C ++（OOP）中的类和封装

Question

We're doing classes in CPP at my course right now using OOP and Im a bit lost with them. I understand the whole process - I think, but I just cant seem to get it right. Im quite new at this so please go easy on me.

Basically, the whole premise for this assigned task is to create the following variables and classes:

1. Create a class named Triangle
2. Encapsulate a, b, c - the triangle side lengths
3. bool Set (double aa, double bb, double cc); - sets the values and returns true or false depending on whether a triangle based on those lengths is possible
4. double Perim(); - calculates the perimeter of the triangle.
5. double Area (); - calculates the triangle area.
6. bool isRect(); - checks whether this is a right-angle triangle.

I hope that makes sense?

Heres what I have so far:

main.cpp file (far from complete, this is just as a placeholder at the moment):

#include <iostream>
#include "triangle.h"

using namespace std;


int main() {
  Triangle t;
  int aa, bb, cc;

  cout <<"Triangle side 1 - " <<endl;
  cin >> aa <<endl;
  cout <<"Triangle side 2 - ";
  cin >> bb <<endl;
  cout <<"Triangle side 3 - ";
  cin >> cc <<endl;


return 0;
}

Triangle.h file:

#include <iostream>
#include "Triangle.cpp"

using namespace std;

#ifndef TRIANGLE_H
#define TRIANGLE_H

class Triangle(){

private:
  double a;
  double b;
  double c;

  double Perim();
  double Area();
  bool IsRect();

public:
  Triangle();
  Triangle(int, int, int);
  bool set(double aa, double bb, double cc);
};

#endif

and Triangle.cpp file (with the formulas to calculate everything)

#include "Triangle.h"

  Triangle::Triangle(){
   a = b = c = 0;
   Perim = 0.0;
   Area = 0.0;
 }

 Triangle::Triangle(int aa, int bb, int cc){

 }

  double Triangle::Perim(){
    return a + b + c;
  }

  double Triangle::Area(){
    s = (a+b+c)/2;
    Area = SQRT(s(s-a)(s-b)(s-c));
    return Area;
  }

  bool Triangle::isRect(){
    return (((a*a) + (b*b)) == (c*c)) ? true : false; //---checks if this is a right angle triangle, and should return true or false.
  }

  bool Triangle::Set(double aa, double bb, double cc){
    a = aa;
    b = bb;
    c = cc;

        if (a + b > c && a + c > b && b + c > a){//if one of these criteria is false, then a triangle is not possible.
          return cout << "A triangle with these parameters is possible.";
        }
        else{
          return cout << "A triangle with these parameters is NOT possible.";
        }
}

Of course this is far from complete, but Im struggling to link everything together. Im trying to get the Main.cpp file, when the person enters the values, that should be passed to other cpp file and calculations made, and once thats done, to return the values to Main cpp through objects (yet to be created). If that makes sense?

Ive been trying to wrap my head around this for a few hours now and I just cant seem to get it right, was hoping someone here would point me in the right direction?

Thank you in advance, and sorry for the messy code..

Answer 1

It's an acceptable start. But there are a couple of issues here.

Let's go through the header first:

You need to get rid of the #include of a cpp file. You should only include headers in a header. And better limit yourself to the headers really required by the code in the header. For instance, <iostream> is not really needed for the class definition. So move this include to the cpp where it is needed.

You also need to get rid of the using clause: this should be used in cpp, and not in headers, because it causes the file including the header to ignore that another namespace is imported, which could later on create conflicts (in larger projects). More advices on how to use headers here .

Then a Triangle is a class and not a function. So you have to define it with class Triangle { ... }; and not class Triangle() {...};

I also would expect the following member functions to be public:

double Perim();    // These are definitively of interest for users of Triangles
double Area();
bool IsRect(); 

Now to the implementation

First, you need to avoid the confusion between member variables and member functions:

  Triangle::Triangle(){
   a = b = c = 0;
   // Perim = 0.0;   // NO !! Perim() is a function, you cannot assign a value to it
   // Area = 0.0;
 }

Then you need to declare the variables that you use:

 double Triangle::Area(){
    auto s = (a+b+c)/2;                // because s deosn't exist
    return std::sqrt(s*(s-a)*(s-b)*(s-c));   // sqrt requires include <cmaths>
                                       // multiplication must be explicit.  
  }

Then you can just return a boolean expression. No need to be more explicit:

bool Triangle::isRect(){
    return ((a*a) + (b*b)) == (c*c); 
  }

Finally, your set function needs some rework: you have to return true or false . You better do not use cout in the return statement, but before returning. Last but not the least, you must perform the test of validity before you assign the member variables.

Finally to main()

Input and output stream are different things. So do not try <<endl on cin !

Once the input work, you can use t.set(...) to use the values entered by the user to change t.

If the t.set(...) returns true, you can display the result of the functions. For example:

 cout << "Area: " << t.Area()<<endl; 

If the result is false, then better inform the user that you can't do no more things with such a triangle.

I suppose that you know for the compilation how to compile the main.cpp and triangle.cpp together.

Answer 2

1) Include header files, not cpp files!

Remove:

    #include "Triangle.cpp"

reason:

cpp files are what gets compiled. They depend on header files, not the other way around. headers can depend on other headers, that happens a lot actually. but they shouldn't ever depend on a cpp file.

2) Wrap headers in ifndef/define blocks.

Move:

#ifndef TRIANGLE_H and #define TRIANGLE_H to the top of the file above everything else. Stylistically you may want to comment what the file does above the ifndef/define block instead of under

Example:

    // This is the triangle header file! Here is where I describe it.
    #ifndef TRIANGLE_H
    #define TRIANGLE_H
    // TODO: Put the includes here
    // TODO: Put your class here
    #endif

Reason:

In C++ you can think of the line: #include <iostream> as copy-pasting the contents of the iostream file overtop of that line. So when you #include "triangle.h" in both main.cpp and triangle.cpp, you're describing what the triangle class will look like to each of those cpp files. main.cpp needs to know so it can create and use a triangle, and triangle.cpp needs to know so it can implement the functions in Triangle correctly.

If you include a file twice, that file is effectively copy-pasted twice. Normally that would be re-defining its contents, which would be a compile error. What the ifndef/define block does is say "hey, if TRIANGLE_H isn't already defined, this is the first time it's been included. So go ahead and define TRIANGLE_H, along with all the file contents. The next time triangle.h is included we'll see that TRIANGLE_H is already defined and we skip right by the contents of that header, instead of re-defining what a Trinagle is.

The reason I suggest you move it to the top, is you're including iostream, triangle.cpp (we talked about that part already) and using namespace std every time triangle.h is included. There's no need for that.

3) Fix the triangle class declaration

Change:

    class Triangle(){

to

    class Triangle {

Reason:

You're close! but you're confusing the syntax of declaring a constructor with the syntax of declaring a class.

4) Make functions you intend to use outside of Triangle public.

Move:

the "public:" access specifier above double Perim();

Reason:

If you want to call the methods: Perim, Area and IsRect from main.cpp (or from anywehre but the inside of the triangle class!) you'll need those methods to be public.

You might be tempted to make everything public, but it's bad form - and your instructor specifically said "encapsulate a, b, c". You can google the encapsulation design pattern to understand why. I won't get into it here.

5) Fill out your triangle constructor

See the method:

    Triangle::Triangle(int aa, int bb, int cc)

Changes:

• In the header file you don't give the three parameters names. This isn't an error, but it's weird. You should probably give those parameters names.

• In your other constructor (the one without parameters) you show that you know how to assign a, b, and ca value! Go ahead and assign them values in this method.

• Consider if you actually want to use int as the types of the parameters. You're storing the side lengths as doubles but you're providing them to the constructor as ints. This isn't an error, but it's weird.

6) Fix the body of the Set method.

See the method:

    Triangle::Set(double aa, double bb, double cc)

Note:

Remember a method is an exchange. You provide it with it's parameters, it does some work and it returns to you some value (unless it 'returns' void). Your method is asking for 3 doubles, and returning a bool. If you're returning a bool, you're agreeing to give back a true or false value. Here you're tring to return whatever cout << "..."; returns. That doesn't sound like what you want (even if it compiles). You don't care about what cout returns, right?

Changes:

Don't cout anything from this function. Just return true if a triangle is possible, and false if it's not possible. You can cout whatever you want from where Set is being called from.

7) Encapsulate a, b, c.

You've made ab and c private. That's half of encapsulation. Your instructor probably wants you to write a getter and setter for these variables in Triangle.

Example:

    double Triangle::GetA() {
        return a;
    }

    void Triangle::SetA(double aa) {
        a = aa;
    }

I'll leave it as an exersize to do the rest.

8) Using your Triangle in main.cpp

If you take your local variable t , you can call functions on it like so:

    t.SetA(1.1);
    cout << t.GetA() << endl; // will print 1.1

    double theValueOfA; // local variable
    theValueOfA = t.GetA(); // store the result of GetA in our new variable

    cout << theValueOfA << endl; // will also print 1.1

With that information, after you're done your calls to cin to get the user inputs, do the following:

• create a boolean variable to store the result of set, we'll use this later.
• call the Set function on t, providing the 3 lengths the user inputted. Store the return of Set in our boolean.
• Write an if statement. Check if the value of that boolean is true. If it is, print "A triangle with these parameters is possible.", otherwise print "A triangle with these parameters is NOT possible."
• If your assignment requires you to do anything else or anything different, such as printing the result of Perim, Area and isRect, do that!