Thread: "Prototype does not match any in class" error

Code:

#include <iostream>using namespace std;


class Rectangle {
    int breadth = 0;
    int length = 0;
    public:
    int area (int, int);
    int perimeter (int, int);
    void SetValues (int, int);
};


Rectangle::area (int l, int h) {
    l = breadth;
    h = length;
    return l * h;
}


Rectangle::SetValues (int First_Value, int Second_Value) {
    breadth = First_Value;
    length = Second_Value;
}


int main () {
// More code to be inserted at a later stage
    return 0;
}

Line 21 gives the error message: "Prototype does not match any in class" error.
Line 10 also gives the error "candidate is void Rectangle::SetValues(int, int)

What I notice is that by changing line 10 to int SetValues (int, int); resolves the error. But why?

Originally Posted by Vespasian_2

What I notice is that by changing line 10 to int SetValues (int, int); resolves the error. But why?

The return types you use when prototyping functions have to match those you use when defining the functions.

Both your function definitions don't have an explicit return type. The syntax is:

Code:

void Rectangle::SetValues(int First_value, int Second_value)
{
 //code
}

Originally Posted by Aslaville

The return types you use when prototyping functions have to match those you use when defining the functions.

Both your function definitions don't have an explicit return type. The syntax is:

Code:

void Rectangle::SetValues(int First_value, int Second_value)
{
 //code
}

Thanks, it works now.

On a related note, why does the following code print out an area of 0 instead of 25?

Code:

#include <iostream>using namespace std;


class Rectangle {
    public:
    int length = 0;
    int breadth = 0;
    int calculate_area ();
    int calculate_perimeter (int, int);
    void SetValues (int, int);
    int area = calculate_area();
};


int Rectangle::calculate_area () {


    return length * breadth;
}


void Rectangle::SetValues (int First_Value, int Second_Value) {
    length = First_Value;
    breadth = Second_Value;
}


int main () {
    Rectangle Rectangle1;
    Rectangle1.SetValues(5, 5);
    cout << Rectangle1.area;
    return 0;
}

Originally Posted by Elkvis

You didn't call .area as a function. You're likely printing some default representation of the address of the function itself.

Which line number?

Originally Posted by Elkvis

Line 31.

But why is that an error? I mean I can call another object Rectangle1 variable, say, length or breadth which doesnt return an area.

Originally Posted by Elysia

Line 11: int area = calculate_area();
This declares a variable area which is initialized when the object is created to the value returned by calculate_area. Since both those are initialized to 0, the result is 0. C++ does not have any kind of declarations where you establish some kind of relationship. The variable area is never recalculated when some variables change. It is initialized first (and only) when the line of code is executed, and that is when the object is first created. Make area a function or call calculate_area.

Okay so if I can confirm what that means is that the value "area" really cannot change during the life of the programme?

Originally Posted by Elysia

The way you've programmed it, it will not change during the life of the programme.

The statement:
int area = calculate_area();
is run once and only once during the whole life of the programme.

I suggest you remove it and call calculate_area() instead:
cout << Rectangle1.calculate_area();

That works.

Even better I set the values within calculate area as follows:

Code:

#include <iostream>

using namespace std;


 class Rectangle {
    public:
    int length = 0;
    int breadth = 0;
    int calculate_area (int, int);
    int calculate_perimeter (int, int);
    void SetValues (int, int);
//    int area = calculate_area();
};


int Rectangle::calculate_area (int First_Value, int Second_Value) {
    length = First_Value;
    breadth = Second_Value;
    return length * breadth;
}


void Rectangle::SetValues (int First_Value, int Second_Value) {
    length = First_Value;
    breadth = Second_Value;
}




int main () {
    Rectangle Rectangle1;
    //Rectangle1.SetValues(5, 5);
    cout << Rectangle1.calculate_area(4, 5);
    return 0;
}

Originally Posted by Elysia

Now calculate_area() is not true to its name. I don't expect calculate_area() to actually store new breath & length if I were to call it, and yet that's what you do. Calculate_area() has one function: to calculate the area and that's it. It should not attempt to do more. Preferably it should use the class's length and breadth members; otherwise there's no point of it being a member function.

Okay, I see what you mean. The function is doing more things than what I described it.

How about this then:

Code:

#include <iostream>

using namespace std;


 class Rectangle {
    public:
    int length = 0;
    int breadth = 0;
    int calculate_area ();
    int calculate_perimeter (int, int);
    void SetValues (int, int);
//    int area = calculate_area();
};


int Rectangle::calculate_area () {
    return length * breadth;
}


int main () {
    Rectangle Rectangle1;
    Rectangle1.length = 1;
    Rectangle1.breadth = 6;
    cout << Rectangle1.calculate_area();



    return 0;
}

Originally Posted by Elysia

This is much better. Note that calculate_area() also should be const.

I really appreciate your input, giving good advice over and above the question(s) asked.

When you say const, you mean for the member function to return a const int

Code:

#include <iostream>

using namespace std;


 class Rectangle {
    public:
    int length = 0;
    int breadth = 0;
    const int calculate_area ();
    int calculate_perimeter (int, int);
    void SetValues (int, int);
//    int area = calculate_area();
};


const int Rectangle::calculate_area () {
    return length * breadth;
}


int main () {
    Rectangle Rectangle1;
    Rectangle1.length = 1;
    Rectangle1.breadth = 6;
    cout << Rectangle1.calculate_area();
    //Rectangle1.SetValues(5, 5);
    //cout << Rectangle1.calculate_area(4, 5);
    //cout << Rectangle1.calculate_area(4, 5);
       // Rectangle1.length = 1;


            //cout << Rectangle1.calculate_area(4, 5);


    return 0;
}