I'm trying to create an iterator which only can dereference to real value types, not to references.
Is this possible using boost::iterator_facade, or does it require me to have values that can be returned by adress\reference.
To be more specfic, my iterator returns a std::pair of references, which means my iterators value_type is not stored anywhere, but created on the fly on dereferencing (like std::map::iterator).
Yes, thing you want is possible. Please, take a look at boost/iterator_facade.hpp (example is for Boost lib of version 1.49.0 but it is ok for its new distributions also):
template <
class Derived
, class Value
, class CategoryOrTraversal
, class Reference = Value&
, class Difference = std::ptrdiff_t
>
class iterator_facade
Template argument Reference is the key. You should just specify Reference when deriving from boost::iterator_facade. For example, your code can look like as the following:
template<typename value_type>
class custom_iterator
: public boost::iterator_facade<
custom_iterator<value_type>,
value_type,
boost::forward_traversal_tag,
value_type
>
{
...
value_type dereference() const{ return value_type(...); }
...
};
Related
In a templated class, I would like to make an alias of a variadic parameter, something like:
template<typename ... TYPES>
class X
{
using Types = TYPES;
};
It is easy to make a std::tuple of it, but is there a syntax for doing the above.
The reason is I would like to reference this from the templated class in another class.
I've been reading the chapter 5 of "C++ Templates, The complete guide", and i've seen the concept of "template template" templates, so tried myself.
In a template class, i've declared my template like this:
template <typename TipoClave, typename TipoDato,
template <class Elem, class Alloc = allocator<Elem>> class Lista = vector>
And this works. My problem comes when I try to use a different container than the default.
My class is as follows:
class Tabla
{
public:
struct Celda {
TipoClave clave;
TipoDato dato;
};
Tabla(unsigned tam)
{
t.resize(tam);
}
///< Some other functions
private:
typedef Lista<Celda> ListaDatos;
Lista<ListaDatos> t;
};
And then, when I try to use it from the main program like this:
int main (void)
{
Tabla<string,Alumno,array> tabla(200);
///< Some stuff
return 0;
}
But this line Tabla<string,Alumno,array> tabla(200); does not compile, getting me an error like:
test_tabla_t.cpp: In function ‘int main()’: test_tabla_t.cpp:20:27:
error: type/value mismatch at argument 3 in template parameter list
for ‘template class Lista> class Tabla’ Tabla
tabla(200);
I've tried to use Tabla<string,Alumno,vector> tabla(200);, and it works, so I don't know how to solve this error.
Assuming you're trying to use std::array, it does not take an Allocator template parameter. Its second argument is the size of the array.
template<
class T,
std::size_t N
> struct array;
You haven't mentioned how experienced you are with C++, but since you're trying to learn more, I'll say that I've been using C++ for nearly a decade and can count on one hand the number of times I've used template template parameters.
In this case, you want Lista to be "something that acts like a sequence container[1]", you might like to read about concepts[2], which may make it into C++20. They allow you to tell the compiler the interface you want a template parameter to have.
In the meantime, it might be easier to declare Lista as a plain template parameter.
[1] http://en.cppreference.com/w/cpp/concept/SequenceContainer
[2] https://en.wikipedia.org/wiki/Concepts_(C%2B%2B)
I was writing wrapper methods for Boost unordered map container. In boost Unordered Map there is a method begin() which returns an iterator to the first element.Actually in my wrapper class i want to return a std::Unordered_map::iterator instead of boost::unordered_map::iterator from my Begin method.
Example code:
template
boost::unordered_map<key, value> m_myMap;
boost::unordered::unordered_map::iterator MyWrapper<>::Begin()
{
return m_myMap.begin();
}
In the above code i want to return std::Unordered_map::iterator
Please help
You can't. C++ is a strongly typed language.
The best you can do is
use std::unordered_map
Use type erasure to hide the implementation (boost::any_iterator or boost::any_range)
My spidy sense tells me that you should take the iterators by deduced template argument type, instead of hard-coding them into your algorithms.
template <typename Iterator>
void foo_algo(Iterator begin, Iterator end, int some_data) {
...
I write a program with my class:
class COrder
{
public:
COrder();
~COrder();
public:
...
CList < CItem > m_oItem;
...
};
which suppose to have list od object of my other class:
class CItem
{
public:
CItem();
~CItem();
public:
int m_i;
double m_d;
CString m_o;
};
and compiler give me error like this in title. Any ideas why ?
In program I use COrder in map:
CMap <CString, LPCTSTR, COrder, COrder> m_map
Quote:
Add copy-constructor and assignment operator to your class COrder.
I add operator= to my class:
COrder& operator=( const COrder oNewOrder )
{
...
m_oItem.AddTail( oNewOrder.m_oItem.GetTail() );
...
return *this;
}
but what you mean by adding "copy-constructor" ?
http://msdn.microsoft.com/en-us/library/ccb3dh5c.aspx i found this but how to implement it in my code. i can't change CList class.
http://www.codeproject.com/Articles/13458/CMap-How-to
Add copy-constructor and assignment operator to your class COrder. This makes the class copyable.
[If class is used in as Key then you need HashKey() and CompareElemenst() in that class]
Also note that STL containers are superior to MFC containers.
You get an error because CMap has default copy-ctor but CMap and CList is derived from CObject and CObject declares private copy constructor and operator=.
So, CMap doesn't offer a copy semantic "out of the box".
I would suggest you to use STL std::map container, which is designed in a
way to implement copy semantic out-of-the-box.
What you don't have with STL out of the box is serialization only.
Note that std::map does not have the confusing ARG_KEY and ARG_VALUE
templates.
std::map just has the Key and Type template arguments (in its basic form).
http://msdn.microsoft.com/en-us/library/s44w4h2s%28VS.80%29.aspx
Or else you can go by the pointer way as Ajay suggested by which you will just shut up the compiler.
The problem statement:
CList<CItem> m_oItem;
And the trigger statement (or some usage):
CMap <CString, LPCTSTR, COrder, COrder> m_map;
Why? Well, CMap would call copy constructor and/or assignment operator for COrder. You didn't provide any, but compiler provides them from your class (i.e. for COrder). This class contains a CList object, which is inherited from CObject. CObject doesn't provide (or better say: Prevents) copy-constructor or assignment operator.
As a result, the compiler raises the error. Unfortunately, the (bad) compiler doesn't give you back-trace of this error.
Best bets for as the solution:
CList < CItem* > m_oItem;
CList<CItem> *m_poItem;
Use or implement your own collection.
I'm new to the bimap functionality of the Boost libraries, and I'm having trouble passing a bimap into another function. My bimap looks like this:
typedef boost::bimap< int, int > bimap_type;
bimap_type bm;
I have an add_values() function that adds a set of values to the bimap:
add_values(int a, int b)
{
bm.insert(bimap_type::value_type(a, b));
}
I then have a function that is meant to set the values of the bimap by getting them from a Singleton Class:
void set_values()
{
MyClass::instance()->get_values(bm);
}
And, in MyClass, get_values() looks like this:
void get_values(bimap_type myBimap)
{
myBimap.add_values(3, 5);
}
However, MyClass does not recognise 'bimap_type'. I try putting the typedef in a separate header file and including that in MyClass, but I get the error message:
'class bimap_type' has no member named 'add_values'
How can I successfully pass the bimap to this Singleton Class in order to fill it with values from the Class? Does anyone know?
Thanks a lot.
Er, boost::bimap itself doesn't have an add_values method and it's hard to tell from these code fragments why you're suddenly expecting one to appear.
Consider renaming your functions: set_values() that calls get_values() that calls add_values() is one confusing call chain...
When you need to modify an object in a function, you have to take it by reference (or a pointer). The idea is that you must work with the same object inside and outside of the function. If you pass by value, function will see a copy, so anything it does with it does not reflect on original object.
// formerly known as add_values()
void initialize(bimap_type& bm, int a, int b)
{
bm.insert(bimap_type::value_type(a, b));
}
And this is how you will call it:
initialize(myBitmap, 3, 5);
Make sure to update your whole call chain to pass by reference where appropriate, because currently your get_values() works with a copy too.