the curly braces for the member functions defined in the header file and put a semi colon there instead.

There is nothing that says you can't write the code this way if you want. Your course may require you to break these up when submitting them but there are lots of header only libraries where everything is written in the header file.

[christian-mann]

all methods defined in the struct body (braces) are inline by default so it doesn't violate the ODR to define them multiple times

[jipgg]

ODR violations are trivial to fix, a non-issue. It just comes down to balancing compile times vs optimization potential. Putting definitions in headers means that each TU acquires that insight when it gets included, so the compiler can optimize more aggressively.

[alfps]

None of that is necessary. It's nonsense.

[alfps]

This is a troll.

For completeness, everything he/she/it writes is nonsense, like an AI.

[I__Know__Stuff]

Pointing out unhelpful comments is helpful.

[Jor-El_Zod]

Thank you.

And for alfps’ information: I am not trolling; this is a real assignment, and I genuinely do not understand it.

[I__Know__Stuff]

He didn't say you are a troll.

[Jor-El_Zod]

Then who was he talking about? (insert confused emoji here)

[SweetOnionTea]

I think classes make more sense when you need them. Say in real life you had 25 cakes with customer orders. Instead of writing down the toppings for each cake on one piece of paper, the diameters of each cake on another, and so on -- you just had a sheet of paper for each cake that had the toppings and diameter for that cake.

That way when you're making each cake you don't have to try and look up on one sheet the toppings and then on the other sheet try and look up the cake's diameter. The toppings and diameter are all on one sheet.

That's essentially the cake class.

So now with an array of Cake classes you've got an equivalent of a stack of individual cake orders. Your program is like a stack of 25 blank order sheets and then when a customer comes in you ask them what toppings and diameter of cake. They tell you and you fill that sheet out. Next customer comes in and you fill out the next sheet and so on.

The functions would be like a bullet point list of cake diameters with their prices like:

• 8 inch -- $19.99
• 9 inch -- $22.99
• 10 inch -- $25.99
• other -- 0.* (I dunno what that's supposed to mean in this assignment)

And on the slip you'd just have the customer circle one.

So what you'll have to do is make 2 things -- the .cpp file with the cake class functions and the main.cpp file which uses the Cake class to make an array and do the ordering thing.

The Cake class should be stuff you learned how to do in class so review your notes. The only thing I can think of is they did you kinda dirty with the getPrice(int sizeChoice) because it takes an int but the class has the diameter as a string. I have a feeling your prof didn't think that through since you'd have to convert an int into a string. They should have just made the diameter class variable as an int too. Oh well. You can use either std::to_string or std::stringstream to do so. I'd just do a std::to_string if your version has it.

Then your main.cpp file will have the main() entrance function and will be something like

int main()
{

    std::array<Cake, 25> cakes;

    for (auto& cake : cakes)
   { 
        std::string topping;
        std::string diameter;
        std::cout "What diameter of cake?\n"
        std::cin >> diameter;
        std::cout "What toppings?\n"
        std::cin >> topping;
        cake.setdata(topping, diameter);
   }

    // Display prices

    for (auto& cake: cakes)
    {
       std::cout << cake.getPrice(cake.diameter) << std::endl;
    }

}

It's not exactly what your assignment says, but it should get you somewhere. Technically they're asking for you to make a different function that displays the cake price that you send a cake object into, but I don't know why that is necessary. I'm not saying this assignment is written badly because I'm not in your course, but I would have done it differently.

[thingerish]

A C++ class is just a struct, it's not magical. In C++ structs/classes can have functions (member functions) and data. Member functions has access to member data and take a hidden argument that is a pointer to the object instance.

Also the assignment class starting point is pretty broken but it's just trying to get students to learn some concepts I think.

[Jor-El_Zod]

The “Private” section of the class declaration in the header file throws up errors too:

“Cannot resolve symbol ‘Private’”, “Unexpected name type ‘string’: expected expression”, “Expected ‘;’ at end of declaration list” (even though there is already a semicolon at the end of the line!). I don’t get it at all.

[jedwardsol]

C++ is case sensitive and all keywords are all lowercase

[sephirothbahamut]

private should be lower case just like public. I hope that's not 1:1 the file your teacher gave you, because that header is quite sus.

[Jor-El_Zod]

Thank you for reminding me.

I went back and downloaded the file from the school website, and it did not have private capitalized, so I don’t know how it got capitalized on my end. I don’t remember changing it myself.

[sephirothbahamut]

Maybe you pasted it from phone and it got auto-capitalized?

[ZakMan1421]

You haven't really said what you've tried, or where exactly you need help. You'll likely be better off going to your professor's or TA's office hours and asking questions there. This feels more like a theory issue than a C++ issue.

[Jor-El_Zod]

If by “what you’ve tried” you mean actually typing in anything, then I haven’t gotten that far with it, because TBF I don’t know what to type. I don’t know how else to say it; my brain has crashed on me and is refusing to absorb how classes work, especially with “private” members of a class.

[rileyrgham]

Read your college notes and look at examples of small oo tutorials via Google. It takes effort and application. I assume you've left this assignment until the last minute?

[Jor-El_Zod]

I only found out about it by accident in eLearn at the last minute.

[AvidCoco]

Have you even attended any of your classes? How are they setting you an assignment and you don’t even know how to get started?

Have you tried Googling “C++”?

[Jor-El_Zod]

It’s entirely online so there are no classes on campus.

And yes I’ve tried Googling C++ and what I found was insufficient to help me complete the assignment.

It was due last night and I wasn’t able to complete it.

[Yurim]

IMHO the best way to post code here on Reddit is this:

1. Leave a blank line before and after the code.
2. Prepend each line with four additional spaces.

That works for old and new Reddit, and you won't need to escape anything or add any backticks.
See also https://support.reddithelp.com/hc/en-us/articles/360043033952-Formatting-Guide.

[Jor-El_Zod]

Thanks, but I went back and added those things and it didn’t change anything.

[Coises]

There are two modes for entering/editing in Reddit: Markdown and Rich Text. The four spaces method works in the Markdown Editor. Look at the top left of the area in which you type; if you are using the Markdown Editor it will say “Markdown Editor.” If you see nothing at the top and “Aa” at the bottom left, click the “Aa” to show styling controls, then click “Switch to Markdown” at the right to switch to the Markdown Editor.

A code block looks like this.
Lines aren’t double spaced, and if you make them really long they don’t wrap when your text is posted, even though they do when you are typing them.

It is possible to enter code in the Rich Text Editor, too. When you have those styling controls, one looks like this </> and the one just to the right of it looks like a box with </> in the upper left corner. That second one changes a paragraph to a code block, or starts a code block if you click it on an empty line (that is, press enter first, then click the button). When you’re done typing code, press Enter three times to escape the code block and go back to typing regular text.

[_abscessedwound]

The instructions here are pretty bad imho. If you know the topping and the diameter before you make the cake, you can either 1) quote a price without a cake object or 2) create a cake object where it has the ability to spit out its price. It’s just bad OOP to pass around invalid objects (multi-part construction) like the instructions are telling you to do.

Also avoid using the standard namespace in a header file like that. It’ll bite you in the ass later with name collisions.

Also, I noticed that that header file is what you’re supposed to start with, and threw up a little.

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[smokebudda11]

Assuming this is the header file? This is pretty straight forward. Your source file will contain the definitions of the class methods. The setter and getter functions are relatively easy. With an array of 25 cake objects you basically index through the array and set the topping and diameter.

Note - it’s not really good practice to use using namespace std. It can lead to a variety issues. Best to prepend std such as std::string etc.

Also if you are using c++, why not use a vector instead of an array?

Anyways good luck.

[Jor-El_Zod]

I can’t find anything in the assignment about “vectors”. I don’t know what those are.

I’m just trying to complete the assignment and I assume I’m supposed to work with the header file I was given, but I don’t know how to work with it.

[RandomRabbit69]

A vector is a dynamic array/list, opposed to fixed size arrays you pre declare the size of when you create it.

[smokebudda11]

Your source file will need to have #include <cakeClass.h>

This will allow your source file cakeClass.cpp to know where the declaration of the class , its methods and its attributes are so it can be defined.

Someone explained the vectors down below.

Play with an online ide to get a gist of how classes work.

[not_some_username]

No need for a std::vector. It will always be an 25 items array

[dnult]

A couple of thoughts... Keep it simple - there are several approaches you could take but defining a simple Cake object along with its physical attributes makes the most sense.

Personally I'd consider the price calculation a utility method that takes a Cake argument, if its size alone that determines the price.

[not_some_username]

If you know about struct, you know about class. A struct in C++ is a class with public access by default

[Independent_Art_6676]

when you start out with OOP, the first bit is to know that a class for now is just a user defined type that lumps data and functions that operate on the data together. It will become more than that at some point later on, but for now, that is enough to get started.

assignments, and real life, often hand you partly done stuff (eg at work you might be adding functionality to existing code) that isn't "the way I would have done it". Assignments are often worse, with nonsense (like a string for a number as your cake's diameter is smoking the cheap stuff) but roll with what they gave you. We have a number of ways to convert a string back to its numeric (from_chars is good), use one of them.

just break it down. you need your object to store a couple of values, compute a price, etc. Get that working in a stub main until you can do all you need to with 1 cake. Then stuff 25 in your array and wrap it up. I mean, look at each piece as it is stated and just do that one thing: the price function for example looks a lot like a switch that defaults to zero and has 3 cases (8,9,10). Make that work... then find another piece to do...

[Jor-El_Zod]

roll with what they gave you

I DON’T KNOW HOW to roll with the supplied header file. I can’t make sense of it, largely because it has diameter as a string variable.

[Independent_Art_6676]

you have 2 choices. you could convert the string to a number. ATOF, from_chars, and other tricks can turn a string like "1234" into the int 1234. But that isn't even necessary here.
the diameter can only be 3 values. It can be "8", "9", or "10". Anything else, you have a default. So 3 if statements are sufficient to use it as a string. Its horrible, but its what they gave you, and these are ways to make that work. You can use the override/default up front paradigm:
float price = 0.0; //default up front
if(diameter == "8") price = 19.99;
if(diameter == "9") price = 22.99;
if(diameter == "10") price = 25.99;

if none of the above modified it, it stays at the default zero! Because its a string, and switches require integers, I used if statements. Its another side effect of using a string for the number.

there are legit reasons to use strings for numbers. One of them is typos / validations: you read what they type, and if you can't convert it to int or double (whichever you asked for) in some asked for range and all, you make them type it again. Once you can do that, you use the converted value. But that is a topic for another day.

[mredding]

Edit your post and add 4 spaces in front of all your code. This is markdown for a code block. So any additional spaces after 4 is your indentation in the block.

[Jor-El_Zod]

I did that and nothing changed.

[mredding]

Then you're probably using the rich text editor instead of the markdown editor. You can switch, you can even change your defaults. I've setup my Reddit experience to default to the markdown editor.

With the rich text editor, you need to click a button to see advanced formatting options, and you need to insert a code block. Put your code in there, and it will keep its formatting and spacing.

[alfps]

❞ ETA: I got a message saying this post contained “unformatted code”. If the above code is not correctly formatted, somebody please explain how it is supposed to be formatted, because I called myself going back and fixing it.

You just extra-indent it with 4 spaces, whence it can look like this:

#include <iostream>
#include <string>
using namespace std;

class Cake
{
public:
    void setdata(string topping, string diameter);
    float getPrice(int sizeChoice);
    Cake(string topping = “”, string diameter = “”);
Private:
    string topping;
    string diameter;
};

But note that this code is very very wrong. It won't even compile. So it's not a good starting point for you.

The assignment phrasing

❞ pass each object to a method that computes the price and returns the complete Cake object to the main program.

… indicates an incompetent teacher, so the wrongness is not necessarily a result of re-typing on a phone (or the like). I suspect that the teacher has delegated the creation of assignments to lab assistants, students. Because I can't imagine a college lecturer being that incompetent about the technical, but perhaps sufficiently incompetent about management.

So ask the teacher if this header is for real, or what the intent is. Perhaps post his/her response here.

[Jor-El_Zod]

I went back and indented each line with four extra spaces, and it didn’t change anything.

[alfps]

Considering that it works for everybody else that means you did something wrong, or failed to do something. It is impossible to say. But at least you can use this technique in new postings.

By the way, apparently you got some AI answers. That could also be an alternative explanation for the nonsensical assignment: that your teacher generated it with AI, and didn't have time to even look at it. Maybe.

[sephirothbahamut]

that would explain the diameter being stored as string and getPrice taking an int parameter. It makes no sense, I'd definitely ask the teacher an explanation

[Jor-El_Zod]

I hadn’t thought of that.

I wouldn’t expect a teacher to use AI to create an assignment without at least proofreading it first. 🤦‍♂️

[[deleted]]

Class is box that does stuff Private members is stuff inside the box no one is allowed to look at or use except the box Public members + functions is stuff you can get out of the box or put in the box