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  • Forcing a templated object to construct from a pointer

    - by SalamiArmi
    I have a fictional class: template<typename T> class demonstration { public: demonstration(){} ... T *m_data; } At some point in the program's execution, I want to set m_data to a big block of allocated memory and construct an object T there. At the moment, I've been using this code: void construct() { *m_data = T(); } Which I've now realised is probably not the best idea... wont work under certain cirumstances, if T has a private assignment operator for example. Is there a normal/better way to do what I'm attempting here?

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  • Constructor initializer list: code from the C++ Primer, chapter 16

    - by Alexandros Gezerlis
    Toward the end of Chapter 16 of the "C++ Primer" I encountered the following code (I've removed a bunch of lines): class Sales_item { public: // default constructor: unbound handle Sales_item(): h() { } private: Handle<Item_base> h; // use-counted handle }; My problem is with the Sales_item(): h() { } line. For the sake of completeness, let me also quote the parts of the Handle class template that I think are relevant to my question (I think I don't need to show the Item_base class): template <class T> class Handle { public: // unbound handle Handle(T *p = 0): ptr(p), use(new size_t(1)) { } private: T* ptr; // shared object size_t *use; // count of how many Handles point to *ptr }; I would have expected something like either: a) Sales_item(): h(0) { } which is a convention the authors have used repeatedly in earlier chapters, or b) Handle<Item_base>() if the intention was to invoke the default constructor of the Handle class. Instead, what the book has is Sales_item(): h() { }. My gut reaction is that this is a typo, since h() looks suspiciously similar to a function declaration. On the other hand, I just tried compiling under g++ and running the example code that uses this class and it seems to be working correctly. Any thoughts?

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  • C++ Template Class Constructor with Variable Arguments

    - by david
    Is it possible to create a template function that takes a variable number of arguments, for example, in this Vector< T, C class constructor: template < typename T, uint C > Vector< T, C >::Vector( T, ... ) { assert( C > 0 ); va_list arg_list; va_start( arg_list, C ); for( uint i = 0; i < C; i++ ) { m_data[ i ] = va_arg( arg_list, T ); } va_end( arg_list ); } This almost works, but if someone calls Vector< double, 3 ( 1, 1, 1 ), only the first argument has the correct value. I suspect that the first parameter is correct because it is cast to a double during the function call, and that the others are interpreted as ints and then the bits are stuffed into a double. Calling Vector< double, 3 ( 1.0, 1.0, 1.0 ) gives the desired results. Is there a preferred way to do something like this?

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  • C++. How to define template parameter of type T for class A when class T needs a type A template parameter?

    - by jaybny
    Executor class has template of type P and it takes a P object in constructor. Algo class has a template E and also has a static variable of type E. Processor class has template T and a collection of Ts. Question how can I define Executor< Processor<Algo> > and Algo<Executor> ? Is this possible? I see no way to defining this, its kind of an "infinite recursive template argument" See code. template <class T> class Processor { map<string,T> ts; void Process(string str, int i) { ts[str].Do(i); } } template <class P> class Executor { Proc &p; Executor(P &p) : Proc(p) {} void Foo(string str, int i) { p.Process(str,i); } Execute(string str) { } } template <class E> class Algo { static E e; void Do(int i) {} void Foo() { e.Execute("xxx"); } } main () { typedef Processor<Algo> PALGO; // invalid typedef Executor<PALGO> EPALGO; typedef Algo<EPALGO> AEPALGO; Executor<PALGO> executor(PALGO()); AEPALGO::E = executor; }

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  • C++ class member functions instantiated by traits

    - by Jive Dadson
    I am reluctant to say I can't figure this out, but I can't figure this out. I've googled and searched Stack Overflow, and come up empty. The abstract, and possibly overly vague form of the question is, how can I use the traits-pattern to instantiate non-virtual member functions? The question came up while modernizing a set of multivariate function optimizers that I wrote more than 10 years ago. The optimizers all operate by selecting a straight-line path through the parameter space away from the current best point (the "update"), then finding a better point on that line (the "line search"), then testing for the "done" condition, and if not done, iterating. There are different methods for doing the update, the line-search, and conceivably for the done test, and other things. Mix and match. Different update formulae require different state-variable data. For example, the LMQN update requires a vector, and the BFGS update requires a matrix. If evaluating gradients is cheap, the line-search should do so. If not, it should use function evaluations only. Some methods require more accurate line-searches than others. Those are just some examples. The original version instantiates several of the combinations by means of virtual functions. Some traits are selected by setting mode bits that are tested at runtime. Yuck. It would be trivial to define the traits with #define's and the member functions with #ifdef's and macros. But that's so twenty years ago. It bugs me that I cannot figure out a whiz-bang modern way. If there were only one trait that varied, I could use the curiously recurring template pattern. But I see no way to extend that to arbitrary combinations of traits. I tried doing it using boost::enable_if, etc.. The specialized state information was easy. I managed to get the functions done, but only by resorting to non-friend external functions that have the this-pointer as a parameter. I never even figured out how to make the functions friends, much less member functions. The compiler (VC++ 2008) always complained that things didn't match. I would yell, "SFINAE, you moron!" but the moron is probably me. Perhaps tag-dispatch is the key. I haven't gotten very deeply into that. Surely it's possible, right? If so, what is best practice?

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  • C++ CRTP(template pattern) question

    - by aaa
    following piece of code does not compile, the problem is in T::rank not be inaccessible (I think) or uninitialized in parent template. Can you tell me exactly what the problem is? is passing rank explicitly the only way? or is there a way to query tensor class directly? Thank you #include <boost/utility/enable_if.hpp> template<class T, // size_t N, class enable = void> struct tensor_operator; // template<class T, size_t N> template<class T> struct tensor_operator<T, typename boost::enable_if_c< T::rank == 4>::type > { tensor_operator(T &tensor) : tensor_(tensor) {} T& operator()(int i,int j,int k,int l) { return tensor_.layout.element_at(i, j, k, l); } T &tensor_; }; template<size_t N, typename T = double> // struct tensor : tensor_operator<tensor<N,T>, N> { struct tensor : tensor_operator<tensor<N,T> > { static const size_t rank = N; }; I know the workaround, however am interested in mechanics of template instantiation for self-education

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  • C++ Function Template With Flexible Return Type

    - by Ignatius Reza
    Let's say that we have a function like so template <class T, class T2> T getMin(T a, T2 b) { if(a < b) return a; return b; } if we call the function like so int a, b; long c; a = getMin(b, c); if c is < a, then the value of c will be type casted to int. Is it possible to make the return type flexible so that it would return an int, or long, or any other type considered smaller by "<" without being type casted?

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  • Why does a Silverlight application show a blank browser screen when created from exported template?

    - by Edward Tanguay
    I created a silverlight app (without website) named TestApp, with one TextBox: <UserControl x:Class="TestApp.MainPage" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="480"> <Grid x:Name="LayoutRoot"> <TextBlock Text="this is a test"/> </Grid> </UserControl> I press F5 and see "this is a test" in my browser (firefox). I select File | Export Template | name it TestAppTemplate and save it. I create a new silverlight app based on the above template. The MainPage.xaml has the exact same XAML as above. I press F5 and see a blank screen in my browser. I look at the HTML source of both of these and they are identical. Everything I have compared in both projects is identical. What do I have to do so that a Silverlight application which is created from my exported template does not show a blank screen? (creating a WPF application from an exported template like this works fine)

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  • Template meta-programming with member function pointers?

    - by wheaties
    Is it possible to use member function pointers with template meta-programming? Such as: class Connection{ public: string getName() const; string getAlias() const; //more stuff }; typedef string (Connection::*Con_Func)() const; template<Con_Func _Name> class Foo{ Connection m_Connect; public: void Foo(){ cout << m_Connect.(*_Name); } }; typedef Foo<&Connection::getName> NamedFoo; typedef Foo<&Connection::getAlias> AliasFoo; Granted, this is rather contrived but is it possible? (yes, there are probably much better ways but humor me.)

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  • Cannot overload function

    - by anio
    So I've got a templatized class and I want to overload the behavior of a function when I have specific type, say char. For all other types, let them do their own thing. However, c++ won't let me overload the function. Why can't I overload this function? I really really do not want to do template specialization, because then I've got duplicate the entire class. Here is a toy example demonstrating the problem: http://codepad.org/eTgLG932 The same code posted here for your reading pleasure: #include <iostream> #include <cstdlib> #include <string> struct Bar { std::string blah() { return "blah"; } }; template <typename T> struct Foo { public: std::string doX() { return m_getY(my_t); } private: std::string m_getY(char* p_msg) { return std::string(p_msg); } std::string m_getY(T* p_msg) { return p_msg->blah(); } T my_t; }; int main(int, char**) { Foo<char> x; Foo<Bar> y; std::cout << "x " << x.doX() << std::endl; return EXIT_SUCCESS; } Thank you everyone for your suggestions. Two valid solutions have been presented. I can either specialize the doX method, or specialize m_getY() method. At the end of the day I prefer to keep my specializations private rather than public so I'm accepting Krill's answer.

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  • [C++] Multiple inheritance from template class

    - by Tom P.
    Hello, I'm having issues with multiple inheritance from different instantiations of the same template class. Specifically, I'm trying to do this: template <class T> class Base { public: Base() : obj(NULL) { } virtual ~Base() { if( obj != NULL ) delete obj; } template <class T> T* createBase() { obj = new T(); return obj; } protected: T* obj; }; class Something { // ... }; class SomethingElse { // ... }; class Derived : public Base<Something>, public Base<SomethingElse> { }; int main() { Derived* d = new Derived(); Something* smth1 = d->createBase<Something>(); SomethingElse* smth2 = d->createBase<SomethingElse>(); delete d; return 0; } When I try to compile the above code, I get the following errors: 1>[...](41) : error C2440: '=' : cannot convert from 'SomethingElse *' to 'Something *' 1> Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast 1> [...](71) : see reference to function template instantiation 'T *Base<Something>::createBase<SomethingElse>(void)' being compiled 1> with 1> [ 1> T=SomethingElse 1> ] 1>[...](43) : error C2440: 'return' : cannot convert from 'Something *' to 'SomethingElse *' 1> Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast The issue seems to be ambiguity due to member obj being inherited from both Base< Something and Base< SomethingElse , and I can work around it by disambiguating my calls to createBase: Something* smth1 = d->Base<Something>::createBase<Something>(); SomethingElse* smth2 = d->Base<SomethingElse>::createBase<SomethingElse>(); However, this solution is dreadfully impractical, syntactically speaking, and I'd prefer something more elegant. Moreover, I'm puzzled by the first error message. It seems to imply that there is an instantiation createBase< SomethingElse in Base< Something , but how is that even possible? Any information or advice regarding this issue would be much appreciated.

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  • How to generate a line break in Django template

    - by Iamamac
    I want to give default value to a textarea. The code is something like this: <textarea>{{userSetting.list | join:"NEWLINE"}}</textarea> where userSetting.list is a string list, each item of whom is expected to show in one line. textarea takes the content between the tags as the default value, preserving its line breaks and not interpreting any HTML tags (which means <br>,\n won't work). I have found a solution: {{userSetting.list | join:" " | wordwrap:0}} (there is no whitespace in the list). But obviously it is NOT a good one. Any help would be appreciated.

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  • template function roundTo int, float -> truncation

    - by Oops
    Hi, according to this question: http://stackoverflow.com/questions/2833730/calling-template-function-without-type-inference the round function I will use in the future now looks like: template < typename TOut, typename TIn > TOut roundTo( TIn value ) { return static_cast<TOut>( value + 0.5 ); } double d = 1.54; int i = rountTo<int>(d); However it makes sense only if it will be used to round to integral datatypes like char, short, int, long, long long int, and it's unsigned counterparts. If it ever will be used with a TOut As float or long double it will deliver s***. double d = 1.54; float f = roundTo<float>(d); // aarrrgh now float is 2.04; I was thinking of a specified overload of the function but ... that's not possible... How would you solve this problem? many thanks in advance Oops

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  • Template deduction based on return type?

    - by Marlon
    I'd like to be able to use template deduction to achieve the following: GCPtr<A> ptr1 = GC::Allocate(); GCPtr<B> ptr2 = GC::Allocate(); instead of (what I currently have): GCPtr<A> ptr1 = GC::Allocate<A>(); GCPtr<B> ptr2 = GC::Allocate<B>(); My current Allocate function looks like this: class GC { public: template <typename T> static GCPtr<T> Allocate(); }; Would this be possible to knock off the extra < A and < B? Thanks

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  • overloading new/delete problem

    - by hidayat
    This is my scenario, Im trying to overload new and delete globally. I have written my allocator class in a file called allocator.h. And what I am trying to achieve is that if a file is including this header file, my version of new and delete should be used. So in a header file "allocator.h" i have declared the two functions extern void* operator new(std::size_t size); extern void operator delete(void *p, std::size_t size); I the same header file I have a class that does all the allocator stuff, class SmallObjAllocator { ... }; I want to call this class from the new and delete functions and I would like the class to be static, so I have done this: template<unsigned dummy> struct My_SmallObjectAllocatorImpl { static SmallObjAllocator myAlloc; }; template<unsigned dummy> SmallObjAllocator My_SmallObjectAllocatorImpl<dummy>::myAlloc(DEFAULT_CHUNK_SIZE, MAX_OBJ_SIZE); typedef My_SmallObjectAllocatorImpl<0> My_SmallObjectAllocator; and in the cpp file it looks like this: allocator.cc void* operator new(std::size_t size) { std::cout << "using my new" << std::endl; if(size > MAX_OBJ_SIZE) return malloc(size); else return My_SmallObjectAllocator::myAlloc.allocate(size); } void operator delete(void *p, std::size_t size) { if(size > MAX_OBJ_SIZE) free(p); else My_SmallObjectAllocator::myAlloc.deallocate(p, size); } The problem is when I try to call the constructor for the class SmallObjAllocator which is a static object. For some reason the compiler are calling my overloaded function new when initializing it. So it then tries to use My_SmallObjectAllocator::myAlloc.deallocate(p, size); which is not defined so the program crashes. So why are the compiler calling new when I define a static object? and how can I solve it?

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  • How can variadic char template arguments from user defined literals be converted back into numeric types?

    - by Pubby
    This question is being asked because of this one. C++11 allows you to define literals like this for numeric literals: template<char...> OutputType operator "" _suffix(); Which means that 503_suffix would become <'5','0','3'> This is nice, although it isn't very useful in the form it's in. How can I transform this back into a numeric type? This would turn <'5','0','3'> into a constexpr 503. Additionally, it must also work on floating point literals. <'5','.','3> would turn into int 5 or float 5.3 A partial solution was found in the previous question, but it doesn't work on non-integers: template <typename t> constexpr t pow(t base, int exp) { return (exp > 0) ? base * pow(base, exp-1) : 1; }; template <char...> struct literal; template <> struct literal<> { static const unsigned int to_int = 0; }; template <char c, char ...cv> struct literal<c, cv...> { static const unsigned int to_int = (c - '0') * pow(10, sizeof...(cv)) + literal<cv...>::to_int; }; // use: literal<...>::to_int // literal<'1','.','5'>::to_int doesn't work // literal<'1','.','5'>::to_float not implemented

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  • Visual Studio - How to use an existing vsproj's project settings as a template for new project?

    - by Jakobud
    There is some software I want to write a plugin for. The software includes some sample plugins. I want to create a new fresh project but I want to use one of the sample plugin vsproj's project settings as a template. It doesn't seem very clear on how to do this. If I do "New Project From Existing Code" that only imports the cpp, h, etc files into the new project. Right now the only way I can see to copy a sample projects settings is to open two instances of VS2005 next to each other and simply mimic the settings... Surely there is a built in method of doing this?

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  • How do i return a template class from a template function?

    - by LoudNPossiblyRight
    It looks logical but for some reason when i uncomment the last cout line, this code does not compile. How do i return a template class? What do i have to do to this code to make it work? #include<iostream> using namespace std; template <int x> class someclass{ public: int size; int intarr[x]; someclass():size(x){} }; template<int x, int y> int somefunc(someclass<x> A, someclass<y> B){ return ( A.size > B.size ? A.size : B.size); } template<int x, int y, int z> someclass<x> anotherfunc(someclass<y> A, someclass<z> B){ return ( A.size > B.size ? A : B); } int main(){ someclass<5> A; someclass<10> B; cout << "SIZE = " << somefunc(A,B) << endl; //cout << "SIZE = " << (anotherfunc(A,B)).size << endl; //THIS DOES NOT COMPILE return 0; }

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  • Function template accepting nothing less than a bidirectional iterator or a pointer

    - by san
    I need a function template that accepts two iterators that could be pointers. If the two arguments are random_access iterators I want the return type to be an object of std::iterator<random_access_iterator_tag, ...> type else a std::iterator<bidirectional_iterator_tag, ...> type. I also want the code to refuse compilation if the arguments are neither a bidirectional iterator, nor a pointer. I cannot have dependency on third party libraries e.g. Boost Could you help me with the signature of this function so that it accepts bidirectional iterators as well as pointers, but not say input_iterator, output_iterator, forward_iterators. One partial solution I can think of is the following template<class T> T foo( T iter1, T iter2) { const T tmp1 = reverse_iterator<T>(iter1); const T tmp2 = reverse_iterator<T>(iter2); // do something } The idea is that if it is not bidirectional the compiler will not let me construct a reverse_iterator from it.

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  • Notepad++ Question

    - by Shane
    I would like a hotkey to Save All in Notepad++ then switch to the browser and refresh the page. I have to do this thousands of times a day and doing the Save All hotkey, then Alt-Tab to the right window, then F5 to refresh again and again, there must be a better solution? I am open to switching editors if there is another that can do this. I have tried using run commands but the problem is although it allows you to make something like [firefox.exe "current file"], if you come down to wanting to open it at a web address like localhost because it needs to run server side, then you have to manually specify the other part of the string, like [chrome.exe "custom var(web address) / filename"] and it always opens a new tab, then creates lots of clutter. Also, if you are editing an include file then it will try to open that instead of the page you want to see when you use the run command, a refresh would be the only realistic method I can think of using.

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  • C++ ambiguous template instantiation

    - by aaa
    the following gives me ambiguous template instantiation with nvcc (combination of EDG front-end and g++). Is it really ambiguous, or is compiler wrong? I also post workaround à la boost::enable_if template<typename T> struct disable_if_serial { typedef void type; }; template<> struct disable_if_serial<serial_tag> { }; template<int M, int N, typename T> __device__ //static typename disable_if_serial<T>::type void add_evaluate_polynomial1(double *R, const double (&C)[M][N], double x, const T &thread) { // ... } template<size_t M, size_t N> __device__ static void add_evaluate_polynomial1(double *R, const double (&C)[M][N], double x, const serial_tag&) { for (size_t i = 0; i < M; ++i) add_evaluate_polynomial1(R, C, x, i); } // ambiguous template instantiation here. add_evaluate_polynomial1(R, C, x, serial_tag());

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  • fancybox image sometimes renders outside box

    - by Colleen
    I have the following django template: <script type="text/javascript" src="{{ STATIC_URL }}js/ jquery.fancybox-1.3.4.pack.js"></script> <link rel="stylesheet" href="{{ STATIC_URL }}css/ jquery.fancybox-1.3.4.css" type="text/css" media="screen" /> {% include "submission-form.html" with section="photos" %} <div class="commentables"> {% load thumbnail %} {% for story in objects %} <div class="image {% if forloop.counter|add:"-1"| divisibleby:picsinrow %}left{% else %}{% if forloop.counter| divisibleby:picsinrow %}right{% else %}middle{% endif %}{% endif %}"> {% if story.image %} {% thumbnail story.image size crop="center" as full_im %} <a rel="gallery" href="{% url post slug=story.slug %}"> <img class="preview" {% if story.title %} alt="{{ story.title }}" {% endif %} src="{{ full_im.url }}" full- image="{% if story.image_url %}{{ story.image_url }}{% else %} {{ story.image.url }}{% endif %}"> </a> {% endthumbnail %} {% else %} {% if story.image_url %} {% thumbnail story.image_url size crop="center" as full_im %} <a rel="gallery" href="{% url post slug=story.slug %}"> <img class="preview" {% if story.title %} alt="{{ story.title }}" {% endif %} src="{{ full_im.url }}" full- image="{{ story.image_url }}"> </a> {% endthumbnail %} {% endif %} {% endif %} </div> {% endfor %} {% if rowid != "last" %} <br style="clear: both" /> {% endif %} {% if not no_more_button %} <p style="text-align: right;" class="more-results"><a href="{% url images school_slug tag_slug %}">more...</a></p> {% endif %} </div> <script> $(document).ready(function(){ function changeattr(e){ var f = $(e.clone()); $(f.children()[0]).attr('src', $(f.children() [0]).attr("full-image")); $(f.children()[0]).attr('height', '500px'); return f[0].outerHTML; } $('.image a').each(function(idx, elem) { var e = $(elem); e.fancybox({ title: $(e.children()[0]).attr('alt'), content: changeattr(e) }); }); }); </script> and I'm occasionally getting weird display errors where the box will either not render anything at all (so it will show up as just a thin white bar, basically) or it will render only about 30 px wide, and position itself halfway down the page. In both cases, if I inspect element, I can see the "shadow" of the full picture, at the right size, with the right url. Image source doesnt' seem to make a difference, I'm getting no errors, and this is happening in both chrome and firefox. Does anyone have any ideas?

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  • Strange Template error : error C2783: could not deduce template argument

    - by osum
    Hi, I have created a simple function with 2 diffrernt template arguments t1, t2 and return type t3. So far no compilation error. But when Itry to call the function from main, I encounter error C2783. I needed to know If the following code is legally ok? If not how is it fixed? please help! template <typename t1, typename t2, typename t3> t3 adder1 (t1 a , t2 b) { return int(a + b); }; int main() { int sum = adder1(1,6.0); // error C2783 could not deduce template argument for t3 return 0; }

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