Well guys, I've been looking for an answer to this error but I haven't got a specific one for my case.
I have a class User, each User has its own list of Computers, the class Computer is composed by these three classes ( Operative Sistem, Memory, and Processor). So Computer has its own toString that calls the specific toString from its components named above.
So...User has his atribute list computerList;
In other class, that I called Controler, I have a function for printing the computer list from a specific user.
Here is my function:
void printComputerList(User* u){
list<Computer*>::iterator itr;
for(itr=u->getComputerList().begin(); itr!=u->getComputerList().end(); itr++){
cout<<(*itr)->toString(); //(*itr) calls its own toString implemented in the class Computer
}
}
So, when I'm running the program, when I choose to print the list that I've already filled
I get the error from the title.
I think it is maybe some kind of confusion between the totrings ?
PD: I can post the rest of the code if it is necesary
Thanks!
There is (at least) one problem with temporary list. Fixed version would look like:
void printComputerList(User* u){
list<Computer*> const computers = u->getComputerList();
list<Computer*>::const_iterator it = computers.begin();
while (it != computers.end())
{
cout << (*it)->toString(); //(*it) calls its own toString implemented in the class Computer
++it;
}
}
Are you sure, that pointers on list are valid (non-null, not dangling ones)?
Related
I have a vector of (pointers to) objects - people. I have a function that adds income to a person. The problem I'm having is to both find a person and accessing that addInc. I still sometimes get confused with pointers/references and more importantly I'm new to OOP. The relevant function is:
bool Population::Income(const string &id, unsigned int amount) {
Person *Candidate = new Person(id);
//find company using lower_bound
iterPeople = lower_bound(m_People.begin(), m_People.end(), Candidate, cmpId);
if ( iterPeople != m_People.end() && (*iterPeople)->m_id == id ) {
*(iterPeople)->addInc(amount);
//request for member 'addInv' in...maybe you meant to use '->'?
delete Candidate;
return true;
}
delete Candidate;
return false;
The rest of the code is HERE. I have two questions:
How do I solve the addInc issue?
About the lower_bound search - that method should be fine with NewPersonor CancelPerson but Income is gonna get called A LOT. Is that method sufficiently quick? Any way to make it more efficient?
BONUS - with addInc also comes MedianNetworth which returns median of all successfully added Incomes. The efficient way to use this is to create two heaps (min and max). My initial plan was to make_heap in the Population class:
make_heap(m_Audits.begin(), m_Audits.end(), cmpInt);
however I cannot make a heap inside the class because of unexpected '(' token - the very same syntax works in main() or inside any function. What am I doing wrong? Obviously I don't want to create heaps inside functions since I would have to create a new heap whenever I wanted to add an entry.
This is a snippet of an open source code. Full source code is available https://github.com/gec/dnp3/blob/master/src/opendnp3/DNP3/ResponseContext.h
ObjectWriteIterator owi = arAPDU.WriteContiguous(apObject, start,stop);
for(size_t i = start; i <= stop; ++i) {
if(owi.IsEnd()) { // out of space in the fragment
this->mStaticWriteMap[arKey] =
boost::bind(&ResponseContext::WriteStaticObjects<T>, this, apObject,
arStart, arStop, arKey, _1); return false;
}
apObject->Write(*owi, arStart->mValue);
++arStart; //increment the iterators
++owi;
}
ObjectWriteIterator::ObjectWriteIterator() :
mpPos(NULL),
mIndex(1),
mStart(0),
mStop(0),
mObjectSize(0)
{}
My question is: I don't understand is where *owi is referring in this context.
owi is an iterator, which is a 'standard' C++ interface for iterating over some collection.
The interface has them use pointer-symantics, so the * operator 'dereferences' the iterator and returns a reference to the value it currently 'points' to, and incrementing it via ++ moves it to the next item in the collection being iterated over.
In this case, it looks like a collection of ObjectWrite objects inside the collection specified by apObject between start and stop (start and stop are also typically defined by iterators set to some location in the collection).
sorry, I was earlier not sure about one can build a self contained 'Mock up' iterator class which use hidden is the header file
inline boost::uint8_t* ObjectWriteIterator::operator*() const
{
if(this->IsEnd()) throw InvalidStateException(LOCATION, "End of
iteration");
return mpPos;
}
in the header file. Sorry for wild goose run. Thanks for the prompt reply and I learned something new about the the core implementation of the iterator as well.
I have a data structure defined up here called this:
typedef list <classSpec*> ClassSpecList;
I'm trying to add stuff into the list here based on functions that return certain values of that match the same data type. In one function, I have a list pointer object defined here and I have another statement that calls a function.
ClassSpecList *answer = 0;
classSpec *thisanswer = parseClass(br);
Basically I'm trying to add the results of what thisanswer returns into my main ClassSpecList. Problem is, when I try
answer->push_back(new classSpec (*thisanswer));
It compiles but I get a seg fault
When I try somethign else like:
answer->insert(ClassSpecList.begin(), *thisanswer);
I keep getting primary expression errors and I do not know why. I even tried it with other list made without typedef and I still get those.
Thank you.
You should initialize the pointer answer first, like :
ClassSpecList *answer = new ClassSpecList;
then you can add thisAnswer into this list.
This should work:
ClassSpecList *answer = new ClassSpecList;
answer->push_back(thisAnswer);
as should this, which is usually recommended:
ClassSpecList answer;
answer.push_back(thisAnswer);
If possible, parseClass shouldn't return a pointer, and you should use typedef list <classSpec> ClassSpecList;.
I'm currently learning C++ and practicing my Knowledge by implementing an simple AddressBook Application. I started with an Entry class and an AddressBook class which implements a STL Map to access the entries by the last names of the persons. Now I arrived at the following code:
Entry AddressBook::get_by_last_name(string last_name){
if(this->addr_map.count(last_name) != 0){
//What can I do here?
} else {
return addr_map[last_name];
}
In Scripting Languages I would just return something like -1, Error Message(A List in Python) to indicate that the Function failed. I don't want throw an exception, because it's part of the application logic. The Calling Class should be able to react to the request by printing something on the console or opening a Message Box. Now I thought about implementing the Scripting Languae Approach in C++ by introducing some kind of an Invalid State to the Class Entry. But isn't that bad practice in C++? Could it be that my whole class design is just not appropriate? I appreciate any help. Please keep in mind that I'm still learning C++.
Some quick notes about your code:
if(this->addr_map.count(last_name) != 0){
//What can I do here?
You probably wanted it the other way:
if(this->addr_map.count(last_name) == 0){
//handle error
But your real problem lies here:
return addr_map[last_name];
Two things to note here:
The operator[] for map can do 2 things: If the element exists, it returns it; If the element doesn't exist, it creaets a new (key,value) pair with the specified key and value's default constructor. Probably not what you wanted. However, if your if statement from before would have been the right way, then the latter would never happen because we would knowthe key exists before hand.
In calling count() before, you effectively tell map to try and find the element. By calling operator[], you are telling map to find it again. So, you're doing twice the work to retrieve a single value.
A better (faster) way to do this involves iterators, and the find method:
YourMap::iterator it = addr_map.find(last_name); //find the element (once)
if (it == addr_map.end()) //element not found
{
//handle error
}
return *it.second; //return element
Now, back to the problem at hand. What to do if last_name is not found?
As other answers noted:
Simplest solution would be to return a pointer (NULL if not found)
Use boost::optional.
Simply return the YourMap::iterator but it seems that you are trying to "hide" the map from the user of AddressBook so that's probably a bad idea.
throw an exception. But wait, now you'll have to first check that calling this method is 'safe' (or handle the exception when appropriate). This check requires a boolean method like lastNameExists which would have to be called before calling get_by_last_name. Of course then we'er back to square 1. We're performing 2 find operations to retrieve a single value. It's safe, but if you're doing A LOT of calls to get_by_last_name then this is potentially a good place to optimize with a different solution (besides, arguably the exception is not very constructive: What's wrong with searching for something that isn't there, huh?).
Create a dummy member for Entryindicating that is not a real Entry but that is very poor design (unmanageable, counter intuitive, wasteful - you name it).
As you can see, the first 2 solutions are by far preferable.
One dead-simple option is to change the return type to Entry* (or const Entry*) and then return either the address of the Entry if found, or NULL if not.
If you use Boost, you could return a boost::optional<Entry>, in which case your success code would be the same, but on not-found you'd say return boost::none. This is fancier, but does about the same thing as using a pointer return type.
Throwing an exception is definitely the 'correct' C++ thing to do, based on your function return type.
You might want a function like this to help you, though:
bool AddressBook::lastNameExists(const string &last_name)
{
return addr_map.count(last_name) > 0;
}
Note that your current code returns the entry 'by value' so modifying the returned entry won't update the map. Not sure if this is by accident or design...
Other answers have given various approaches, most of them valid. I didn't see this one yet:
You could add a second parameter with a default value:
Entry AddressBook::get_by_last_name(string last_name, const Entry& default_value){
if(this->addr_map.count(last_name) == 0){
return default_value;
} else {
return addr_map[last_name];
}
In this particular instance, there might not be a sensible default value for a non-existing last name, but in many situations there is.
In C++ you have several ways of signalling that an issue happened in your function.
You can return a special value which the calling code will recognize as an invalid value. This can be a NULL pointer if the function should return a pointer, or a negative value if your function returns an index in an array, or, in the case of a custom class (e.g. your Entry class) you can define a special Entry::invalid value or something similar that can be detected by the calling function.
Your calling code could look like
if ( entryInstance->get_by_last_name("foobar") != Entry::invalid)
{
// here goes the code for the case where the name is valid
} else {
// here goes the code for the case where the name is invalid
}
On the other hand you can use the C++ exceptions mechanism and make your function throw an exception. For this youcan create your own exception class (or use one defined in the standard library, deriving from std::exception). Your function will throw the exception and your calling code will have to catch it with a try...catch statement.
try
{
entryInstance->get_by_last_name("foobar")
}
catch (Exception e)
{
// here goes the code for the case where the name is invalid
}
// here goes the code for the case where the name is valid
Apart from the fact that you could have more than one entry per surname.
Eliminate the getter, and you've solved the problem, or at least shifted it elsewhere.
Tell the AddressBook to display people with given surnames. If there aren't any it can do nothing.
AddressBookRenderer renderer;
AddressBook contacts;
contacts.renderSurnames("smith", renderer);
contacts.renderCompletions("sm", renderer);
//etc
You can do what std::map (and the other containers do).
You return an iterator from your search function.
If the search does not find a value that is useful return an iterator to end().
class AddressBook
{
typedef <Your Container Type> Container;
public:
typedef Container::iterator iterator;
iterator get_by_last_name(std::string const& lastName) {return addr_map.find[lastName];}
iterator end() {return addr_map.end();}
};
Your address book is a container like object.
Not finding an item in a search is likely to happen but it does not have enough context to incorporate error handling code (As the address book could be used from lots of places and each place would have different error handling ideas).
So you must move the test for not found state out of your address book.
just like "Python" we return a marker. In C++ this is usually an iterator to end() which the calling code can check and take the appropriate action.
AddressBook& ab = getAddressBookRef();
AddressBook::iterator find = ab.get_by_last_name("cpp_hobbyist");
if (find != ab.end())
{
Entity& person *find; // Here you have a reference to your entity.
// you can now manipulate as you want.
}
else
{
// Display appropriate error message
}
Alright, I'm trying to write a program in C++ that deals with a double-linked list. Specifically, the list contains a collection of artwork objects as its nodes. In this particular function, I'm trying to remove nodes through the "sell" command, but I'm getting this error on several lines:
Error: lvalue required as left operand of assignment
I've done my research and I've found that this type of error commonly arises when you try to assign values when you actually want to compare them, or if you're trying to assign values to a constant. However, I don't think that's my problem. Here's the problem code:
// Sell At Function: This function sells the specified artwork.
void CR_ArtCollection::sell_at(string title, ostream& log)
{
CR_ArtWorks* walker = first;
while(walker != NULL)
{
if(title == walker->get_title())
{
walker->get_next()->get_prev() = walker->get_prev(); // Error
walker->get_prev()->get_next() = walker->get_next(); // Error
delete walker;
walker = NULL;
}
else walker = walker->get_next();
}
}
If anyone can point me in the right direction, I would be incredibly appreciative.
It's quite obvious - get_prev and get_next return r-values. That means you can't assign to them.
Check your interface for a method similar to set_next and set_prev and call it as:
walker->get_next()->set_prev(walker->get_prev());
walker->get_prev()->set_next(walker->get_next());
As the names suggest - get_xxxx, those methods are there so you can get the values, not also set them.
Alright, if a function returns a primitive type such as a integer or a pointer it is not legal to assign to the result of the function. That's the error you have.
One possibility would be to change your get_prev and get_next function to return references to pointers.
But I don't suggest you do that. Your code clearly needs redesigning. You should add an erase function to your linked list class. That way the pointer manipulation code will be in the CR_ArtWorks class where it belongs, instead of the CR_ArtCollection class where it doesn't.