Reference to an empty vector(Return Default value )

[spider84]

I am trying to find a way to return an empty vector.
just for practice.
Let s say I have a class (Truck) that is include in an other class that we call Class (Car).
Truck.h

Class Truck{
     public :  
        Truck(); //Empty Constructor
        void addTruck();
     private:
        string brand;
}

Car.h

Class Car{
     public:
        Car();
        void addCar();
        Truck& findByName(const string& brandName);
     private:
         vector<Truck>myCars;//Vector
        Truck defaultName;
}

I want to return a default vector if I don't find the brand of the truck

Car.cpp

Truck::Truck(){
      brand="Empty";
}
Car::Car(){}
Truck& findByName(const string& brandName){
      for(int i=0; i<Car::myCars.size(); i++)
			if(Car::myCars[i].getBrand() == brandName)
				return Car::myCar[i];
		
		return Truck::defaultName;
}

How can i call the function from Main.cpp ??

[laserlight]

It sounds like you want to return a default truck, not vector, perhaps something like this:

class Truck {
public:
    static Truck defaultTruck;

    Truck(); //Empty Constructor
    void addTruck();
private:
    string brand;
};

class Car {
public:
    Car();
    void addCar();
    Truck& findByName(const string& brandName);
private:
    vector<Truck> myCars; //Vector
};

// ...

Truck Truck::defaultTruck;

Truck& findByName(const string& brandName) {
    for (int i = 0; i < myCars.size(); i++)
        if (myCars[i].getBrand() == brandName)
            return myCar[i];

    return Truck::defaultTruck;
}

But I think that it would be a wrong approach. If you must return a reference (or even an object), perhaps it would be better to throw an exception if the truck of the given brand name is not found. If not, you could return say, an iterator to the element of the internal vector, except that instead of directly returning a vector<Truck>::iterator, you return an iterator of a typedef type, then also provide a way to retrieve the one-past-the-end iterator so that the caller can test for it. There's also the option of returning a pointer with a null pointer used to indicate "not found".

[2kaud]

For classes that require a find method, probably the best way is don't! The STL algorithm provides find() methods (see http://www.cplusplus.com/reference/algorithm/). IMO the way forward for class(s) that require find methods is to write the class so that the STL algorithms work. This means that the class needs to provide iterators.

NB I realise this is practice, but having a class called Car return a reference to an object called Truck? Doesn't really seem right..

[2kaud]

For returning an iterator, consider

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

class Truck {
public:
	Truck() {};
	Truck(const string& brd) : brand(brd) {};

	void addTruck(const string& brd) {
		brand = brd;
	}

	string getBrand() const
	{
		return brand;
	}

	typedef vector<Truck> vt;
	typedef vt::const_iterator cvti;

private:
	string brand;
};

class Car {
public:
	Car() {};

	void addCar(const Truck& trk) {
		myCars.push_back(trk);
	}

	Truck::cvti findByName(const string& brandName) const;

	Truck::cvti end() const
	{
		return myCars.cend();
	}

	void display() const
	{
		for (const auto& c : myCars)
			cout << c.getBrand() << endl;
	}

private:
	Truck::vt myCars;
};

 Truck::cvti Car::findByName(const string& brandName) const {
	for (Truck::cvti vi = myCars.cbegin(); vi != myCars.cend(); ++vi)
		if (vi->getBrand() == brandName)
			return vi;

	return myCars.cend();
}

int main()
{
	Car myCars;
	myCars.addCar(Truck("qwerty"));
	myCars.addCar(Truck("asdfgh"));
	myCars.addCar(Truck("zxcvbn"));

	myCars.display();

	auto f = myCars.findByName("qwerty");
	cout << "Finding: " << "qwerty" << endl;
	if (f != myCars.end())
		cout << "Found: " << f->getBrand() << endl;
	else
		cout << "Not found" << endl;

	f = myCars.findByName("123");
	cout << "Finding: " << "123" << endl;
	if (f != myCars.end())
		cout << "Found: " << f->getBrand() << endl;
	else
		cout << "Not found" << endl;
}

[GCDEF]

Why would Truck be a member of Car? Seems like there may be a design problem there.

[laserlight]

For returning an iterator, consider

findByName (or should it be findByBrand?) should be a non-member non-friend function, at least after a corresponding begin() member is added to the interface of Car. I know my earlier post was more along having a member function for finding, but in retrospect that seems unnecessarily restrictive compared to your more general suggestion that "the way forward for class(s) that require find methods is to write the class so that the STL algorithms work".

I realise this is practice, but having a class called Car return a reference to an object called Truck? Doesn't really seem right..

Why would Truck be a member of Car?

Could be an unusual usage of "Car", e.g., to refer to trucks stored on a railroad car.

[2kaud]

To use the STL find(), consider

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

class Truck {
public:
	Truck() {};
	Truck(const string& brd) : brand(brd) {};

	void addTruck(const string& brd) {
		brand = brd;
	}

	string getBrand() const
	{
		return brand;
	}

	bool operator==(const string& st) const {
		return st == brand;
	}

	typedef vector<Truck> vt;
	typedef vt::const_iterator cvti;

private:
	string brand;
};

class Car {
public:
	Car() {};

	void addCar(const Truck& trk) {
		myCars.push_back(trk);
	}

	Truck::cvti findByName(const string& brandName) const;

	Truck::cvti end() const
	{
		return myCars.cend();
	}

	Truck::cvti begin() const
	{
		return myCars.cbegin();
	}

	void display() const
	{
		for (const auto& c : myCars)
			cout << c.getBrand() << endl;
	}

private:
	Truck::vt myCars;
};

 Truck::cvti Car::findByName(const string& brandName) const {
	for (Truck::cvti vi = myCars.cbegin(); vi != myCars.cend(); ++vi)
		if (vi->getBrand() == brandName)
			return vi;

	return myCars.cend();
}

int main()
{
	Car myCars;
	myCars.addCar(Truck("qwerty"));
	myCars.addCar(Truck("asdfgh"));
	myCars.addCar(Truck("zxcvbn"));

	myCars.display();

	auto f = find(myCars.begin(), myCars.end(), "qwerty");
	//auto f = myCars.findByName("qwerty");
	cout << "Finding: " << "qwerty" << endl;
	if (f != myCars.end())
		cout << "Found: " << f->getBrand() << endl;
	else
		cout << "Not found" << endl;

	f = find(myCars.begin(), myCars.end(), "123");
	//f = myCars.findByName("123");
	cout << "Finding: " << "123" << endl;
	if (f != myCars.end())
		cout << "Found: " << f->getBrand() << endl;
	else
		cout << "Not found" << endl;

}

[spider84]

him
I don;t want to use the STL find(), i just want to go through my Vector an find The information.
mainCar.cpp

int main(){
      Truck myTruck;
      Truck returnTruck;
      Cars myCars;
      string brandTruck;
     cout<<"Inout brand :";
         getline(cin, brandTruck);
     returnTruck = Truck.addTruck(brandTruck);

}

After adding this code to my Main .cpp file, i am getting the error :
invalid use of non static data member return Truck:efaultName;

[2kaud]

Post your complete program so that we can see what is happening.

[spider84]

Here is the full Code :
Truck.cpp

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

Truck::Truck(){ //Truck's Default Constructor
	Truck::brand = "default";
}
Truck::Truck(const string & brnd){
	brand=brnd;
}

Car::Car(){ } //Car's Default Constructor

void Car::addCar(){ //To add an new car/truck
	string brnd;
	cout<<"Name Brand:";
		getline(cin,brnd);
		
	Truck newTruck(brnd);
		
	Car::myTruck.push_back(newTruck);
}

Truck& findByName(const string& br) {
		
		for(int i=0; i<Car::myTruck.size(); i++)
			if(Car::myTruck[i].getTrcuk() == br)
				return Car::myTruck[i];
		
		return Car::myDefaultTruck;
	
}


void Car::listTrucks(){
	for(int i=0; i<myTruck.size(); i++)
		cout<<Car::myTruck[i].brand;
}

void Truck::setTruck(string trk){
	brand = trk;
}

string Truck::getTruck(){
	return brand;
}

UseTruck.cpp

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

int main(){
	string nt, brand;
	Truck t1;
	Car c1;
	char ch;
	int op;
	do{
	cout<<"Chose option!!"<<endl;
		cout<<"-1- Add a truck"<<endl;
		cout<<"-2- find truck"<<endl;
		cout<<"-3- list All trucks"<<endl;
		cin>>op;
		switch (op){
			case 1: 
				t1.addTruck();
			break;
			
			case 2:
				cout<<"input Brand to find : ";
				getline(cin,brand);
				t1 = c1.findByName(brand);
			break;
			
			case 3:
				c1.listTrucks();
			break;
			
			default:
				cout<<"Not an option";
			break;	
		}
		
		cout<<"Continue??? (Y/n)";
			cin>>ch;
	}while(ch!='n' && ch!='N');
	return 0;
}

Truck.h

#ifndef TRUCK_H
#define TRUCK_H

#include <iostream>
#include <vector>

using namespace std;

class Truck{
     public :  
        Truck(); //Empty Constructor
        Truck(const string&);
        void addTruck();
        
        void setTruck(string brand);
        string getTruck();
        
        friend class Car;
        
     private:
        string brand;
};

#endif

Car.h

#ifndef CAR_H
#define CAR_H

#include "Truck.h"

using namespace std;

class Car{
     public :  
        Car(); //Empty Constructor
        void addCar();
        void listTrucks();
        Truck& findByName(const string& name);
        friend class Car;
        
     private:
        vector <Truck> myTruck;
        Truck myDefaultTruck;
};

#endif

I compile it on command line on MAC

[2kaud]

What does Truck::addTruck() do?

Putting all the code together, this compiles for VS2015 and works as expected

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

class Truck {
public:
	Truck(); //Empty Constructor
	Truck(const string&);
	void addTruck() {}

	void setTruck(string brand);
	string getTruck();

	friend class Car;

private:
	string brand;
};

class Car {
public:
	Car(); //Empty Constructor
	void addCar();
	void listTrucks();
	Truck& findByName(const string& name);
	friend class Car;

private:
	vector <Truck> myTruck;
	Truck myDefaultTruck;
};

Truck::Truck() { //Truck's Default Constructor
	Truck::brand = "default";
}
Truck::Truck(const string & brnd) {
	brand = brnd;
}

Car::Car() { } //Car's Default Constructor

void Car::addCar() { //To add an new car/truck
	string brnd;
	cout << "Name Brand:";
	//getline(cin, brnd);
	cin >> brnd;

	Truck newTruck(brnd);

	Car::myTruck.push_back(newTruck);
}

Truck& Car::findByName(const string& br) {

	for (int i = 0; i<Car::myTruck.size(); i++)
		if (Car::myTruck[i].getTruck() == br)
			return Car::myTruck[i];

	return Car::myDefaultTruck;
}

void Car::listTrucks() {
	for (int i = 0; i<myTruck.size(); i++)
		cout << Car::myTruck[i].brand << endl;
}

void Truck::setTruck(string trk) {
	brand = trk;
}

string Truck::getTruck() {
	return brand;
}

int main() {
	string nt, brand;
	Truck t1;
	Car c1;
	char ch;
	int op;
	do {
		cout << "Chose option!!" << endl;
		cout << "-1- Add a truck" << endl;
		cout << "-2- find truck" << endl;
		cout << "-3- list All trucks" << endl;
		cin >> op;
		switch (op) {
		case 1:
			//t1.addTruck();
			c1.addCar();
			break;

		case 2:
			cout << "input Brand to find : ";
			//getline(cin, brand);
			cin >> brand;
			t1 = c1.findByName(brand);
			if (t1.getTruck() == "default")
				cout << "not found" << endl;
			else
				cout << "found" << endl;
			break;

		case 3:
			c1.listTrucks();
			break;

		default:
			cout << "Not an option";
			break;
		}

		cout << "Continue??? (Y/n)";
		cin >> ch;
	} while (ch != 'n' && ch != 'N');
	return 0;
}

[spider84]

Truck::addTruck() does nothing it s from a previous activity that I experimented. I apologize for that

Thank you for your replay.
I just tried the code you posted. it works ! what did you change? I can't see what you changed ?

[2kaud]

A bit of a general clean up of the code gives

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

class Truck {
public:
	Truck() {};
	Truck(const string& brd) : brand(brd) {}
	void setTruck(const string& brand);
	string getTruck() const;

private:
	string brand;
};

class Car {
public:
	Car() {};
	void addCar();
	void listTrucks() const;
	const Truck& findByName(const string& name) const;

private:
	vector <Truck> myTruck;
	static Truck myDefaultTruck;
};

Truck Car::myDefaultTruck;

void Car::addCar() 
{
	string brnd;

	cout << "Name Brand:";
	cin >> brnd;

	myTruck.emplace_back(brnd);
}

const Truck& Car::findByName(const string& br) const {

	for (auto& t : myTruck)
		if (t.getTruck() == br)
			return t;

	return myDefaultTruck;
}

void Car::listTrucks() const {
	for (const auto& t : myTruck)
		cout << t.getTruck() << endl;
}

void Truck::setTruck(const string& trk) {
	brand = trk;
}

string Truck::getTruck() const {
	return brand;
}

int main() {
	string nt, brand;
	Car c1;
	char ch;
	int op;

	do {
		cout << "Chose option!!" << endl;
		cout << "-1- Add a truck" << endl;
		cout << "-2- find truck" << endl;
		cout << "-3- list All trucks" << endl;
		cin >> op;
		switch (op) {
		case 1:
			c1.addCar();
			break;

		case 2:
			cout << "input Brand to find : ";
			cin >> brand;
			if (c1.findByName(brand).getTruck().empty())
				cout << "not found" << endl;
			else
				cout << "found" << endl;
			break;

		case 3:
			c1.listTrucks();
			break;

		default:
			cout << "Not an option";
			break;
		}

		cout << "Continue??? (Y/n)";
		cin >> ch;
	} while (ch != 'n' && ch != 'N');
	return 0;
}

[2kaud]

I just tried the code you posted. it works ! what did you change? I can't see what you changed ?

If you compare line for line you'll see the small changes needed!

[spider84]

Yes got it ! thank you so much for the help.
Having multiple functions with the same names in different classes could get your brain washed !
Will never do it again ! lol
thank you 2kaud
And thank you everyone for the answers