C++ Method wont work

Question

So I created a method in order to start the process for adding, multiplying and etc polynomials, however, when attemepting to run start(), the compiler runs, however the box remains blank, even though it shouldn't. Not exactly sure what I'm doing wrong.

Any ideas?

Here is my code.

Here is my header

#ifndef _POLY_GUARD 
#define _POLY_GUARD 

#include <iostream>
using namespace std;

class Polynomial
{
public:
    Polynomial(int coef, int exp);
    void start();   
    friend ostream & operator << (ostream &out, const vector<int> &c);
    friend istream & operator >> (istream &in, const Polynomial &c);  
    void addPolynomials();    
    void multiplyPolynomials();
    void evaluatePolynomial();
    int findCoefficient();
    int findLeadingExponent();    
};

#endif

This is the source code.

#include "Polynomial.h"
#include <utility>
#include <iostream>
#include <vector>
#include <string>
using namespace std;  

void Polynomial::start()
{
    int choice;

    std::cout << "What do you wish to do?" << std::endl;
    std::cout << "1. Add two polynomials" << std::endl;
    std::cout << "2. Multiply two polynomials" << std::endl;
    std::cout << "3. Evaluate one polynomial at a given value" << std::endl;
    std::cout << "4. Find Coefficent for a given polynomial and given exponent" << std::endl;
    std::cout << "5. Find the leading exponent for a given polynomial" << std::endl;
    std::cout << "6. Exit " << std::endl;

    std::cin >> choice;

    if (choice < 1 || choice > 6)
    {
        do
        {
            std::cout << "Invalid entry: please reenter choice" << std::endl;
            std::cin >> choice;   
        } while (choice < 1 || choice > 6);
    }

    if (choice == 1)
    {       
    }
 }

And finally, here is my main

#include "Polynomial.h"
#include <string>
#include <vector>
#include <utility>

int main()
{
    Polynomial start();
    system("pause");
}

My first idea is for you to use a debugger and edit your post with the results of your debugging session. — Thomas Matthews, Apr 26 '18 at 01:21
`Polynomial start()` declares a function. Nothing gets called except your call to `system()`. — 1201ProgramAlarm, Apr 26 '18 at 01:25
@RetiredNinja But `Polynomial` doesn't have default constructor. — songyuanyao, Apr 26 '18 at 01:33
@RetiredNinja Thank you! I can't believe I forgot a default constructor! — Nom OnTheCookie, Apr 26 '18 at 01:33
If you're in the market for a good introductory text on C++, I'd suggest C++ Primer 5th Edition. In my experience it is far better than most texts used to teach introductory courses. It's also comparatively cheap. — eric, Apr 26 '18 at 01:48

score 1 · Answer 1 · answered Apr 26 '18 at 01:34

According to this page, it is possible to declare a function within another function.

So your main function declares a function named start that returns Polynomial object, and calls system("pause"), then returns.

try dividing declaration and calling method start:

Polynomial p;
p.start();

or

Polynomial(1, 2).start(); // pass valid parameters

eric · Accepted Answer · 2018-04-26T01:43:12.090

Read the comments above as they are just as helpful as the example below.

So you have a class Polynomial from which you can create (instantiate) objects of that particular type.

class Polynomial {
public:
    /// default constructor
    Polynomial() = default;

    /// constructor with your coef and exp parameters
    /// when invoked it will use its arguments to initialize
    /// the data members coef and exp.
    Polynomial(int coef, int exp)
        : coef(coef)
        , exp(exp){};

    /// your member function start()
    void start();

private:
    /// your private data members that 
    /// are initialized upon construction
    /// when calling the appropriate constructor
    int coef;
    int exp;
};

In you main function, as others have mentioned you can construct an object of type Polynomial, call it app or whatever:

int main()
{
    /// app is your object of type Polynomial
    /// its coef and exp are initialized
    /// using your arguments 4 and 5 respectively.
    Polynomial app(4, 5);
    /// now you can call you member function
    app.start();

    return 0;
}

score 0 · Answer 3 · answered Apr 26 '18 at 02:42

0

Because

Polynomial start();

Is a most vexing parse.

https://en.wikipedia.org/wiki/Most_vexing_parse

answered Apr 26 '18 at 02:42

Jay Wai Tan

169
7

C++ Method wont work

3 Answers3