I don't understand pointers and references very well yet, but I have a class with static methods and variables that will be referenced from main and other classes. I have a variable defined in main() that I want to pass to a variable in this class with static functions. I want those functions to change the value of the variable that is seen in the main() scope.
This is an example of what I am trying to do, but I get compiler errors...
class foo
{
public:
static int *myPtr;
bool somfunction() {
*myPtr = 1;
return true;
}
};
int main()
{
int flag = 0;
foo::myPtr = &flag;
return 0;
}
Provide the definition of the static variable outside the class as:
//foo.h
class foo
{
public:
static int *myPtr; //its just a declaration, not a definition!
bool somfunction() {
*myPtr = 1;
//where is return statement?
}
}; //<------------- you also forgot the semicolon
/////////////////////////////////////////////////////////////////
//foo.cpp
#include "foo.h" //must include this!
int *foo::myPtr; //its a definition
Beside that, you also forgot the semicolon as indicated in the comment above, and somefunction needs to return a bool value.
#include <iostream>
using namespace std;
class foo
{
public:
static int *myPtr;
bool somfunction() {
*myPtr = 1;
return true;
}
};
//////////////////////////////////////////////////
int* foo::myPtr=new int(5); //You forgot to initialize a static data member
//////////////////////////////////////////////////
int main()
{
int flag = 0;
foo::myPtr = &flag;
return 0;
}
Related
Here's the code:
#include <iostream>
using namespace std;
class Zaix
{
private:
static int mor;
public:
static int beri;
static void setmor(int lip)
{
Zaix::mor=lip;
}
static int getmor(void)
{
return mor;
}
};
int Zaix::beri=3;
int main()
{
cout<<Zaix::beri<<endl;
Zaix::beri++;
cout<<Zaix::beri<<endl;
Zaix::setmor(6);
return 0;
}
Now, line 4 of main() function Zaix::setmor(6); somehow invalidates line 11 of the code presented Zaix::mor=lip;. With this line commented out, the whole thing compiles OK, with it present, compiler gives this error:
undefined reference to Zaix::mor"
Any idea why that is?
Define the variable outside class as well.
int Zaix::mor;
For assignment:
int Zaix::mor = 4;
In C++ we need to define all the static member variable of a class outside of it else we get a linking error. You just need to do like below:-
int Zaix::mor;// Just add this line below int Zaix::beri = 3;
I am struggling knowing how to create a class. I want to create a "Player" class and all I want to do is pass in the name while I'll have the other variables start at 0 until they are updated when a game is run (later in the program)
Player::Player(string name_in)
{
name = name_in;
int numOfWins = 0;
int numOfLoses = 0;
int numOfDraws = 0;
int totalMatches = 0;
}
Right now there are lots of errors around numOfWins, numOfLoses, numOfDraws and totalMatches. What can I do to fix this?
Perhaps the error is in your int ... part of assignments, which essentially creates a new local variable in a constructor.
Try this version:
#include <string>
using namespace std;
class Player
{
string name;
int numOfWins;
int numOfLoses;
int numOfDraws;
int totalMatches;
public:
Player(string name_in)
{
name = name_in;
numOfWins = 0;
numOfLoses = 0;
numOfDraws = 0;
totalMatches = 0;
}
};
You should declare other instance variables in the class declaration, rather than declaring them as locals (which is completely useless).
// This part goes in the header
class Player {
string name;
int numOfWins;
int numOfLoses;
int numOfDraws;
int totalMatches;
public:
Player(string name_in);
};
Now in the constructor you could use initialization lists:
// This part goes into the CPP file
Player::Player(string name_in)
// Initialization list precedes the body of the constructor
: name(name_in), numOfWins(0), numOfLoses(0), numOfDraws(0), totalMatches(0) {
// In this case, the body of the constructor is empty;
// there are no local variable declarations here.
}
Kinda vague, but I'll take a crack at it. You Probably want:
class Player{
string name;
int numOfWins;
int numOfLosses;
int numOfDraws;
int totalMatches;
Player(string name_in)
};
Player::Player(string name_in){
name = name_in;
numOfWins = 0;
numOfLosses = 0;
numOfDraws = 0;
totalMatches = 0;
}
Haven't used C++ in a while, so this may be faulty.
The errors you get, at least from the snippet you posted are caused for you can't declare variables in constructor - you declare them in class body and initialize in constructor or using another function.
#include <string>
class Player {
public:
Player( std::string const& name_in) : name( name_in),
numOfWins(), numOfLoses(),
numOfDraws(), totalMatches()
{} // constructor
// will initialize variables
// numOfWins() means default
// initialization of an integer
private:
std::string name;
int numOfWins;
int numOfLoses;
int numOfDraws;
int totalMatches;
};
usage:
int main() {
Player( "player_one");
return 0;
}
I have a simple class containing a static attribute. There are two static methods in this class: one to get the static attribute and the other to initialize it. Yet when call the static method the compiler reports an error.
The class:
class Sudoku {
Cell Grid[9][9];
int CurrentLine;
int CurrentColumn;
void deleteValInColumn(int val, int col);
void deleteValInRow(int val, int row);
void deleteValInBox(int val, int x, int y);
static int unsetted; //!
public:
static void IniUnsetted() { //!
unsetted = 0;
}
static int GetUns() { //!
return unsetted;
}
Sudoku(ini InitGrid[9][9]);
void Calculate_Prob_Values();
Cell getCell(int x, int y);
QVector<int> getPossibleValues(int x, int y);
bool SolveIt();
};
This is the error I get:
In member function 'bool Sudoku::SolveIt()':
no return statement in function returning non-void [-Wreturn-type]
In function `ZN6Sudoku6GetUnsEv':
undefined reference to `Sudoku::unsetted` error: ld returned 1 exit status
You will need to define the static variable, even if it is not initialized explicitly. That is what is missing in your code. You should have provided a simple example to reproduce the issue, but for your convenience I am providing one which works.
main.cpp
class Foo {
public:
static int si;
static void bar();
};
int Foo::si = 0; // By default, it will be initialized to zero though.
void Foo::bar() {
Foo::si = 10;
};
int main()
{
Foo::bar();
return 0;
}
Note: I would suggest to get someone to review your code because "unsetted" is incorrect English. If we are at it, you would probably need to fix your indentation as well.
In your code there is no definition of unsetted, there is only declaration.
The solution is to put somewhere in your cpp file a line like this:
int Sudoku::unsetted
The reason for that is that each instantiation of Sudoku class will use the same unsetted member so it cannot be defined for each of them, so it's up to programmer to define it in one place only.
In your cpp file, define the static variable (ideally with an initialization):
int Sudoku::unsetted = 0;
If you are declaring any static variable in class, then you should define that variable outside the class also.
Example:
class A
{
public:
static int x; // declaration
};
int A::x; // definition
I have written a class as shown below:
#include<iostream>
using namespace std;
class A
{
static int cnt;
static void inc()
{
cnt++;
}
int a;
public:
A(){ inc(); }
};
int main()
{
A d;
return 0;
}
I want to call the function inc through the constructor, but when i compile i am getting an error as:
/tmp/ccWR1moH.o: In function `A::inc()':
s.cpp:(.text._ZN1A3incEv[A::inc()]+0x6): undefined reference to `A::cnt'
s.cpp:(.text._ZN1A3incEv[A::inc()]+0xf): undefined reference to `A::cnt'
I am unable to understand what the error is... plz help...
Static field is not defined - Take a look at Why are classes with static data members getting linker errors?.
#include<iostream>
using namespace std;
class A
{
static int cnt;
static void inc(){
cnt++;
}
int a;
public:
A(){ inc(); }
};
int A::cnt; //<---- HERE
int main()
{
A d;
return 0;
}
Inside the class static int cnt; is only declared, and need to be defined. In C++ you usually declare in your .h .hpp files and then define your static class members in your .c and .cpp files.
In your case, you need to add
int A::cnt=0; // = 0 Would be better, otherwise you're accessing an uninitialized variable.
I don't know how to initalize a static class member without creating an object of this class.
Here is my code:
namespace {
class CPassant : public thread
{
private:
static unsigned LastID;
public:
CPassant (unsigned pDelaiArr = 0, unsigned pDelaiDep = 0)
{
(blabla)
}
static void setLastID (unsigned Valeur)
{
LastID = Valeur;
/* error : undefined reference to `(anonymous
namespace)::CPassant::LastID' */
} // setLastID ()
}; // class CPassant
} // anonym namespace
int main ()
{
CPassant::CPassant ().setLastID(0);
// doesn't work too:
// unsigned CPassant::LastID = 0;
return 0;
}
Thanks
NB: I've already looked at those answers, but none of them worked:
stackoverflow.com/ initialize-a-static-member-an-array-in-c
stackoverflow.com/ how-to-initialize-a-static-member
Do this in your cpp file:
unsigned CPassant::LastID = 0;
This is called defining the static class member, If you dont do this you will end up getting linker errors. You just declared the static member but did not define it.
Note that access specifiers do not matter here while defining the static member.
You have declared, but not defined, the static member. You must define it. Here is one way:
namespace {
/* ... */
unsigned CPassant::LastID = 0;
}; // anonym namespace
you have to do
unsigned CPassant::LastID = 0;
in the .cpp file..
The problem with your initialization of LastID is that it's outside of the namespace you declared it. Put it in the same namespace and it will work.