The class called Universes uses a data member of type States, whilst States uses an object of type Universes. I'm using Visual C++ 2010 Express (if that makes any difference).
States.h:
class Universes;
extern Universes universe;
class States
{
public:
int relations;
States();
};
States::States()
{
relations = universe.state_no;
}
Universes.h
#include "States.h"
class Universes
{
public:
States state;
int state_no;
};
Test.cpp
#include "stdafx.h"
#include <iostream>
#include <conio.h>
#include "Universes.h"
using namespace System;
int main(array<System::String ^> ^args)
{
Universes universe;
_getch();
return 0;
}
I keep getting the following errors:
States.h(16): error C2027: use of undefined type 'Universes'
States.h(1) : see declaration of 'Universes'
States.h(16): error C2228: left of '.state_no' must have class/struct/union
At the point where you try to access universe.state_no, the Universes class is incomplete (it's forward-declared).
A clean way to fix this is to move the definition of States::States into States.cpp, and make sure States.cpp #includes Universes.h.
Related
I'm trying to defined an external constructor when porting a class to python, by using make_constructor absolutely fails. When I try:
#include <boost/python/numpy.hpp>
using boost::python;
class foo
{
int i;
public:
foo(int i) : i(i){}
};
foo foo_create(int i){return foo(i);}
BOOST_PYTHON_MODULE(bar)
{
class_<foo>("foo")
.def("__init__", make_constructor(&foo_create));
}
I get the following error
error: no type named ‘element_type’ in ‘class foo’
I tried using noinit and init() with the same result. What am I doing wrong?
Awe found the problem, part of it being the really sparse documentation on make_construction. I needed to return a ptr to a new instance like so (in this case I made them shared pointers):
#include <boost/python/numpy.hpp>
#include <memory>
using boost::python;
class foo
{
int i;
public:
foo(int i) : i(i){}
};
std::shared_ptr<foo> foo_create(int i){return std::shared_ptr<foo>(foo(i));}
BOOST_PYTHON_MODULE(bar)
{
class_<foo, std::shared_ptr<foo>>("foo")
.def("__init__", make_constructor(&foo_create));
}
The documentation on make_constructor is really sparse, but there is some discussion here: https://wiki.python.org/moin/boost.python/HowTo under point "9".
Iam confused with the below question I did the program as per my understanding but it crashes what am I doing wrong? If someone can please assist me it would be much appreciated.
my main.cpp looks like this:
#include <iostream>
#include <iomanip>
#include "Number.h"
using namespace std;
int main()
{
Number n1(10);
Number n2 = n1;
n2.printNum();
n2.addOne();
n1 = n2;
n1.printNum();
return 0;
}
Then my header file looks like this:
#include <iostream>
using namespace std;
class Number
{
int *p;
public:
Number(int);
void addOne();
void printNum();
};
And the below parts for the constructor I need to complete there where it shows comments that's the part I should complete:
#include <iostream>
#include "Number.h"
using namespace std;
Number::Number(int a1)
{
*p = a1;//write the code needed to initialise the value of the member variable with a1
}
void Number::printNum()
{
cout << "The number is " << *p << endl;
}
void Number::addOne()
{
*p++;//write the code needed to increment the value of the member variable by one.
}
Then the question asks the below what should I do to the code to use the BIG THREE?
Consider the following program. Complete the class definition (where you are asked to) and check the output. You can see that that program works without error once it is completed. However, experts suggest that in any class that uses pointers and the new operator it is better to follow the rule of The Big Three. Modify the class definition to follow the rule of The Big Three and submit the new program and the output. Demonstrate the use of this pointer.
Thank you
Rohan
In my one c++ file named "two_elements.cpp", I am defining two classes within same file, and the code goes like below:
#include <iostream>
#include <cstream>
class A{
public:
void methodA(){
//do something
}
};
class B{
public:
A a_obj;
a_obj.methodA();
};
This throws the following error:
error: unknown type name 'a_obj'
I folowed the following stack overflow links comprehensively, but could not find a work around:
Unknown type name class
unknown type name 'class'
C++ - 2 classes 1 file
Please note most of these questions had classes in the individual header files.
I found the solution:
#include <iostream>
#include <cstream>
class A{
public:
void methodA(){
//do something
}
};
class B{
public:
A a_obj;
B(){
a_obj.methodA();
}
};
An instance needs to be called inside a constructor or a function in c++.
I am trying to get acquainted with boost thread and signals. I have therefore implemented this very simple code consisting of a class (Class1) implementing a thread. I'd like this class to provide services as result of signals reception. To this end I have just started to exploit the signal boost library but I am getting this error:
/home/andrea/libs/boost_1_50_0/boost/bind/mem_fn.hpp:359:22: error: invalid use of non-static member function
when I try to compile it in the Eclipse environment with gcc. Is there anything wrong with the singleton or is the binding to the instance method?
Here is Class1.cpp
#include "Class1.hpp"
#include <boost/thread.hpp>
#include <boost/date_time.hpp>
#include "Package1.hpp"
Class1::Class1(){
boost::thread thread(boost::bind(&Class1::classifierBehavior,this));
};
void Class1::classifierBehavior(){
service.run();
Package1Signals::getInstance()->signal1.connect(boost::bind(&Class1::method1, boost::ref(*this)));
};
void Class1::method1(Signal1 signal1){}
And Package1.hpp
#ifndef PACKAGE1_HEADER
#define PACKAGE1_HEADER
#include <boost/signal.hpp>
struct Signal1{
int foo;
};
class Package1Signals{
private:
Package1Signals();
static Package1Signals * instance;
public:
boost::signal<void (Signal1)> signal1;
static Package1Signals * getInstance(){
if(!instance){
instance = new Package1Signals();
}
return instance;
};
};
#endif
Your binder should have 1 argument:
boost::bind(&Class1::methpod1, boost::ref(*this), _1)
I'm working on a plug-in protocol of sorts. The idea is that there's a base clase PCOperatorBase and that plugins would subclass this base class to provide specific functionality through a "process" virtual method that they override. The subclasses also should have a static struct member that hold typical plugin info: plug name, type and subtype. This info is used by another class (a PCOperatorManager) to know what types of plugins is dealing with. And I thought about having it be a static member so that there's no need to instantiate a plug to merely find out about the type of operator it is.
I have the following classes:
PCOperatorBase.h the superclass from which to derive all other plugs:
#ifdef PCOPERATORBASE_EXPORTS
#define PCOPERATORBASE_API __declspec(dllexport)
#else
#define PCOPERATORBASE_API __declspec(dllimport)
#endif
class PCOPERATORBASE_API PCOperatorBase
{
public:
typedef struct OperatorInfo
{
wchar_t* name;
wchar_t* type;
wchar_t* subtype;
} OperatorInfo;
PCOperatorBase(void);
virtual ~PCOperatorBase(void);
virtual int process(int* inBuffer, int* outBuffer, int bufferSize);
};
And then, for instance, a subclass:
BlackNWhite.h: a RGB->black / white operator -- yes, these are going to be graphics plugs, and disregard the types for the in / out buffers.. they are merely placeholders at this point.
#ifdef BLACKNWHITE_EXPORTS
#define BLACKNWHITE_API __declspec(dllexport)
#else
#define BLACKNWHITE_API __declspec(dllimport)
#endif
// This class is exported from the BlackNWhite.dll
class BLACKNWHITE_API CBlackNWhite : PCOperatorBase
{
public:
static PCOperatorBase::OperatorInfo* operatorInfo;
CBlackNWhite(void);
virtual ~CBlackNWhite(void);
//virtual int process(int* inBuffer, int* outBuffer, int bufferSize);
};
BLACKNWHITE_API CBlackNWhite* getOperatorInstance();
BLACKNWHITE_API const PCOperatorBase::OperatorInfo* getOperatorInfo();
And here the implementation file:
BlackNWhite.cpp:
#include "stdafx.h"
#include "BlackNWhite.h"
BLACKNWHITE_API CBlackNWhite* getOperatorInstance()
{
return new CBlackNWhite();
}
BLACKNWHITE_API const PCOperatorBase::OperatorInfo* getOperatorInfo()
{
return CBlackNWhite::operatorInfo;
}
CBlackNWhite::CBlackNWhite()
{
}
CBlackNWhite::~CBlackNWhite()
{
}
Now, I've tried a few approaches but I can't get the DLL to compile, because of the static member.The linker throws:
\BlackNWhite.lib and object c:\Projects\BlackNWhite\Debug\BlackNWhite.exp
1>BlackNWhite.obj : error LNK2001: unresolved external symbol "public: static struct PCOperatorBase::OperatorInfo * CBlackNWhite::operatorInfo" (?operatorInfo#CBlackNWhite##2PAUOperatorInfo#PCOperatorBase##A)
1>c:\Projects\BlackNWhite\Debug\BlackNWhite.dll : fatal error LNK1120: 1 unresolved externals
I thought that since the struct is defined inside the base class and the base class is exporting, the struct would export too. But I guess I'm wrong?
So how should I be doing it?
And regardless, is there a better approach to having the plugs' factory export their name,type and subtype without the need for instantiation than a static class member? For example, a resource file? or even another extern "C" function could return this info.. But I just felt that since it's C++ it makes the most sense to encapsulate this data (which is about the class as a factory itself) within the class, whether through a member or a method.
It doesn't have anything to do with DLLs, you declared the static member but you forgot to define it. Add this line to BlackNWhite.cpp:
PCOperatorBase::OperatorInfo* CBlackNWhite::operatorInfo = NULL;