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  • C++: Constructor/destructor unresolved when not inline?

    - by Anamon
    In a plugin-based C++ project, I have a TmpClass that is used to exchange data between the main application and the plugins. Therefore the respective TmpClass.h is included in the abstract plugin interface class that is included by the main application project, and implemented by each plugin. As the plugins work on STL vectors of TmpClass instances, there needs to be a default constructor and destructor for the TmpClass. I had declared these in TmpClass.h: class TmpClass { TmpClass(); ~TmpClass(); } and implemented them in TmpClass.cpp. TmpClass::~TmpClass() {} TmpClass::TmpClass() {} However, when compiling plugins this leads to the linker complaining about two unresolved externals - the default constructor and destructor of TmpClass as required by the std::vector<TmpClass> template instantiation - even though all other functions I declare in TmpClass.h and implement in TmpClass.cpp work. As soon as I remove the (empty) default constructor and destructor from the .cpp file and inline them into the class declaration in the .h file, the plugins compile and work. Why is it that the default constructor and destructor have to be inline for this code to compile? Why does it even maatter? (I'm using MSVC++8).

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  • Should this work?

    - by Noah Roberts
    I am trying to specialize a metafunction upon a type that has a function pointer as one of its parameters. The code compiles just fine but it will simply not match the type. #include <iostream> #include <boost/mpl/bool.hpp> #include <boost/mpl/identity.hpp> template < typename CONT, typename NAME, typename TYPE, TYPE (CONT::*getter)() const, void (CONT::*setter)(TYPE const&) > struct metafield_fun {}; struct test_field {}; struct test { int testing() const { return 5; } void testing(int const&) {} }; template < typename T > struct field_writable : boost::mpl::identity<T> {}; template < typename CONT, typename NAME, typename TYPE, TYPE (CONT::*getter)() const > struct field_writable< metafield_fun<CONT,NAME,TYPE,getter,0> > : boost::mpl::false_ {}; typedef metafield_fun<test, test_field, int, &test::testing, 0> unwritable; int main() { std::cout << typeid(field_writable<unwritable>::type).name() << std::endl; std::cin.get(); } Output is always the type passed in, never bool_.

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  • I have a bunch of template parameters that I want to hide from my users. How can I do this?

    - by Alex
    I have a superclass which is defined in terms of a few internal types it uses. Subclassing is performed as so: template <class InternalType1, class InternalType2> class Super { ... } class Sub : Super <interalTypeClass1, interalTypeClass2> { ... } But when I want to write a function that takes a pointer to the superclass, this happens : template <class InternalType1, class InternalType2> void function(Super<InternalType1, InternalType2>* in) { ... } The user really shouldn't know anything about the inside classes, and should really just concern himself with the use of the function. Some of these template lists become very very large, and expecting the user to pass them every time is wasteful, in my opinion. Any suggestions? EDIT: The function needs to know the internal types in use, so unless there is a way to access template types at compile time, I think there is no solution? Potential solution: Have each class do the following: #define SubTemplateArgs <SubTypeName, SubInternalType1, SubInternalType2> ?

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  • avoiding enums as interface identifiers c++ OOP

    - by AlasdairC
    Hi I'm working on a plugin framework using dynamic loaded shared libraries which is based on Eclipse's (and probally other's) extension-point model. All plugins share similar properties (name, id, version etc) and each plugin could in theory satisfy any extension-point. The actual plugin (ie Dll) handling is managed by another library, all I am doing really is managing collections of interfaces for the application. I started by using an enum PluginType to distinguish the different interfaces, but I have quickly realised that using template functions made the code far cleaner and would leave the grunt work up to the compiler, rather than forcing me to use lots of switch {...} statements. The only issue is where I need to specify like functionality for class members - most obvious example is the default plugin which provides a particular interface. A Settings class handles all settings, including the default plugin for an interface. ie Skin newSkin = settings.GetDefault<ISkin>(); How do I store the default ISkin in a container without resorting to some other means of identifying the interface? As I mentioned above, I currently use a std::map<PluginType, IPlugin> Settings::defaults member to achieve this (where IPlugin is an abstract base class which all plugins derive from. I can then dynamic_cast to the desired interface when required, but this really smells of bad design to me and introduces more harm than good I think. would welcome any tips edit: here's an example of the current use of default plugins typedef boost::shared_ptr<ISkin> Skin; typedef boost::shared_ptr<IPlugin> Plugin; enum PluginType { skin, ..., ... } class Settings { public: void SetDefault(const PluginType type, boost::shared_ptr<IPlugin> plugin) { m_default[type] = plugin; } boost::shared_ptr<IPlugin> GetDefault(const PluginType type) { return m_default[type]; } private: std::map<PluginType, boost::shared_ptr<IPlugin> m_default; }; SkinManager::Initialize() { Plugin thedefault = g_settings.GetDefault(skinplugin); Skin defaultskin = boost::dynamic_pointer_cast<ISkin>(theskin); defaultskin->Initialize(); } I would much rather call the getdefault as the following, with automatic casting to the derived class. However I need to specialize for every class type. template<> Skin Settings::GetDefault<ISkin>() { return boost::dynamic_pointer_cast<ISkin>(m_default(skin)); }

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  • WPF: In Expression Blend, how to change Foreground of a custom button with a Border on different states?

    - by SubmarineX
    In Expression Blend 4, I want to change the Foreground of a custom button on different states. I'm just able to change the Background and BorderBrush. Just like this: If the state is "Normal", the color of text "Button" is Black, while the state is "Pressed", the color of text "Button" is White. On Brushes Panel under Properties Panel, there're 3 properties but no Foreground property: Who can help me? I'm so perplexed. Edit I find ContentControl have a Foreground property, but ContentPresenter haven't. Wether I should use ContentControl instead of ContentPresenter?

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  • Does template class/function specialization improves compilation/linker speed?

    - by Stormenet
    Suppose the following template class is heavily used in a project with mostly int as typename and linker speed is noticeably slower since the introduction of this class. template <typename T> class MyClass { void Print() { std::cout << m_tValue << std::endl;; } T m_tValue; } Will defining a class specialization benefit compilation speed? eg. void MyClass<int>::Print() { std::cout << m_tValue << std::endl; }

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  • Getting a type for a template instantiation?

    - by ebo
    I have the following situation: I have a object of type MyClass, which has a method to cast itself to it's base class. The class includes a typedef for it's base class and a method to do the downcast. template <class T, class B> class BaseClass; template <class T> class NoAccess; template <class T> class MyClass : public BaseClass<T, NoAccess<T> > { private: typedef BaseClass<T, NoAccess<T> > base; public: base &to_base(); }; I need to pass the result of a base call to a functor Operator: template <class Y> class Operator { Operator(Y &x); }; Operator<???> op(myobject.to_base()); Is there a easy way to fill the ??? provided that I do not want to use NoAccess?

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  • In the generic programming/TMP world what exactly is a model / a policy and a "concept" ?

    - by Hassan Syed
    I'd like to know the precise yet succinct definitions of these three concepts in one place. The quality of the answer should depend on the following two points. Show a simple code snippet to show how and what the concept/technique is used for. Be simple enough to understand so that a programmer without any exposure to this area can grasp it. Note: There are probably many correct answers since each concept has many different facets. If there are a lot of good answers I will eventually turn the question into CW and aggregate the answers. -- Post Accept Edit -- Boost has a nice article on generic programming concepts

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  • How can I have multiple navigation paths with Django, like a simplifies wizard path and a full path?

    - by Zeta
    Lets say I have an application with a structure such as: System set date set name set something Other set death ray target calibrate and I want to have "back" and "next" buttons on a page. The catch is, if you're going in via the "wizard", I want the nav path to be something like "set name" - "set death ray target" - "set name". If you go via the Advanced options menu, I want to just iterate options... "set date" - "set name" - "set something" - "set death ray target" - calibrate. So far, I'm thinking I have to use different URIs, but that's that. Any ideia how this could be done? Thanks.

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  • Django template context not working with imported class

    - by Andy Hume
    I'm using Django's templating on appengine, and am having a problem whereby a class I'm importing from another package is not correctly being made available to the template context. Broadly speaking, this is the code. The prop1 is not available in the template in the first example below, but is in the second. MyClass is identical in both cases. This does not work: from module import MyClass context = MyClass() self.response.out.write(template.render(path, context)) This does: class MyClass(object): def __init__(self): self.prop1 = "prop1" context = MyClass() self.response.out.write(template.render(path, context)) If I log the context in the above code I get: <module.MyClass object at 0x107b1e450> when it's imported, and: <__main__.MyClass object at 0x103759390> when it's defined in the same file. Any clues as to what might cause this kind of behaviour?

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  • What does "static" mean in the context of declaring global template functions?

    - by smf68
    I know what static means in the context of declaring global non-template functions (see e.g. What is a "static" function?), which is useful if you write a helper function in a header that is included from several different locations and want to avoid "duplicate definition" errors. So my question is: What does static mean in the context of declaring global template functions? Please note that I'm specifically asking about global, non-member template functions that do not belong to a class. In other words, what is the difference between the following two: template <typename T> void foo(T t) { /* implementation of foo here */ } template <typename T> static void bar(T t) { /* implementation of bar here */ }

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  • C++ Types Impossible to Name

    - by Kirakun
    While reading Wikipedia's page on decltype, I was curious about the statement, Its [decltype's] primary intended use is in generic programming, where it is often difficult, or even impossible, to name types that depend on template parameters. While I can understand the difficulty part of that statement, what is an example where there is a need to name a type that cannot be named under C++03? EDIT: My point is that since everything in C++ has a declaration of types. Why would there ever be a case where it is impossible to name a type? Furthermore, aren't trait classes designed to yield type informations? Could trait classes be an alternative to decltype?

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  • Specify a base classes template parameters while instantiating a derived class?

    - by DaClown
    Hi, I have no idea if the title makes any sense but I can't find the right words to descibe my "problem" in one line. Anyway, here is my problem. There is an interface for a search: template <typename InputType, typename ResultType> class Search { public: virtual void search (InputType) = 0; virtual void getResult(ResultType&) = 0; }; and several derived classes like: template <typename InputType, typename ResultType> class XMLSearch : public Search<InputType, ResultType> { public: void search (InputType) { ... }; void getResult(ResultType&) { ... }; }; The derived classes shall be used in the source code later on. I would like to hold a simple pointer to a Search without specifying the template parameters, then assign a new XMLSearch and thereby define the template parameters of Search and XMLSearch Search *s = new XMLSearch<int, int>(); I found a way that works syntactically like what I'm trying to do, but it seems a bit odd to really use it: template <typename T> class Derived; class Base { public: template <typename T> bool GetValue(T &value) { Derived<T> *castedThis=dynamic_cast<Derived<T>* >(this); if(castedThis) return castedThis->GetValue(value); return false; } virtual void Dummy() {} }; template <typename T> class Derived : public Base { public: Derived<T>() { mValue=17; } bool GetValue(T &value) { value=mValue; return true; } T mValue; }; int main(int argc, char* argv[]) { Base *v=new Derived<int>; int i=0; if(!v->GetValue(i)) std::cout<<"Wrong type int."<<std::endl; float f=0.0; if(!v->GetValue(f)) std::cout<<"Wrong type float."<<std::endl; std::cout<<i<<std::endl<<f; char c; std::cin>>c; return 0; } Is there a better way to accomplish this?

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  • Constant template parameter class manages to link externally

    - by the_drow
    I have a class foo with an enum template parameter and for some reason it links to two versions of the ctor in the cpp file. enum Enum { bar, baz }; template <Enum version = bar> class foo { public: foo(); }; // CPP File #include "foo.hpp" foo<bar>::foo() { cout << "bar"; } foo<baz>::foo() { cout << "baz"; } I'm using msvc 2008, is this the standard behavior? Are only type template parameters cannot be linked to cpp files?

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  • In Jeditable, how do I make it so that when I click the div to edit, the text box content has initial value that is processed?

    - by TIMEX
    When the user clicks on the div, jeditable will make a text box. However, I want the initial text to be done with function stripTags(), instead of what's on the page. The reason is that I'm using some URL techniques to turn plain text links into URLs. When the user clicks on the div, jeditable is turning them into <a href=>..</a> Is there a "beforeSubmit" option in jeditable? http://www.appelsiini.net/projects/jeditable

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  • static member specialization of templated child class and templated base class

    - by b3nj1
    I'm trying to have a templated class (here C) that inherits from another templated class (here A) and perform static member specialization (of int var here), but I cant get the right syntax to do so (if it's possible #include <iostream> template<typename derived> class A { public: static int var; }; //This one works fine class B :public A<B> { public: B() { std::cout << var << std::endl; } }; template<> int A<B>::var = 9; //This one doesn't works template<typename type> class C :public A<C<type> > { public: C() { std::cout << var << std::endl; } }; //template<> template<typename type> int A<C<type> >::a = 10; int main() { B b; C<int> c; return 0; } I put an example that works with a non templated class (here B) and i can get the static member specialization of var, but for C that just doesn't work. Here is what gcc tells me : test.cpp: In constructor ‘C<type>::C()’: test.cpp:29:26: error: ‘var’ was not declared in this scope test.cpp: At global scope: test.cpp:34:18: error: template definition of non-template ‘int A<C<type> >::a’ I'm using gcc version 4.6.3, thanks for any help

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  • signature output operator overload

    - by coubeatczech
    hi, do you know, how to write signature of a function or method for operator<< for template class in C++? I want something like: template <class A class MyClass{ public: friend ostream & operator<<(ostream & os, MyClass<A mc); } ostream & operator<<(ostream & os, MyClass<A mc){ // some code return os; } But this just won't compile. Do anyone know, how to write it correctly?

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  • C++ : Swapping template class elements of different types?

    - by metamemetics
    template< class T1, class T2 > class Pair { T1 first; T2 second; }; I'm being asked to write a swap() method so that the first element becomes the second and the second the first. I have: Pair<T2,T1> swap() { return Pair<T2,T1>(second, first); } But this returns a new object rather than swapping, where I think it needs to be a void method that changes its own data members. Is this possible to do since T1 and T2 are potentially different class types? In other words I can't simply set temp=first, first=second, second=temp because it would try to convert them to different types. I'm not sure why you would potentially want to have a template object that changes order of its types as it seems that would cause confusion but that appears to be what I'm being asked to do.

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  • C++ template type deduction problem

    - by hamishmcn
    motivation: I would like to create a utility class so that instead of having to write: if( someVal == val1 || someVal == val2 || someVal == val3 ) I could instead write: if( is(someVal).in(val1, val2, val3) ) which is much closer to the mathematical 'a is an element of (b,c,d)' and also would save on a lot of typing when the variable name 'someVal' is long. Here is the code I have so far (for 2 and 3 values): template<class T> class is { private: T t_; public: is(T t) : t_(t) { } bool in(const T& v1, const T& v2) { return t_ == v1 || t_ == v2; } bool in(const T& v1, const T& v2, const T& v3) { return t_ == v1 || t_ == v2 || t_ == v3; } }; However it fails to compile if I write: is(1).in(3,4,5); instead I have to write is<int>(1).in(3,4,5); Which isn't too bad, but it would be better if somehow the compiler could figure out that the type is int with out me having to explicitly specify it. Is there anyway to do this or I am stuck with specifying it explicitly?

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  • Updating several records at once using Django

    - by 47
    I want to create a list of records with checkboxes on the left side....kinda like the inbox in Gmail. Then if a user selects some or all of these checkboxes, then the selected record(s) can be updated (only one field will be updated BTW), possibly by clicking a button. I'm stuck on how to do this though....ideas?

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  • template; Point<2, double>; Point<3, double>

    - by Oops
    Hi, I want to create my own Point struct it is only for purposes of learning C++. I have the following code: template <int dims, typename T> struct Point { T X[dims]; Point(){} Point( T X0, T X1 ) { X[0] = X0; X[1] = X1; } Point( T X0, T X1, T X2 ) { X[0] = X0; X[1] = X1; X[2] = X2; } Point<dims, int> toint() { //how to distinguish between 2D and 3D ??? Point<dims, int> ret = Point<dims, int>( (int)X[0], (int)X[1]); return ret; } std::string str(){ //how to distinguish between 2D and 3D ??? std::stringstream s; s << "{ X0: " << X[0] << " | X1: " << X[1] << " }"; return s.str(); } }; int main(void) { Point<2, double> p2d = Point<2, double>( 12.3, 45.6 ); Point<3, double> p3d = Point<3, double>( 12.3, 45.6, 78.9 ); Point<2, int> p2i = p2d.toint(); //OK Point<3, int> p3i = p3d.toint(); //m??? std::cout << p2d.str() << std::endl; //OK std::cout << p3d.str() << std::endl; //m??? std::cout << p2i.str() << std::endl; //m??? std::cout << p3i.str() << std::endl; //m??? char c; std::cin >> c; return 0; } of couse until now the output is not what I want. my questions is: how to take care of the dimensions of the Point (2D or 3D) in member functions of the Point? many thanks in advance Oops

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  • std::basic_string full specialization (g++ conflict)

    - by SoapBox
    I am trying to define a full specialization of std::basic_string< char, char_traits<char>, allocator<char> > which is typedef'd (in g++) by the <string> header. The problem is, if I include <string> first, g++ sees the typedef as an instantiation of basic_string and gives me errors. If I do my specialization first then I have no issues. I should be able to define my specialization after <string> is included. What do I have to do to be able to do that? My Code: #include <bits/localefwd.h> //#include <string> // <- uncommenting this line causes compilation to fail namespace std { template<> class basic_string< char, char_traits<char>, allocator<char> > { public: int blah() { return 42; } size_t size() { return 0; } const char *c_str() { return ""; } void reserve(int) {} void clear() {} }; } #include <string> #include <iostream> int main() { std::cout << std::string().blah() << std::endl; } The above code works fine. But, if I uncomment the first #include <string> line, I get the following compiler errors: blah.cpp:7: error: specialization of ‘std::basic_string<char, std::char_traits<char>, std::allocator<char> >’ after instantiation blah.cpp:7: error: redefinition of ‘class std::basic_string<char, std::char_traits<char>, std::allocator<char> >’ /usr/include/c++/4.4/bits/stringfwd.h:52: error: previous definition of ‘class std::basic_string<char, std::char_traits<char>, std::allocator<char> >’ blah.cpp: In function ‘int main()’: blah.cpp:22: error: ‘class std::string’ has no member named ‘blah’ Line 52 of /usr/include/c++/4.4/bits/stringfwd.h: template<typename _CharT, typename _Traits = char_traits<_CharT>, typename _Alloc = allocator<_CharT> > class basic_string; As far as I know this is just a forward delcaration of the template, NOT an instantiation as g++ claims. Line 56 of /usr/include/c++/4.4/bits/stringfwd.h: typedef basic_string<char> string; As far as I know this is just a typedef, NOT an instantiation either. So why are these lines conflicting with my code? What can I do to fix this other than ensuring that my code is always included before <string>?

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  • C++ creating generic template function specialisations

    - by Fire Lancer
    I know how to specialise a template function, however what I want to do here is specialise a function for all types which have a given method, eg: template<typename T> void foo(){...} template<typename T, if_exists(T::bar)>void foo(){...}//always use this one if the method T::bar exists T::bar in my classes is static and has different return types. I tried doing this by having an empty base class ("class HasBar{};") for my classes to derive from and using boost::enable_if with boost::is_base_of on my "specialised" version. However the problem then is that for classes that do have bar, the compiler cant resolve which one to use :(. template<typename T> typename boost::enable_if<boost::is_base_of(HasBar, T>, void>::type f() {...} I know that I could use boost::disable_if on the "normal" version, however I do not control the normal version (its provided by a third party library and its expected for specialisations to be made, I just don't really want to make explicit specialisations for my 20 or so classes), nor do I have that much control over the code using these functions, just the classes implementing T::bar and the function that uses it. Is there some way to tell the compiler to "always use this version if possible no matter what" without altering the other versions?

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