Is a getter method `class ClassName ClassName() { ... }` legal C++? [duplicate] - c++

I don't even know where to go with this. Google wasn't very helpful. As with my previous question. I'm using TextMate's Command+R to compile the project.
game.h:16:error: declaration of ‘Player* HalfSet::Player() const’
players.h:11:error: changes meaning of ‘Player’ from ‘class Player’
game.h:21:error: ‘Player’ is not a type
player.h file (partial)
#ifndef PLAYERS_H
#define PLAYERS_H
using namespace std;
#include <string>
#include <vector>
#include <istream>
#include <iomanip>
#include "generics.h"
class Player{ //Line 11
public:
//getters
long Id() const;
string FirstName() const;
string LastName() const;
string Country() const;
//setters
void setId(long id);
void setFirstName(string s);
void setLastName(string s);
void setCountry(string s);
//serializing functions
void display(ostream &out);
void read(istream &in);
void write(ostream &out);
//Initalizers
Player();
Player(istream &in);
Player(string firstName, string lastName);
Player(string firstName, string lastName, string country);
Player(long id, string firstName, string lastName, string country);
~Player();
private:
long _id;
string _firstName;
string _lastName;
string _country;
};
game.h file (partial)
#ifndef GAME_H
#define GAME_H
#include "generics.h"
#include "players.h"
#include <string>
#include <vector>
#include <istream>
#include <iomanip>
using namespace std;
class HalfSet{
public:
//getters
Player* Player() const; //Line 16
int GamesWon() const;
int TotalPoints() const;
int Errors() const;
//setters
void setPlayer(Player* p);
void setGamesWon(int games);
void setTotalPoints(int points);
void setErrors(int errors);
//Serialization
void display(ostream &out) const;
void read(istream &in) const;
void write(ostream &out) const;
//Initalizers
HalfSet();
~HalfSet();
private:
Player* _player;
int _gamesWon;
int _points;
int _errors;
};
What is going on here?

In C++ you cannot name a function the same name as a class/struct/typedef. You have a class named "Player" and so the HalfSet class has a function named "Player" ("Player *Player()"). You need to rename one of these (probably changing HalfSet's Player() to getPlayer() or somesuch).

The current answer to this question is just incorrect, it says:
In C++ you cannot name a function the same name as a class/struct/typedef
Name hiding of a class by a function is allowed, if we go to the draft Pre C++11 standard section 3.3.7 Name hiding it says:
A class name (9.1) or enumeration name (7.2) can be hidden by the name
of an object, function, or enumerator declared in the same scope. If a
class or enumeration name and an object, function, or enumerator are
declared in the same scope (in any order) with the same name, the
class or enumeration name is hidden wherever the object, function, or
enumerator name is visible.
So the fact that you have the function and a class named Player is not the issue, in fact the following code is valid:
class Player
{
} ;
Player* Player() ;
and we can use an elaborated type specifier to un-hide the class type.
As far as I can tell this is violating section 3.3.6 Class scope paragraph 2 which says:
A name N used in a class S shall refer to the same declaration in its
context and when re-evaluated in the completed scope of S. No
diagnostic is required for a violation of this rule.
So in this case Player changes meaning from class to a function, it is not clear to me it was intended so strictly but I can see how it could be read that way. This seems to be the message gcc uses when it detects this violation as we can see from a similar question.
Using an elaborated type specifier prevents the change of meaning:
class Player* Player() const ;

Your problem is that names are looked up in scopes. Within the declaration of HalfSet::setPlayer(Player*), the unqualified name Player needs to be looked up. The first scope tried is class HalfSet. In that scope, the lookup of Player finds function HalfSet::Player, not global class ::Player.
The solution is to use a qualified name, ::Player. This tells the compiler which scope to use for lookup (global) which in turn means HalfSet::Player is not even considered.

I had this error on my code I can't comprehend this
A name N used in a class S shall refer to the same declaration in its context and when re-evaluated in the completed scope of S. No diagnostic is required for a violation of this rule.
But this what I did so it fixed my error
previous code
#include "a.h"
class System {
public:
car car; /*This line is problem, Can't declare object with same name as
class*/
};
Modified code
#include "a.h"
class System {
public:
car Car; //Here is the difference, Just changed first letter of object
};

Related

C++ class declaration after using it

I want to create method with an argument which links to Enemy which is declared later.
Here is my code:
#include <iostream>
#include <vector>
using namespace std;
class Weapon{
public:
int atk_points;
string name;
string description;
void Attack(Entity target){
};
};
class Armor{
public:
int hp_points;
string name;
string description;
int block_chance;
};
class Entity{
public:
int hp;
int atk;
string name;
vector<Weapon> weapons;
vector<Armor> armors;
};
I tried to search for answers, but nothing I found was helpful.
Here's error log:
prog.cpp:9:15: error: ‘Entity’ has not been declared
void Attack(Entity target){
The problem is that the compiler doesn't know what Entity is at the point where you have used as a parameter type. So you need to tell the compiler that Entity is a class type.
There are 2 ways to solve this both of which are given below:
Method 1
To solve this you need to do 2 things given below:
Provide a forward declaration for the class Entity.
Make the parameter of Attack to be a reference type so that we can avoid unnecessary copying the argument and also since we're providing a member function's definition instead of just declaration.
class Entity; //this is the forward declaration
class Weapon{
public:
int atk_points;
string name;
string description;
//------------------------------v------------>target is now an lvalue reference
void Attack(const Entity& target){
};
};
Working demo
Method 2
Another way to solve this is that you can provide just the declaration for the member function Attack' inside the class and then provide the definition after the class Entity's definition as shown below:
class Entity; //forward declaration
class Weapon{
public:
int atk_points;
string name;
string description;
//------------------------------v----------->this time using reference is optional
void Attack(const Entity& target); //this is a declaration
};
//other code here as before
class Entity{
public:
int hp;
int atk;
string name;
vector<Weapon> weapons;
vector<Armor> armors;
};
//implementation after Entity's definition
void Weapon::Attack(const Entity& target)
{
}
Working demo
You can't.
You must declare it earlier. You can, however, define it later, in some circumstances.
To forward declare a class, write this before it is used:
class Entity;
In c++, such code cannot compile:
class A {
void fooa(B) {}
};
class B {
void foob(A) {}
};
However, such code can be compiled, we can change code in the way:
class A { };
class B { };
void fooa(A *, B) {}
void foob(B *, A) {}
It works and nothing is recursive.
So, I don't think change to reference is a good idea. The directly way to do that is just to use some trick. For example, change Entity to auto. Like that: void Attack(auto target).
What's more, with c++20, you can define a concept attackable and make Entity is attackable, I like it much.

Why am I getting a "no matching function" error?

Pretty new to C++, I was given an assignment that is basically about utilizing two different classes, however when creating my header files and c++ files and attempting to compile I get an error that reads no matching function for call to 'Owner::Owner()'. Since I am not super familiar with C++ yet I am assuming that this issue has something to do with my constructors and the way I am trying to call them, my assignment details what I think my issue is about but I am having trouble understanding exactly what needs to be done. I will provide the assignment details on the issue, as well as the code and compilation error below. Sorry for all the information I've just been stuck with this issue for a while and I can't seem to figure out a solution.
Transcribed Error
In constructor 'Dog::Dog(std::__cxx11::string, int)':
Dog.cpp:23:46: error: no matching function for call to 'Owner::Owner()' Dog::Dog(std::string unsetBreed, int unsetAge){
Assignment Details
Now you will write a program that consists of two classes, a Dog class and an Owner class. Their specification is shown in the UML diagram below. Notice that in our design, every Dog has an Owner class member. Class Owner is immutable, as mentioned above. An immutable class is just a class whose members cannot be changed(mutated) after an object was instantiated. Therefore, the Owner class does not have any setter methods. The Owner’s class attributes must be set at the time of creation(in the Owner’s constructor). You will call Owner’s constructor from inside Dog’s constructor. Do not forget to do it inside each constructor in class Dog.
Dog.h File
#ifndef DOG_H_INCLUDED
#define DOG_H_INCLUDED
#include <iostream>
#include "Owner.h"
class Dog {
//-----------------------//
private:
std::string breed;
int age;
Owner owner;
static int dogCount;
//-----------------------//
public:
Dog();
Dog(std::string, int);
std::string getBreed();
int getAge();
void setBreed(std::string);
void setAge(int);
void printDogInfo();
int getDogCount();
};
#endif // DOG_H_INCLUDED
Owner.h File
#ifndef OWNER_H_INCLUDED
#define OWNER_H_INCLUDED
#include <iostream>
class Owner {
//-----------------------//
private:
std::string name;
int age;
//-----------------------//
public:
Owner(std::string, int);
std::string getName();
int getAge();
//-----------------------//
};
#endif // OWNER_H_INCLUDED
Dog.cpp File
#include <iostream>
#include "Owner.cpp"
#include "Owner.h"
#include "Dog.h"
//---------------SETTERS------------------//
void Dog::setBreed(std::string dogBreed){dogBreed = breed;}
void Dog::setAge(int dogAge){dogAge = age;}
//--------------GETTERS------------------//
std::string Dog::getBreed(){return breed;}
int Dog::getAge(){return age;}
int Dog::getDogCount(){return dogCount;}
//--------------OTHERS-------------------//
Dog::Dog(std::string unsetBreed, int unsetAge){
Owner::Owner(std::string unsetName, int unsetOwnerAge);
Dog::setBreed(unsetBreed);
Dog::setAge(unsetAge);
}
void Dog::printDogInfo(){
Dog::getBreed();
Dog::getAge();
}
Owner.cpp File
#include <iostream>
#include "Owner.h"
#include "Dog.h"
//--------------GETTERS------------------//
int Owner::getAge(){return age;}
std::string Owner::getName(){return name;}
//--------------OTHERS-------------------//
Owner::Owner(std::string unsetName, int unsetOwnerAge){
Owner::getName();
Owner::getAge();
}
The problem here is that you don't have a constructor that receives 0 parameters.
You have 2 options:
1 - Define other constructor:
//header file
class Owner {
...
public:
Owner(std::string, int);
Owner();
...
};
//cpp file
...
Owner::Owner(){
name = "Jhon Doe";
age = 18;
}
...
2 - Define default params:
//header file
class Owner {
...
public:
Owner(std::string unsetName = "John Doe", int unsetAge = 18);
...
};
//cpp file
...
Owner::Owner(std::string unsetName = "John Doe", int unsetAge = 18);
Owner::getName();
Owner::getAge();
}
...
Note: I don't know why you are calling getter in the constructors.
Looks like your Owner class dosen't have a default constructor try including Owner::Owner() constructor.

Class Pointer as a Data Member of Another Object

just getting an odd error and I'm not entirely sure as to why.
I have 4 files (two headers and two implementations). The issue is inside the headers:
The main file ONLY includes the Station.h, which is why Stations.h is included inside it.
Station.h
#ifndef STATION_H
#define STATION_H
#include "Stations.h"
#include <string>
enum PassType{student, adult};
class Station{
std::string station_name;
unsigned int student_passes;
unsigned int adult_passes;
public:
Station();
void set(const std::string&, unsigned, unsigned);
void update(PassType, int);
unsigned inStock(PassType) const;
const std::string& getName() const;
};
#endif
Stations.h
#ifndef STATIONS_H
#define STATIONS_H
#include "Station.h"
namespace w2{
class Stations{
Station *station;
public:
Stations(char *);
void update() const;
void restock() const;
void report() const;
~Stations();
};
}
#endif
It doesn't know what Station is. I'm getting the following error:
./Stations.h:9:2: error: unknown type name 'Station'; did you mean 'Stations'?
Station *station;
What exactly am I missing here?
You are #includeing Stations.h in Station.h. As a result, the compiler sees class Stations before class Station. In this case, it doesn't appear that Station requires Stations, so you can simply remove the include.
If Station did need to know about Stations, then you'd have to use a forward declaration in one of the headers or the other (and be careful not to use the forward-declared class in a way that required the full definition).
Don't forget to put a semicolon after you declare the Stations class:
class Stations {
Station *station;
};
U need to do the forward declaration.
Remove the #include "Stations.h" from the Station.h
#ifndef STATIONS_H
#define STATIONS_H
#include "Station.h"
namespace w2{
class Station;
class Stations{
Station *station;
public:
Stations(char *);
void update() const;
void restock() const;
void report() const;
~Stations();
};
}
#endif

forward declaration on c++ with a list inside

I have two clases Pet and Person
Here is the Person.h:
#ifndef PERSON_H
#define PERSON_H
#include <list>
class Pet;
class Person
{
public:
Person();
Person(const char* name);
Person(const Person& orig);
virtual ~Person();
bool adopt(Pet& newPet);
void feedPets();
private:
char* name_;
std::list<Pet> pets_;
};
#endif
And here is the Pet.h
#ifndef PET_H
#define PET_H
#include <list>
#include "Animal.h"
class Person;
class Pet : public Animal
{
public:
Pet();
Pet(const Pet& orig);
virtual ~Pet();
std::list<Pet> multiply(Pet& pet);
private:
std::string name_;
Person* owner_;
};
#endif
The problem that i have is this:
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/list.tcc:129: error: invalid use of undefined type `struct Pet'
Person.h:13: error: forward declaration of `struct Pet'
I fixed trying to put this std::list<Pet>* pets_; but when i tried to call list functions always have a link problem. My question is how a have to include a list inside a class that contains objects from another class.
The standard requires that, except where explicitly stated, you use complete types with the library templates. This basically inhibits your design (where each object maintains by value a list of the other type).
You can work around this by using [smart] pointers (either a pointer to the container or container of pointers).

Are the two forms of default class constructor equivalent?

When I try the two forms of constructor in classes of derivation hierarchy, the result turns out to be different. Could anybody tell me why? Below are the test code.
//Person.h
#ifndef PERSON_H_
#define PERSON_H_
#include<string>
using std::string;
class Person{
private:
string firstname;
string lastname;
public:
Person(const char *fn="NoName", const char *ln="NoName"); //A
Person(const string &fn, const string &ln);
virtual ~Person(){}
};
class Gunslinger:virtual public Person{
private:
int notchnum;
public:
Gunslinger(const char*f="unknown",const char*n="unknown",int not=0);//B
virtual ~Gunslinger(){}
};
class PokerPlayer:virtual public Person{
public:
PokerPlayer(const char*fn="unknown", const char*ln="unknown");//C;
virtual ~PokerPlayer(){}
};
class BadDude:public Gunslinger,public PokerPlayer{
public:
BadDude(const char*fn="unknown", const char*ln="unknown", int notc=0);//D
};
#endif
//PersonDefinition.cpp
#include"Person.h"
#include<iostream>
#include<cstdlib>
using std::cout;
using std::endl;
using std::cin;
Person::Person(const char*fn, const char*ln):firstname(fn),lastname(ln){
}
Person::Person(const string &fn,const string &ln):firstname(fn),lastname(ln){
}
Gunslinger::Gunslinger(const char*fn,const char*ln, int not):Person(fn,ln),notchnum(not){
}
PokerPlayer::PokerPlayer(const char*fn,const char*ln):Person(fn,ln){
}
BadDude::BadDude(const char*fn, const char*ln, int notc):Person(fn,ln),PokerPlayer(fn, ln),Gunslinger(fn,ln,notc){
}
//PersonTest.cpp
#include<iostream>
#include "Person.h"
int main(){
Person a("Jack","Husain");
PokerPlayer b("Johnson","William",8);
Gunslinger c("Mensly","Sim");
}
So, here is the problem. The program above fails to compile with the default constructor with default value for all argument and throws an error message saying that "expected ',' or '...' before '!' token", but if I replace the default constructor in Line A,B,C,D with the form without argument, the program compiles and run successfully.Could anyone tell me why? Below is the error message.
You did not implement all the constructors. For instance, you declared a constructor PokerPlayer::PokerPlayer(char*, char*) but you are trying to create a PokerPlayer with PokerPlayer b("Johnson","William",8); (i.e. you never declared a constructor which takes the third argument). The declaration you want for that previous line to work is PokerPlayer::PokerPlayer(char*, char*, int);
Furthermore, you have the exact opposite problem when trying to declare a GunSlinger. Your GunSlinger class requires the third parameter and you are trying to declare it without that argument.
Even though your base class supports several types of constructors, each derived class must also have every constructor you wish to use on it explicitly declared/implemented (with the exception of the default constructor).
EDIT
Here is some semi-functional code:
class PokerPlayer : public Person
{
...
PokerPlayer(char* fname, char* lname, int val);
...
};
Implementation
PokerPlayer::PokerPlayer(char* fname, char* lname, int val) : Person(fname, lname, val)
{
// Anything else we should do...
}