I have a problem using the std::map, specifically when using find.
I have the following code.
class MyClass
{
update(const QVariant&);
QVariant m_itemInfo;
std::map<QVariant, int> m_testMap;
}
void update(const QVariant& itemInfo)
{
if(m_itemInfo != itemInfo)
{
// The items are not equal
m_itemInfo = itemInfo;
}
if(m_testMap.find(itemInfo) == m_testMap.end())
{
// TestMap doesnt contain key itemInfo.
m_testMap.insert(std::make_pair(itemInfo, 1));
}
// More code
}
The function update is called several times (with different itemInfo objects) in my code. Now when I start to debug it, I see that the first time update is called, both the first and the second if loop are entered. So far so good. However the second time update is called I do see that the first if loop is called, but the second is skipped! What am I missing here?
I guess the problem that the first and second QVariants that you pass to your Update method have different type (for example, bool and uint). std::map::find doesn't use !=operator to compare keys, it uses operator < (less) by default. If two compared QVariant values have different types operators != and < may work contradictory.
std::map::find compares keys in the following way:
Two keys are considered equivalent if the container's comparison object returns false reflexively (i.e., no matter the order in which the elements are passed as arguments).
i.e. std::map::find considers that v1 is equal to v2
if(!(v1<v2) && !(v2>v1)) { //is TRUE !!!
}
To solve your problem, you should define a less comparison for std:map.
class QVariantLessCompare {
bool operator()(const QVariant& v1, QVariant& v2) const {
// ==== You SHOULD IMPLEMENT appropriate comparison here!!! ====
// Implementation will depend on type of QVariant values you use
//return v1 < v2;
}
};
And use QVariantCompare in a such way:
std::map<QVariant, int, QVariantLessCompare> m_testMap;
A more paradigmatic solution is to use QMap which correctly implements the comparison of most QVariant types. It won't do userTypes() out of the box, but this still might suit your application.
A cleaner version of the solution proposed by Володин Андрей, that builds, might look like:
struct QVariantLessCompare {
bool operator()(const QVariant& v1,const QVariant& v2) const
{
return v1.toInt() < v2.toInt();
}
};
Related
I have a QMap<QString, myStruct> with
myStruct {
QString firstname;
QString lastname;
QString status;
}
How can I sort this QMap according to priority order: status then firstname then lastname?
As far as I understand, you'd like to retrieve the values of the map sorted in the mentioned way, but still have access to the key. Right?
Quickly speaking, a map is a collection of <key, value> pairs automatically sorted by key, then you may try a list of <value, key> pairs manually sorted by value instead. Something like QList<QPair<myStruct, QString>>, while overriding the operator< for myStruct.
struct myStruct {
QString firstname;
QString lastname;
QString status;
bool operator<(const myStruct& o) const {
return std::tie(status, firstname, lastname) <
std::tie(o.status, o.firstname, o.lastname);
}
};
QMap<QString, myStatus> map; // your original map
QList<QPair<myStatus, QString>> inv;
// Populate the inverted list
for (auto k : map.keys()) {
inv.append(QPair<myStatus, QString>(map[k], k));
}
std::sort(std::begin(inv), std::end(inv));
for (auto p : inv) {
qDebug() << p.first.status << p.first.firstname << p.first.lastname << p.second;
}
Of course, it is a one-time use structure that doesn't keep updated with your original map, but you mentioned that the map is fixed (constant?) so it may not be a problem then.
BTW, a QMap can be used for the inverse look-up but only in the case the values of the myStruct part are also unique (so they can be used also as a key), otherwise you may overwrite values when constructing the inverse map.
Note: The std::tie is used just to simplify the sorting condition for tuples (so you'd need to include <tuple>).
UPDATE
Answering one of your comments: Yes, you can also specify your own comparison predicate and then avoid overriding the operator<, but I think it is harder to read and less re-usable:
std::sort(std::begin(inv), std::end(inv),
[](const QPair<myStatus, QString>& lhs, const QPair<myStatus, QString>& rhs) {
return std::tie(lhs.first.status, lhs.first.firstname, lhs.first.lastname) <
std::tie(rhs.first.status, rhs.first.firstname, rhs.first.lastname);
});
Of course, you can implement that comparison lambda as you want, I've used the std::tie again to simplify the logic in the post. The downside is that if you need to generate the inverse map in several places you'd have to repeat the lambda expression everywhere (or create a function to create the inverse map of course).
As a side note and in case you are curious, lhs and rhs refers to left-hand side and right-hand side respectively, in this case they are used as lhs < rhs by the sorting algorithm for comparing the elements.
Finally, if you'd want to avoid the std::tie you'd have to make the comparisons manually (code below modifies the operator< of the first version):
bool operator<(const myStruct& o) const {
if (status < o.status) return true;
if (status > o.status) return false;
// status == o.status, move to next attribute
if (firstname < o.firstname) return true;
if (firstname > o.firstname) return false;
// firstname== o.firstname, move to next attribute
if (lastname < o.lastname) return true;
if (lastname > o.lastname) return false;
return false; // are equal
}
You can't sort a QMap manually, you'll have to use a QList (or QVector) for that and use std::sort on it. Use QMap::values() to extract the values (structs) from the map into a list, then implement a compare function/method and call it with std::sort. See cbucharts answer for some hints how to do this.
Keeping map and list in sync when the values change is a different issue, if this is a requirement you should create a separate question, adding a MCVE and more details on what you tried.
[UPDATE: My problem is solved! Lots of thanks to Mike Seymour and Niall and all you guys!]
My code has errors in the for loop and I do not know how to fix it :(
MyClass::ITECH7603Class(set<Student>* students) {
/* Initialize dynamically the group field */
group = new map<string, Student>();
for (set<Student>::iterator it = students->begin(); it != students->end(); it++) {
addStudent(it);
}
}
void MyClass::addStudent(Student* studentPtr) {
string fullName = studentPtr->getName() + " " + studentPtr->getSurname();
group->insert(pair<string, Student>(fullName, *studentPtr));
}
So the main idea is to loop through all students in the set, and add each student into a map group. Any help? Thank you very much!
for (set<Student>::iterator it = students->begin; it != students->end; it++) {
addStudent(it);
}
should be:
for (set<Student>::iterator it = students->begin(); it != students->end(); it++) {
//^^ //^^
addStudent(it);
}
addStudent takes a pointer, while it is an iterator, so can't be passed directly.
You should change addStudent to take either a value or a pointer/reference to const:
// option 1
void addStudent(Student);
addStudent(*it);
// option 2
void addStudent(Student const &);
addStudent(*it);
// option 3
void addStudent(Student const *);
addStudent(&*it);
If, as you say in a comment, you must leave it taking a mutable pointer, then you'll need some grotesquery to deal with the fact that elements of the set are immutable:
// nasty option
addStudent(const_cast<Student*>(&*it));
// slightly less nasty option
Student copy = *it;
addStudent(©);
Beware that the first option will give undefined behaviour if the function uses the dodgy pointer to make any modification to the Student object stored in the set. The second makes a temporary copy, which can be modified without breaking the set. This is fine as long as addStudent only stores a copy of the object passed to it, not the pointer itself, which will become invalid when copy is destroyed.
In c++11 you can use range for sytax:
for (const auto &student : *students)
{
addStudent(it);
}
Then change addStudent function signature to accept reference:
void MyClass::addStudent(const Student &student) {
While you've gotten answers that "fix" your code to the extent of compiling and producing results that you apparently find acceptable, I don't find them very satisfying in terms of code style. I would do this job rather differently. In particular, my code to do this wouldn't have a single (explicit) loop. If I needed to do approximately what you're asking for, I'd probably use code something like this:
std::pair<std::string, Student> make_mappable(Student &stud) {
return std::make_pair(stud.getName() + " " + stud.getSurName(), stud);
}
std::map<std::string, Student> gen_map(std::set<Student> const &input) {
std::map<std::string, Student> ret;
std::transform(input.begin(), input.end(),
std::inserter(ret, ret.end()),
make_mappable);
return ret;
}
There definitely would not be any new in sight, nor would there be any passing a pointer to a Student.
OTOH, since the data you're using as the key for your map is data that's already in the items in the set, it may more convenient all around to continue to use a set, and just specify a comparison function based on the student's name:
struct by_given_name {
bool operator()(Student const &a, Student const &b) const {
if (a.getName() < b.getName())
return true;
if (b.getName() < a.getName())
return false;
return a.getSurName() < b.getSurName();
}
};
std::set<Student, by_given_name> xform(std::set<Student> const &in) {
return std::set<Student, by_given_name>{in.begin(), in.end()};
}
For what its worth, a Live Demo of the latter.
Whether the latter is practical will typically depend on one other factor though: your ability to create a Student from only a name/surname. If you can't do that, searching by name will be inconvenient (at best), so you'd want to use a map.
I realize this probably isn't much (if any) help in completely what's apparently home-work for a class--but even if your class prevents you from actually turning in decent code, it seems worthwhile to me to at least try to learn to write decent code in addition to what it requires. If you do pass the class and get a job writing code, you'd probably rather your coworkers didn't want to hurt you.
Sorry for the format, I never really posted to a forum like this, so I have to learn the how to a bit.
My problem is:
I'm writing a template class, and I'd like to access my containers via multiple kind of [] operators. I read a bit in this subject, so I've been able to make one overloading, but I need some more:
So in my header file, relevant things about my container:
template <class T>
class version_controlled_vector
{
int rev;
bool vector_state_changed;
std::vector< std::string > revision;
std::vector< std::vector<T> > v;
//first one works ok, im satisfied with it:
std::vector<T>& operator[] (const int idx)
{
return v[idx];
}
//im not sure how to define the further one(s?):
T& operator[](const int idx2) const
{
return v[idx2];
}
//...and ofc some other code
};
//to have these usages at my main.cpp:
version_controlled_vector<int> mi;
version_controlled_vector<std::string> ms;
//this works, and i d like to keep it,
5 == mi[ 0 ][ 0 ];
//and i d like to have these two usages too:
//getting the first character of the stored string:
'H' == ms[ 0 ][ 0 ]; // with the first overload from the header ms[0][0][0]
works to get the first character of the string for eg "Hello"
but, i have to use the ms[0][0] format to achieve this
//and this:
4 == mi[ 0 ]; // i d like this as if it d behave like 4 == mi[0][0];
I don't really get how can I use the single[] when I made an overload to use the [][]
The only solution I have read about is maybe const-overloading, but I'm not sure at all, I'm quite a weakie.
Thanks for ideas!
I think you are muddying the interface of the class. The expectations from the class are:
Get the i-th value from the j-th version.
Get the i-th value from the latest version.
Get the j-th version.
You have the option of using the overloaded operator[] function to get those values but, it will be better to have functions that reflect the interface.
// Get the versionIndex-th version.
std::vector<T>& getVersion(int versionIndex);
// Get the itemIndex-th value from the versionIndex-th version.
T& getItem(int versionIndex, int itemIndex);
// Get the itemIndex-th value from the latest version.
T& getItem(int itemIndex);
Then, the implementation would be simpler and less confusing.
std::vector<T>& getVersion(int versionIndex)
{
// Make sure to add out of bound checks
return v[versinIndex];
}
T& getItem(int versionIndex, int itemIndex)
{
// Make sure to add out of bound checks
return v[versinIndex][itemIndex];
}
T& getItem(int itemIndex);
{
// Make sure to add out of bound checks
return v.back()[itemIndex];
}
Given these, the only operator[] that makes sense, at least to me, is one that returns the i-th value from the latest version.
T& operator[](int itemIndex);
{
// Make sure to add out of bound checks
return v.back()[itemIndex];
}
It's a bit tricky one, you need to realise that when you write
version_controlled_vector<int> mi;
5 == mi[0][0];
during the second fetch you're no longer accessing your version_controlled_vector class but the inner property of it and it's type is std::vector<T> which has its own subscript operator that you call in the second [0].
To control the subscript operator of the second fetch you need to create another class that derives from std::vector<T> and has overloaded subscript operator. Then you should use this class instead of the std::vector in the implementation of version_controlled_vector.
Update
I have created an qt bugticket hoping the documentation will be extended.
Original Question
Believing an Question from 2010 and the Qt Documentation, the operator==() doesn't work with custom types.
Quote:
bool QVariant::operator==(const QVariant & v) const
Compares this QVariant with v and returns true if they are equal; otherwise returns false.
QVariant uses the equality operator of the type() it contains to check for equality. QVariant will try to convert() v if its type is not the same as this variant's type. See canConvert() for a list of possible conversions.
Warning: This function doesn't support custom types registered with qRegisterMetaType().
I've tried to reproduce the repro case from the Stackoverflow Question from 2010 and the comparison worked without any problems for me.
I also went a step further and tried comparisons using an own class which also worked perfectly.
To reproduce, put the following code into any header:
enum MyEnum { Foo, Bar };
Q_DECLARE_METATYPE(MyEnum)
class MyClass
{
int value;
public:
MyClass() : value(0)
{
}
MyClass(int a) : value(a)
{
}
bool operator==(const MyClass &) const
{
Q_ASSERT(false); // This method seems not to be called
return false;
}
bool operator!=(const MyClass &) const
{
Q_ASSERT(false); // This method seems not to be called
return true;
}
};
Q_DECLARE_METATYPE(MyClass)
And the following code into any function:
QVariant var1 = QVariant::fromValue<MyEnum>(Foo);
QVariant var2 = QVariant::fromValue<MyEnum>(Foo);
Q_ASSERT(var1 == var2); // Succeeds!
var1 = QVariant::fromValue<MyEnum>(Foo);
var2 = QVariant::fromValue<MyEnum>(Bar);
Q_ASSERT(var1 != var2); // Succeeds!
QVariant obj1 = QVariant::fromValue<MyClass>(MyClass(42));
QVariant obj2 = QVariant::fromValue<MyClass>(MyClass(42));
Q_ASSERT(obj1 == obj2); // Succeeds!
obj1 = QVariant::fromValue<MyClass>(MyClass(42));
obj2 = QVariant::fromValue<MyClass>(MyClass(23));
Q_ASSERT(obj1 != obj2); // Succeeds!
I would guess that in newer qt versions the size of a type is aquired when the Q_DECLARE_METATYPE is used so the QVariant can compare values of unknown types bytewise.
But that's only a guess and I don't want to risk the stability of my application by guessing what qt does instead of relying on the documentation.
Can I find out, how the QVariant compares unknown types? I would prefer relying on specification than on implementation.
I'm afraid you'll need to rely on the code (and, being behaviour, it can't be changed without breaking), and not documentation. There's a surprise just below, though.
Here's the relevant code.
QVariant::operator== for types with unregistered operators will just use memcmp. The relevant snippet (in 5.1) is this:
bool QVariant::cmp(const QVariant &v) const
{
QVariant v1 = *this;
QVariant v2 = v;
if (d.type != v2.d.type)
// handle conversions....
return handlerManager[v1.d.type]->compare(&v1.d, &v2.d);
}
handlerManager is a global object that gets used to perform type-aware manipulations. It contains an array of QVariant::Handler objects; each of such objects contains pointers to perform certain operations on the types they know how to handle:
struct Handler {
f_construct construct;
f_clear clear;
f_null isNull;
f_load load;
f_save save;
f_compare compare;
f_convert convert;
f_canConvert canConvert;
f_debugStream debugStream;
};
Each and every of those members is actually a pointer to a function.
The reason for having this array of global objects is a bit complicated -- it's for allowing other Qt libraries (say, QtGui) to install custom handlers for the types defined in those libs (f.i. QColor).
The operator[] on the handlerManager will perform some extra magic, namely get the right per-module handler given the type:
return Handlers[QModulesPrivate::moduleForType(typeId)];
Now the type is of course a custom type, so the Handler returned here is the one the Unknown module. That Handler will use the customCompare function in qvariant.cpp, which does this:
static bool customCompare(const QVariant::Private *a, const QVariant::Private *b)
{
const char *const typeName = QMetaType::typeName(a->type);
if (Q_UNLIKELY(!typeName) && Q_LIKELY(!QMetaType::isRegistered(a->type)))
qFatal("QVariant::compare: type %d unknown to QVariant.", a->type);
const void *a_ptr = a->is_shared ? a->data.shared->ptr : &(a->data.ptr);
const void *b_ptr = b->is_shared ? b->data.shared->ptr : &(b->data.ptr);
uint typeNameLen = qstrlen(typeName);
if (typeNameLen > 0 && typeName[typeNameLen - 1] == '*')
return *static_cast<void *const *>(a_ptr) == *static_cast<void *const *>(b_ptr);
if (a->is_null && b->is_null)
return true;
return !memcmp(a_ptr, b_ptr, QMetaType::sizeOf(a->type));
}
Which, apart from a bit of error checking and handling shared and null variants in a special way, uses memcmp on the contents.
... only if the type is not a pointer type, it seems. Wonder why there's that code there...
Good news!
Starting with Qt 5.2, you can use QMetaType::registerComparator (see here) to make Qt invoke operator< and operator== on your custom type. Just add to your main:
qRegisterMetaType<MyClass>();
QMetaType::registerComparators<MyClass>();
And voilà, you'll hit the assert in your equality operator. QVariant::cmp now is:
QVariant v1 = *this;
QVariant v2 = v;
if (d.type != v2.d.type)
// handle conversions, like before
// *NEW IMPORTANT CODE*
if (v1.d.type >= QMetaType::User) {
// non-builtin types (MyClass, MyEnum...)
int result;
// will invoke the comparator for v1's type, if ever registered
if (QMetaType::compare(QT_PREPEND_NAMESPACE(constData(v1.d)), QT_PREPEND_NAMESPACE(constData(v2.d)), v1.d.type, &result))
return result == 0;
}
// as before
return handlerManager[v1.d.type]->compare(&v1.d, &v2.d);
I am working on a simple hash table in C++. I have methods to insert, delete, and search the hash table for the specified key. I know that the C++ map STL container can handle my situation, but I would kind of like to code my own as an educational exercise.
Basically I have a hash table that will take a single string and map it to a vector of other strings. This is easy to do in a method because calling a .Add() or .Delete() will behave as expected. I would however like to create an overloaded [] operator to the class that is able to do these operations on the vector.
For example, if I want to add an item to the vector I can write something like this:
hashTable[string1] = newString;
This will set the new string as a member of my vector. The same can be said for delete and search.
hashTable[string1] = "";
cout << hashTable[string1] << endl;
My major problem is that I do not know how to overload the [] operator to gain this functionality. I have this function coded up right now. It works on a basic 1 to 1 string match, but not on a string to vector match.
//Return a reference to a vector to update then reassign?
vector& HashClass::operator[](const string index)
{
assert(size >= 0 && size < maxSize);
Hash(key);
return hashTable[index];
}
I think I'm most stuck on the idea of having a vector return that later needs to be assigned. As the user, I would find this kludgy.
This question is closely related to another question: what behavior do
you want when you access a non-existant value other than in an
assignment? In other words, what do you want to happen when you write:
std::cout << hashTable[string] << std::endl;
and string is not present in the table?
There are two possible approaches: you can consider it an error, and
throw an exception, or abort, or something similar; or you can return
some sort of default, built with the default constructor, or provided by
the client earlier.
The standard map and unordered_map take the second approach, using the
default constructor to construct a new value. This allows a very simple
solution: if operator[] isn't present, you insert it, initializing it
with the default value. Then you return a reference to it;
hashTable[string] = newString; assigns through the reference to an
already existing value.
In many use cases, the first approach will be preferable (perhaps with a
contains function, so you can test up front whether the operator[]
will find something or not). To implement the first approach, you must
first implement specific functions for each type of access:
template <typename Key, typename Value>
class HashTable
{
public:
Value* get( Key const& key ) const;
void set( Key const& key, Value const& value );
};
(I generally make these public; there's no reason to forbid their use by
a client.)
Then, you define operator[] to return a proxy, as follows:
template <typename Key, typename Value>
class HashTable
{
public:
class Proxy
{
HashTable* myOwner;
Key myKey;
public:
Proxy( HashTable* owner, Key const& key )
: myOwner( owner )
, myKey( key )
{
}
operator Value const&() const
{
Value const* result = myOwner->get( myKey );
if ( result == NULL ) {
// Desired error behavior here...
}
return *result;
}
Proxy const& operator==( Value const& value ) const
{
myOwner->set( myKey, value );
return *this;
}
};
Value* get( Key const& key ) const;
void set( Key const& key, Value const& value );
Proxy operator[]( Key const& key )
{
return Proxy( this, key );
}
};
Thus, when you write:
hashTable[key] = newString;
, the proxy's operator= will call hashTable.put( key, newString );
in other contexts, however, it will call the implicit type conversion on
the proxy, which calls hashTable.get( key ).
In some cases, even if you desire to return a default value, it may be
preferable to use this solution: the get function is not required to
insert anything into the hash table, so the table doesn't fill up with
all of the misses, and you can overload the operator[] on const, so
you can use it on a const hash table as well. Also, it doesn't
require the value type to have a default constructor.
It does have one disadvantage with respect to the solution used in the
standard; since you can't overload operator., you can't make the proxy
behave like a reference, and things like:
hashTable[string].someFunction();
don't work. A work-around is to overload operator-> in the proxy, but
this leads to a somewhat unnatural syntax:
hashTable[string]->someFunction(); // But the hash table contains
// values, not pointers!!!
(Don't be mislead by the implicit conversion to a reference. An
implicit conversion will not be considered for a in an expression
a.b.)
In C++, [] access to associative containers is generally given the semantics of default-constructing an object of the mapped type, inserting it with the key, and returning a reference to the inserted mapped object.
So your operator[] would be implemented as:
string& HashClass::operator[](const string index)
{
assert(size >= 0 && size < maxSize);
Hash(key);
vector &v = hashTable[index];
if (index in v) {
...
} else {
v.push_back(string());
return v.back();
}
}