I am very new to pointers, so I have no idea what is going on with them.
I am trying to get a master class pass a pointer of itself to its worker/s, and I have no idea why it doesn't work.
#include <iostream>
#include <Windows.h>
using namespace std;
class BigOne {
public:
LittleOne workers[1] = {LittleOne(this)};
int increase = 0;
};
class LittleOne {
BigOne* master;
public:
LittleOne(BigOne*);
void Increment();
};
LittleOne::LittleOne(BigOne* upOne) {
master = upOne;
}
void LittleOne::Increment() {
master->increase++;
}
BigOne Outer;
int main() {
cout << Outer.increase << endl;
Outer.worker.Increment();
cout << Outer.increase << endl;
system("PAUSE");
}
This is my problem boiled down to its core components.
The problem isn't with pointers. It's mostly here:
class BigOne {
public:
LittleOne workers[1] = {LittleOne(this)};
int increase = 0;
};
When you define workers, what is LittleOne? How is it laid out in memory? How is it initialized? The compiler can't know, it hasn't seen the class definition yet. So you must flip the definitions around:
class BigOne;
class LittleOne {
BigOne* master;
public:
LittleOne(BigOne*);
void Increment();
};
class BigOne {
public:
LittleOne workers[1] = {LittleOne(this)};
int increase = 0;
};
The forward declaration allows us to define members that accept and return pointers. So the class LittleOne can have its definition written before BigOne. And now BigOne can define members of type LittleOne by value.
The issue is with forward declaration.
You can refer the following link for more details and explanation.
What are forward declarations in C++?
Related
How can such a code work correctly when the IWindow pointer clearly has an address to a ISheet class which has no method Say?
#include <iostream>
using namespace std;
class IWindow
{
private:
int p;
double f;
public:
void Say() { cout << "Say in IWindow"; }
};
class ISheet
{
public:
void foo() { cout << "ISheet::foo"; }
};
int main()
{
ISheet *sh = new ISheet();
int ptr = (int)sh;
IWindow *w = (IWindow*)ptr;
w->Say();
sh->foo();
return 0;
}
When compiled in Visual Studio 2015 it runs and executes with no problems, but I was expecting to get an error on line w->Say(). How is this possible?
It works by the grace of the almighty Undefined Behavior. Your functions don't try to access any data members of the containing class, they just write something to std::cout, which anyone can do.
What you've effectively done is
#include <iostream>
void IWindow_Say(void*)
{
std::cout << "Say in IWindow";
}
int main()
{
IWindow_Say(0xdeadbeef); // good luck with that pointer
}
You never used the pointer (which became this in your original example) so no side-effects were observed.
I wrote a small piece of code to test friend functions. It worked fine for methods that didn't belong to a specific class but when I tried to put it into a class all it can access is the public variables (just as any object would).
#include <iostream>
#include <conio.h>
using namespace std;
class something{
int ip = 100;
public:
int x = 100;
void getIP();
friend void cIP::changeIP(something);
};
void something::getIP(){
cout << ip << endl;
}
class cIP{
public:
int i;
cIP();
cIP(int nIP);
something some;
void changeIP(something s);
};
cIP::cIP(){
i = 100;
}
cIP::cIP(int nIP){
i = nIP;
}
void cIP::changeIP(something s){
s.ip = i;
}
s.ip brings up the error member is inaccessible.
I have made a map of functions. all these functions are void and receive single string parameter.
code:
void f1(string params){...}
void f2(string params){...}
void f3(string params){...}
map<string , void*> funcMap;
funcMap["f1"] =(void*)&f1;
funcMap["f2"] =(void*)&f2;
funcMap["f3"] =(void*)&f3;
how do i call a function?
I tried the next code, but id doesn't work:
void (*func)(string) = &funcMap[commandType];
func(commandParam);
I get this error message:
Server.cpp:160:46: error: cannot convert ‘void**’ to ‘void (*)(std::string) {aka void (*)(std::basic_string<char>)}’ in initialization
using pfunc = void (*)(string);
map<string, pfunc> funcMap;
funcMap["f1"] = f1; //and so forth
And then call:
pfunc f = funcMap[commandType];
(*f)(commandParam);
In general, why throw away type safety? If it's a map of function pointers, declare it to be one.
Why not just have those as separate classes.
Then have the methods as virtual.
You can then have a map between the string and the base class.
i.e.
class Someoperation
{
virtual void Doit() = 0;
};
map<string, Someopertion> ops;
Then
class MyOp : public Someoperation
{
void Doit() { /* Some code here */}
};
Just add objects
ops["Hello"] = MyOp();
then call it
ops["Hello"].Doit();
&funcMap[commandType]
Just drop the &. Your compile error was useful here. It had a void** on the right which is because you took the address of a function pointer. You don't want two levels of indirection there.
Try C++ style. It has overhead for allocation and inheritance, but it's more flexible and extensible if you'll need some more functionality in the future.
#include <iostream>
#include <string>
#include <unordered_map>
#include <memory>
using namespace std;
class Someoperation {
public:
virtual void Doit() = 0;
};
class MyOp1 : public Someoperation {
public:
void Doit() final { cout << "MyOp1" << endl; }
};
class MyOp2 : public Someoperation {
public:
void Doit() final { cout << "MyOp2" << endl; }
};
int main() {
unordered_map<string, unique_ptr<Someoperation> > ops;
ops["1"] = unique_ptr<Someoperation>(new MyOp1);
ops["2"] = unique_ptr<Someoperation>(new MyOp2);
ops["1"]->Doit(); // Out: MyOp1
ops["2"]->Doit(); // Out: MyOp2
return 0;
}
#include <iostream>
using namespace std;
class B
{
public:
int getMsg(int i)
{
return i + 1;
}
};
class A
{
B b;
public:
void run()
{
taunt(b.getMsg);
}
void taunt(int (*msg)(int))
{
cout << (*msg)(1) << endl;
}
};
int main()
{
A a;
a.run();
}
The above code has a class B inside a class A, and class A has a method taunt that takes a function as an argument. class B's getMsg is passed into taunt...The above code generated the following error message: "error: no matching function for call to 'A::taunt()'"
What's causing the error message in the above code? Am I missing something?
Update:
#include <iostream>
using namespace std;
class B
{
public:
int getMsg(int i)
{
return i + 1;
}
};
class A
{
B b;
public:
void run()
{
taunt(b.getMsg);
}
void taunt(int (B::*msg)(int))
{
cout << (*msg)(1) << endl;
}
};
int main()
{
A a;
a.run();
}
t.cpp: In member function 'void A::run()':
Line 19: error: no matching function for call to 'A::taunt()'
compilation terminated due to -Wfatal-errors.
I'm still getting the same error after changing (*msg)(int) to (B::*msg)(int)
b.getMsg is not the correct way to form a pointer to member, you need &B::getMsg.
(*msg)(1) is not the correct way to call a function through a pointer to member you need to specify an object to call the function on, e.g. (using a temporary) (B().*msg)(1).
The right way to do such things in OOP is to use interfaces so all you need to do is to define an interface and implement it in B class after that pass the pointer of instance which implements this interface to your method in class A.
class IB{
public:
virtual void doSomething()=0;
};
class B: public IB{
public:
virtual void doSomething(){...}
};
class A{
public:
void doSomethingWithB(IB* b){b->doSomething();}
};
This works in VS 2010. The output is the same on all lines:
#include <iostream>
#include <memory>
#include <functional>
using namespace std;
using namespace std::placeholders;
class A
{
public:
int foo(int a, float b)
{
return int(a*b);
}
};
int main(int argc, char* argv[])
{
A temp;
int x = 5;
float y = 3.5;
auto a = std::mem_fn(&A::foo);
cout << a(&temp, x, y) << endl;
auto b = std::bind(a, &temp, x, y);
cout << b() << endl;
auto c = std::bind(std::mem_fn(&A::foo), &temp, _1, y);
cout << c(5) << endl;
}
Basically, you use std::mem_fn to get your callable object for the member function, and then std::bind if you want to bind additional parameters, including the object pointer itself. I'm pretty sure there's a way to use std::ref to encapsulate a reference to the object too if you'd prefer that. I also included the _1 forwarding marker just for another way to specify some parameters in the bind, but not others. You could even specify everything BUT the class instance if you wanted the same parameters to everything but have it work on different objects. Up to you.
If you'd rather use boost::bind it recognizes member functions and you can just put it all on one line a bit to be a bit shorter: auto e = boost::bind(&A::foo, &temp, x, y) but obviously it's not much more to use completely std C++11 calls either.
I am using inheritance for my code. ChangeRequest is my base class. The code is as follows.
ChangeRequest.h
#include <iostream>
#include <string>
using namespace std;
class ChangeRequest
{
int swapDay;
int newDay;
public:
void setSwapDay(int newDay);
int getSwapDay();
void setNewDay(int newDay);
int getNewDay();
};
ChangeRequest.cpp
#include "ChangeDutyRequest.h"
void ChangeRequest::setSwapDay(int newDay)
{
swapDay = newDay;
}
int ChangeRequest::getSwapDay()
{
return swapDay;
}
void ChangeRequest::setNewDay(int day)
{
newDay = day;
}
int ChangeRequest::getNewDay()
{
return newDay;
}
The code below is for the derived class. SwapDuty
SwapDuty.h
#include <iostream>
#include <string>
#include "ChangeRequest.h"
using namespace std;
class SwapDuty: public ChangeRequest
{
string requester;
public:
void setRequester(string member);
string getRequester();
};
SwapDuty.cpp
#include "SwapDuty.h"
void SwapDuty::setRequester(string member)
{
requester = member;
}
string SwapDuty::getRequester()
{
return requester;
}
when I compile and access the requester attribute using getRequester(). I get the following error.
'class ChangeRequest' has no member named 'getRequester'
This is how I used my code
Can someone please tell me what I am doing wrong? Thanks in advance
SwapDuty newSwapDutyRequest;
for(int i = 0; i < tempList.size(); i++ )
{
if(tempList[i].getPersonToPerform().getName() == loginMember)
{
newSwapDutyRequest.setRequester(loginMember);
newSwapDutyRequest.setSwapDay(swapDay);
newSwapDutyRequest.setNewDutyDay(daySwapWith);
break;
}
}
changeList.push_back(newSwapDutyRequest);
cout << changeList[1].getRequester() << endl;
What is type of changeList?
Although you have created an object of the derived class, I suspect that you are pushing it into the container of the base class. Possibly you are getting few warnings before this error as well, because of pushing derived object into the container of type base.
If you want to make a container of base class and push in the derived class objects you need to work with the pointers to the objects not the objects themselves.
Where is your class ChangeDutyRequest? You don't show it. Maybe you forgot to inherit it from the correct base or invoke it incorrectly?
Show fuller code sample