Error: variable has incomplete type void

[passiontrip]

Hello I'm fresh into c++ and I'm really struggling with this error. I'm tasked to have all the outputs for my values of area, perimeter, and diagonal in a display function. I created one and am trying to invoke the function within my main file but I keep encountering the error in the title.

What exactly does the error mean in this situation and how should I go about fixing it? Thanks in advance.

The display function is being called in int main() on line 43.
The display function was created on line 72.

#include <iostream>
#include <iomanip> //using setprecision(1)
#include <math.h>
using namespace std;
 
//functions
double findArea(double sideOne, double sideTwo);
double findPerimeter(double sideOne, double sideTwo);
double findDiagonal(double sideOne, double sideTwo);
void display(double sideOne, double sideTwo, double area, double perimeter, double diagonal);
 
//Main
int main()
{   //Variables
    double sideOne, sideTwo;
 
    //Gathering the number for boths sides from the user
    cout << "Please enter the length of side one." << endl;
    cin >> sideOne;
 
    cout << "\nPlease enter the length of the side two." << endl;
    cin >> sideTwo;
 
    //Validating that the values are greater than zero
    if (sideOne <=0 || sideTwo <= 0)
    {
        cout << "Both values for the length need to be greater than zero." << endl;
        exit(0);
    }
 
    //setting precision to one decimal place
    cout << fixed << setprecision(1);
    // Declaring the variable area and invoking the findArea function to calculate and assign the value
    double area = findArea(sideOne,sideTwo);
 
    //Invoking the perimeter function and outputting value
    double perimeter = findPerimeter(sideOne,sideTwo);
 
    //Invoking diagnal function
    double diagonal = findDiagonal(sideOne,sideTwo);
 
 
    void display(area, perimeter, diagonal);
    {
        cout << area << endl; // this line is just here for testing. Definitely wrong.
    }
    return 0;
}
 
//Calculating Area
double findArea( double sideOne, double sideTwo)
{
    double area = sideOne * sideTwo;
    return area;
}
 
//Calculating Perimeter
double findPerimeter(double sideOne, double sideTwo)
{
    double perimeter = ((sideOne * 2) + (sideTwo * 2));
    return perimeter;
}
 
//Calculating diagnal
double findDiagonal(double sideOne, double sideTwo)
{
    double diagonalTotal = ((sideOne *sideOne) + (sideTwo*sideTwo));
    double diagonal = pow(diagonalTotal,0.5);
    return diagonal;
}
 
void display(double sideOne, double sideTwo, double area, double perimeter, double diagonal)
{
    cout << "The area is: " << area << endl << endl;
    cout << "The perimeter is: " << perimeter << endl<< endl;
    cout << "The diagonal is: " << diagonal << endl;
}

[JLBorges]

int main()
{   //Variables
    double sideOne, sideTwo;
 
    //Gathering the number for boths sides from the user
    cout << "Please enter the length of side one." << endl;
    cin >> sideOne;
 
    cout << "\nPlease enter the length of the side two." << endl;
    cin >> sideTwo;
 
    //Validating that the values are greater than zero
    if (sideOne <=0 || sideTwo <= 0)
    {
        cout << "Both values for the length need to be greater than zero." << endl;
        exit(0);
    }
 
    //setting precision to one decimal place
    cout << fixed << setprecision(1);
    // Declaring the variable area and invoking the findArea function to calculate and assign the value
    double area = findArea(sideOne,sideTwo);
 
    //Invoking the perimeter function and outputting value
    double perimeter = findPerimeter(sideOne,sideTwo);
 
    //Invoking diagnal function
    double diagonal = findDiagonal(sideOne,sideTwo);
 
    /***************** commented out *******************
    void display(area, perimeter, diagonal);
    {
        cout << area << endl; // this line is just here for testing. Definitely wrong.
    }
    *********************************************************/
    display( sideOne, sideTwo, area, perimeter, diagonal ); // *** added

    return 0;
}

[passiontrip]

Hey thanks so much! I take it when you use a void function you don't need to put void in the main function you just invoke it with the name of the function and parameters it's using?

[JLBorges]

Yes. Just invoke it, like we invoke the other functions that return values.