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  • Determining if types alias to the same underlying type in C++

    - by emchristiansen
    I'd like to write a templated function which changes its behavior depending on template class types passed in. To do this, I'd like to determine the type passed in. For example, something like this: template <class T> void foo() { if (T == int) { // Sadly, this sort of comparison doesn't work printf("Template parameter was int\n"); } else if (T == char) { printf("Template parameter was char\n"); } } Is this possible?

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  • What is causing this template-related compile error? (c++)

    - by Setien
    When I try to compile this: #include <map> #include <string> template <class T> class ZUniquePool { typedef std::map< int, T* > ZObjectMap; ZObjectMap m_objects; public: T * Get( int id ) { ZObjectMap::const_iterator it = m_objects.find( id ); if( it == m_objects.end() ) { T * p = new T; m_objects[ id ] = p; return p; } return m_objects[ id ]; } }; int main( int argc, char * args ) { ZUniquePool< std::string > pool; return 0; } I get this: main.cpp: In member function ‘T* ZUniquePool<T>::Get(int)’: main.cpp:12: error: expected `;' before ‘it’ main.cpp:13: error: ‘it’ was not declared in this scope I'm using GCC 4.2.1 on Mac OS X. It works in VS2008. I'm wondering whether it might be a variation of this problem: http://stackoverflow.com/questions/1364837/why-doesnt-this-c-template-code-compile But as my error output is only partially similar, and my code works in VS2008, I am not sure. Can anyone shed some light on what I am doing wrong?

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  • Imbricated C++ template

    - by gregseth
    I have the following pattern: template <int a, int b> class MyClass { template <int c> MyClass<a, c> &operator*(MyClass<c, b> const &other) const; }; // ../.. template <int a, int b> template <int c> MyClass<a, c> &MyClass<a, b>::operator*(MyClass<c, b> const &other) const { MyClass<a, c> result; // ..do stuff.. return result; } It doesn't compile, the error message is Error C2975. invalid template argument 'number', constant expression expected. If I replace template <int c> by template <int c, int d> and use it accordignly, it works fine. But I want d to be the same value as b. My questions: Why the example doesn't work? How can I enforce d to be the same than b? Thanks.

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  • template; operator (int)

    - by Oops
    Hi, regarding my Point struct already mentioned here: http://stackoverflow.com/questions/2794369/template-class-ctor-against-function-new-c-standard is there a chance to replace the function toint() with a cast-operator (int)? namespace point { template < unsigned int dims, typename T > struct Point { T X[ dims ]; //umm??? template < typename U > Point< dims, U > operator U() const { Point< dims, U > ret; std::copy( X, X + dims, ret.X ); return ret; } //umm??? Point< dims, int > operator int() const { Point<dims, int> ret; std::copy( X, X + dims, ret.X ); return ret; } //OK Point<dims, int> toint() { Point<dims, int> ret; std::copy( X, X + dims, ret.X ); return ret; } }; //struct Point template < typename T > Point< 2, T > Create( T X0, T X1 ) { Point< 2, T > ret; ret.X[ 0 ] = X0; ret.X[ 1 ] = X1; return ret; } }; //namespace point int main(void) { using namespace point; Point< 2, double > p2d = point::Create( 12.3, 34.5 ); Point< 2, int > p2i = (int)p2d; //äähhm??? std::cout << p2d.str() << std::endl; char c; std::cin >> c; return 0; } I think the problem is here that C++ cannot distinguish between different return types? many thanks in advance. regards Oops

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  • template specilization using member enums

    - by Altan
    struct Bar { enum { Special = 4 }; }; template<class T, int K> struct Foo {}; template<class T> struct Foo<T::Special> {}; Usage: Foo<Bar> aa; fails to compile using gcc 4.1.2 It complains about the usage of T::Special for partial specilization of Foo. If Special was a class the solution would be to a typename in front of it. Is there something equivalent to it for enums (or integers)? Thanks, Altan

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  • specyfic syntax question

    - by bua
    Hi there, Is it possible to create template to the initialization like: template <typename C> typename C::value_type fooFunction(C& c) {...}; std::vector<string> vec_instance; fooFunction(cont<0>(vec_instance)); fooFunction(cont<1>(vec_instance)); In general i'm interested is it possible to specify template using integer (ie. 0) instead of true type name. And how to achieve above?

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  • allocating extra memory for a container class.

    - by sil3nt
    Hey there, I'm writing a template container class and for the past few hours have been trying to allocate new memory for extra data that comes into the container (...hit a brick wall..:| ) template <typename T> void Container<T>::insert(T item, int index){ if ( index < 0){ cout<<"Invalid location to insert " << index << endl; return; } if (index < sizeC){ //copying original array so that when an item is //placed in the middleeverything else is shifted forward T *arryCpy = 0; int tmpSize = 0; tmpSize = size(); arryCpy = new T[tmpSize]; int i = 0, j = 0; for ( i = 0; i < tmpSize; i++){ for ( j = index; j < tmpSize; j++){ arryCpy[i] = elements[j]; } } //overwriting and placing item and location index elements[index] = item; //copying back everything else after the location at index int k = 0, l = 0; for ( k =(index+1), l=0; k < sizeC || l < (sizeC-index); k++,l++){ elements[k] = arryCpy[l]; } delete[] arryCpy; arryCpy = 0; } //seeing if the location is more than the current capacity //and hence allocating more memory if (index+1 > capacityC){ int new_capacity = 0; int current_size = size(); new_capacity = ((index+1)-capacityC)+capacityC; //variable for new capacity T *tmparry2 = 0; tmparry2 = new T[new_capacity]; int n = 0; for (n = 0; n < current_size;n++){ tmparry2[n] = elements[n]; } delete[] elements; elements = 0; //copying back what we had before elements = new T[new_capacity]; int m = 0; for (m = 0; m < current_size; m++){ elements[m] = tmparry2[m]; } //placing item elements[index] = item; } else{ elements[index] = item; } //increasing the current count sizeC++; my testing condition is Container cnt4(3); and as soon as i hit the fourth element (when I use for egsomething.insert("random",3);) it crashes and the above doesnt work. where have I gone wrong?

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  • Problems Expanding an Array in C++

    - by dxq
    I'm writing a simulation for class, and part of it involves the reproduction of organisms. My organisms are kept in an array, and I need to increase the size of the array when they reproduce. Because I have multiple classes for multiple organisms, I used a template: template <class orgType> void expandarray(orgType* oldarray, int& numitems, int reproductioncount) { orgType *newarray = new orgType[numitems+reproductioncount]; for (int i=0; i<numitems; i++) { newarray[i] = oldarray[i]; } numitems += reproductioncount; delete[] oldarray; oldarray = newarray; newarray = NULL; } However, this template seems to be somehow corrupting my data. I can run the program fine without reproduction (commenting out the calls to expandarray), but calling this function causes my program to crash. The program does not crash DURING the expandarray function, but crashes on access violation later on. I've written functions to expand an array hundreds of times, and I have no idea what I screwed up this time. Is there something blatantly wrong in my function? Does it look right to you? EDIT: Thanks for everyone's help. I can't believe I missed something so obvious. In response to using std::vector: we haven't discussed it in class yet, and as silly as it seems, I need to write code using the methods we've been taught.

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  • Detect if class has overloaded function fails on Comeau compiler

    - by Frank
    Hi Everyone, I'm trying to use SFINAE to detect if a class has an overloaded member function that takes a certain type. The code I have seems to work correctly in Visual Studio and GCC, but does not compile using the Comeau online compiler. Here is the code I'm using: #include <stdio.h> //Comeau doesnt' have boost, so define our own enable_if_c template<bool value> struct enable_if_c { typedef void type; }; template<> struct enable_if_c< false > {}; //Class that has the overloaded member function class TestClass { public: void Func(float value) { printf( "%f\n", value ); } void Func(int value) { printf( "%i\n", value ); } }; //Struct to detect if TestClass has an overloaded member function for type T template<typename T> struct HasFunc { template<typename U, void (TestClass::*)( U )> struct SFINAE {}; template<typename U> static char Test(SFINAE<U, &TestClass::Func>*); template<typename U> static int Test(...); static const bool Has = sizeof(Test<T>(0)) == sizeof(char); }; //Use enable_if_c to only allow the function call if TestClass has a valid overload for T template<typename T> typename enable_if_c<HasFunc<T>::Has>::type CallFunc(TestClass &test, T value) { test.Func( value ); } int main() { float value1 = 0.0f; int value2 = 0; TestClass testClass; CallFunc( testClass, value1 ); //Should call TestClass::Func( float ) CallFunc( testClass, value2 ); //Should call TestClass::Func( int ) } The error message is: no instance of function template "CallFunc" matches the argument list. It seems that HasFunc::Has is false for int and float when it should be true. Is this a bug in the Comeau compiler? Am I doing something that's not standard? And if so, what do I need to do to fix it?

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  • Can the template parameters of a constructor be explicitly specified?

    - by sth
    A constructor of a class can be a template function. At the point where such a constructor is called, the compiler usually looks at the arguments given to the constructor and determines the used template parameters from them. Is there also some syntax to specify the template parameters explicitly? A contrived example: struct A { template<typename T> A() {} }; Is there a way to instantiate this class? What is the syntax to explicitly specify the constructor's template parameters? My use case would be a problem were the compiler doesn't seem to find the correct templated constructor. Explicitly specifying the template parameters would probably generate more useful error messages or even resolve the problem.

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  • Magento CSS not Loading in New Template

    - by vulgarbulgar
    I have posted on the template creator's site, as well as Magento support, but no one has responded. The CSS is not loading at all on the custom theme I have installed, which is supposedly compatible with the current version of Magento. You can view the page here: shop.dearearth.net This is a fresh installation of Magento and the theme, with a fresh database. I have a feeling it should be a relatively quick fix. Thanks for looking.

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  • Specializing function template for both std::string and char*

    - by sad_man
    As the title says I want to specialize a function template for both string and char pointer, so far I did this but I can not figure out passing the string parameters by reference. #include <iostream> #include <string> template<typename T> void xxx(T param) { std::cout << "General : "<< sizeof(T) << std::endl; } template<> void xxx<char*>(char* param) { std::cout << "Char ptr: "<< strlen(param) << std::endl; } template<> void xxx<const char* >(const char* param) { std::cout << "Const Char ptr : "<< strlen(param)<< std::endl; } template<> void xxx<const std::string & >(const std::string & param) { std::cout << "Const String : "<< param.size()<< std::endl; } template<> void xxx<std::string >(std::string param) { std::cout << "String : "<< param.size()<< std::endl; } int main() { xxx("word"); std::string aword("word"); xxx(aword); std::string const cword("const word"); xxx(cword); } Also template<> void xxx<const std::string & >(const std::string & param) thing just does not working. If I rearranged the opriginal template to accept parameters as T& then the char * is required to be char * & which is not good for static text in code. Please help !

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  • C++, what does this syntax mean?

    - by aaa
    i found this in this file: http://www.boost.org/doc/libs/1_43_0/boost/spirit/home/phoenix/core/actor.hpp What does this syntax means? struct actor ... { ... template <typename T0, typename T1> typename result<actor(T0&,T1&)>::type // this line thank you

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  • C++ Iterators and inheritance

    - by jomnis
    Have a quick question about what would be the best way to implement iterators in the following: Say I have a templated base class 'List' and two subclasses "ListImpl1" and "ListImpl2". The basic requirement of the base class is to be iterable i.e. I can do: for(List<T>::iterator it = list->begin(); it != list->end(); it++){ ... } I also want to allow iterator addition e.g.: for(List<T>::iterator it = list->begin()+5; it != list->end(); it++){ ... } So the problem is that the implementation of the iterator for ListImpl1 will be different to that for ListImpl2. I got around this by using a wrapper ListIterator containing a pointer to a ListIteratorImpl with subclasses ListIteratorImpl2 and ListIteratorImpl2, but it's all getting pretty messy, especially when you need to implement operator+ in the ListIterator. Any thoughts on a better design to get around these issues?

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  • Declare variables that depend on unknown type in template functions.

    - by rem
    Suppose I'm writing a template function foo that has type parameter T. It gets an object of type T that must have method bar(). And inside foo I want to create a vector of objects of type returned by bar. In GNU C++ I can write something like that: template<typename T> void foo(T x) { std::vector<__typeof(x.bar())> v; v.push_back(x.bar()); v.push_back(x.bar()); v.push_back(x.bar()); std::cout << v.size() << std::endl; } How to do the same thing in Microsoft Visual C++? Is there some way to write this code that works in both GNU C++ and Visual C++?

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  • C++: How to require that one template type is derived from the other

    - by Will
    In a comparison operator: template<class R1, class R2> bool operator==(Manager<R1> m1, Manager<R2> m2) { return m1.internal_field == m2.internal_field; } Is there any way I could enforce that R1 and R2 must have a supertype or subtype relation? That is, I'd like to allow either R1 to be derived from R2, or R2 to be derived from R1, but disallow the comparison if R1 and R2 are unrelated types.

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  • C++ Template Question

    - by user323422
    see following code and please clear doubts1. as ABC is template why it not showing error when we put defination of ABC class member function in test.cpp 2.if i put test.cpp code in test.h , then it working fine // test.h template <typename T> class ABC { public: void foo( T& ); void bar( T& ); }; // test.cpp template <typename T> void ABC<T>::foo( T& ) {} // definition template <typename T> void ABC<T>::bar( T& ) {} // definition template void ABC<char>::foo( char & ); // 1 // main.cpp #include "test.h" int main() { ABC<char> a; a.foo(); // working a.bar(); // link error }

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  • template function error..

    - by sil3nt
    Hi there, I have function which takes in an parameter of a class called "Triple", and am returning the averge of 3 values of type float. template <typename ElemT> float average(Triple ElemT<float> &arg){ float pos1 = arg.getElem(1); float pos2 = arg.getElem(2); float pos3 = arg.getElem(3); return ( (pos1+pos2+po3) /3 ); } when i try compiling this i get q2b.cpp:32: error: template declaration of `float average' q2b.cpp:32: error: missing template arguments before "ElemT" not quite sure what this means.

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  • Class templating std::set key types

    - by TomFLuff
    I have a class to evaluate set algebra but wish to template it. At the minute it looks a bit like this set.h: template<typename T> class SetEvaluation { public: SetEvaluation<T>(); std::set<T> evaluate(std::string in_expression); } set.cpp template<typename T> std::set<T> SetEvaluation<T>::evaluate(std::string expression) { std::set<T> result; etc etc... } But i'm getting undefined reference errors when compiling. Is it possible to declare the return type as std::set<T> and then pass std::string as the class template param. There are no errors in the class but only when I try to instantiate SetEvaluation<std::string> Can anyone shed light on this problem? thanks

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  • Usage of CRTP in a call chain

    - by fhw72
    In my widget library I'd like to implement some kind of call chain to initialize a user supplied VIEW class which might(!) be derived from another class which adds some additional functionality like this: #include <iostream> template<typename VIEW> struct App { VIEW view; void init() {view.initialize(); } }; template<typename DERIVED> struct SpecializedView { void initialize() { std::cout << "SpecializedView" << std::endl; static_cast<DERIVED*>(this)->initialize(); } }; struct UserView : SpecializedView<UserView> { void initialize() {std::cout << "UserView" << std::endl; } }; int _tmain(int argc, _TCHAR* argv[]) { // Cannot be altered to: App<SpecializedView<UserView> > app; App<UserView> app; app.init(); return 0; } Is it possible to achieve some kind of call chain (if the user supplied VIEW class is derived from "SpecializedView") such that the output will be: console output: SpecializedView UserView Of course it would be easy to instantiate variable app with the type derived from but this code is hidden in the library and should not be alterable. In other words: The library code should only get the user derived type as parameter.

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