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  • Cannot initialize non-const reference from convertible type

    - by Julien L.
    Hi, I cannot initialize a non-const reference to type T1 from a convertible type T2. However, I can with a const reference. long l; const long long &const_ref = l; // fine long long &ref = l; // error: invalid initialization of reference of // type 'long long int&' from expression of type // 'long int' Most problems I encountered were related to r-values that cannot be assigned to a non-const reference. This is not the case here -- can someone explain? Thanks.

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  • How to const declare the this pointer sent as parameter

    - by Tomas
    Hi, I want to const declare the this pointer received as an argument. static void Class::func(const OtherClass *otherClass) { // use otherClass pointer to read, but not write to it. } It is being called like this: void OtherClass::func() { Class::func(this); } This does not compile nad if i dont const declare the OtherClass pointer, I can change it. Thanks.

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  • Const Functions and Interfaces in C++

    - by 58gh1z
    I'll use the following (trivial) interface as an example: struct IObject { virtual ~IObject() {} virtual std::string GetName() const = 0; virtual void ChangeState() = 0; }; Logic dictates that GetName should be a const member function while ChangeState shouldn't. All code that I've seen so far doesn't follow this logic, though. That is, GetName in the example above wouldn't be marked as a const member function. Is this laziness/carelessness or is there a legitimate reason for this? What are the major cons of me forcing my clients to implement const member functions when they are logically called for?

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  • How to iterate properly across a const set?

    - by Jared
    I'm working on a program that's supposed to represent a graph. My issue is in my printAdjacencyList function. Basically, I have a Graph ADT that has a member variable "nodes", which is a map of the nodes of that graph. Each Node has a set of Edge* to the edges it is connected to. I'm trying to iterate across each node in the graph and each edge of a node. void MyGraph::printAdjacencyList() { std::map<std::string, MyNode*>::iterator mit; std::set<MyEdge*>::iterator sit; for (mit = nodes.begin(); mit != nodes.end(); mit++ ) { std::cout << mit->first << ": {"; const std::set<MyEdge*> edges = mit->second->getEdges(); for (sit = edges.begin(); sit != edges.end(); sit++) { std::pair<MyNode*, MyNode*> edgeNodes = *sit->getEndpoints(); } } std::cout << " }" << std::endl; } getEdges is declared as: const std::set<MyEdge*>& getEdges() { return edges; }; and get Endpoints is declared as: const std::pair<MyNode*, MyNode*>& getEndpoints() { return nodes; }; The compiler error I'm getting is: MyGraph.cpp:63: error: request for member `getEndpoints' in `*(&sit)->std::_Rb_tree_const_iterator<_Tp>::operator-> [with _Tp = MyEdge*]()', which is of non-class type `MyEdge* const' MyGraph.cpp:63: warning: unused variable 'edgeNodes' I have figured out that this probably means I'm misusing const somewhere, but I can't figure out where for the life of me. Any information would be appreciated. Thanks!

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  • Converting a const char* into a double

    - by Koning Baard
    I am trying to convert a const char* to a double precision floating point number: int main(const int argc, const char *argv[]) { int i; double numbers[argc - 1]; for(i = 1; i < argc, i += 1) { /* -- Convert each argv into a double and put it in `number` */ } /* ... */ return 0; } Can anyone help me? Thanks

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  • How to push_back without operator=() for const members?

    - by WilliamKF
    How to push_back() to a C++ std::vector without using operator=() for which the default definition violates having const members? struct Item { Item(int value) : _value(value) { } const int _value; } vector<Item> items; items.push_back(Item(3)); I'd like to keep the _value const since it should not change after the object is constructed, so the question is how do I initialize my vector with elements without invoking operator=()?

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  • const keyword in Objective-c

    - by user392412
    int main(int argc, char *argv[]) { @autoreleasepool { const int x = 1; const NSMutableArray *array1 = [NSMutableArray array]; const NSMutableString *str1 = @"1"; NSString * const str2 = @"2"; // x = 2; compile error [array1 addObject:@"2"]; // ok // [str1 appendString:@"2"]; // runtime error // Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attempt to mutate immutable object with appendString:' // str2 = @"3"; compile error } } my Question is Why array1 addObject is legal and why str1 appendString is forbidden?

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  • Template neglects const (why?)

    - by Gabriel
    Does somebody know, why this compiles?? template< typename TBufferTypeFront, typename TBufferTypeBack = TBufferTypeFront> class FrontBackBuffer{ public: FrontBackBuffer( const TBufferTypeFront front, const TBufferTypeBack back): ////const reference assigned to reference??? m_Front(front), m_Back(back) { }; ~FrontBackBuffer() {}; TBufferTypeFront m_Front; ///< The front buffer TBufferTypeBack m_Back; ///< The back buffer }; int main(){ int b; int a; FrontBackBuffer<int&,int&> buffer(a,b); // buffer.m_Back = 33; buffer.m_Front = 55; } I compile with GCC 4.4. Why does it even let me compile this? Shouldn't there be an error that I cannot assign a const reference to a non-const reference?

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  • Const-Qualification of Main's Parameters in C++

    - by pt2cv
    The C++ standard mandates that all conforming implementations support the following two signatures for main: int main(); int main(int, char*[]); In case of the latter signature, would the addition of (top-level) const-ness break any language rules? For example: int main(const int argc, char** const argv); From my understanding, top-level const qualification doesn't affect the function's signature hash, so it should be legal as far as the specification is concerned. Also, did anyone ever encounter an implementation which rejected this type of modification?

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  • Const Discards Qualifers: C++

    - by user991673
    I'm using OpenGL to render camera perspectives, and a one point in my code I'm trying to take the direction of the light (shown here as type "Vector4") and multiply it by a matrix of type "Matrix4x4" that represents the Modelview transformation (sorry if this is not making any sense, this is for a school project, as such I'm still learning about this stuff) Anyway, my code goes as follows... Vector4 lightDirection = data->dir * follow->getModelviewMatrix().getInverse().getTranspose(); data->dir = lightDirection; setLight(*data); this give me the following error: passing 'const vec4<double>' as 'this' argument of 'vec4<T>& vec4<T>::operator=(const vec4<T>&)[with T = double]' discards qualifiers Again, much of this code is prewritten for the class (namely the vector and matrix types) but if someone could just help me decipher what the error means it would be much appreciated! I can give more information as needed. I figured 'data' or 'data-dir' were const, however I can find no mention of either of them to be. 'dir' is of type SceneLightData, and when its added on I'm doing this: void Scene::addLight(const SceneLightData &sceneLight) { SceneLightData light = sceneLight; m_lights.push_back(&light); } The error occurs on this line: data->dir = lightDirection; EDIT problem solved. thanks everyone! solution: void Scene::addLight(const SceneLightData &sceneLight) { SceneLightData* light = new SceneLightData; *light = sceneLight; m_lights.push_back(light); } and SceneLightData* data = m_lights[i]; data->dir = data->dir * follow->getModelviewMatrix().getInverse().getTranspose(); setLight(*data);

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  • C++: combine const with template arguments

    - by awn
    The following example is working when I manualy replace T wirh char *, but why is not working as it is: template <typename T> class A{ public: A(const T _t) { } }; int main(){ const char * c = "asdf"; A<char *> a(c); } When compiling with gcc, I get this error: test.cpp: In function 'int main()': test.cpp:10: error: invalid conversion from 'const char*' to 'char*' test.cpp:10: error: initializing argument 1 of 'A<T>::A(T) [with T = char*]'

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  • 47 memory leaks. STL pointers.

    - by icelated
    I have a major amount of memory leaks. I know that the Sets have pointers and i cannot change that! I cannot change anything, but clean up the mess i have... I am creating memory with new in just about every function to add information to the sets. I have a Cd/ DVD/book: super classes of ITEM class and a library class.. In the library class i have 2 functions for cleaning up the sets.. Also, the CD, DVD, book destructors are not being called.. here is my potential leaks.. library.h #pragma once #include <ostream> #include <map> #include <set> #include <string> #include "Item.h" using namespace std; typedef set<Item*> ItemSet; typedef map<string,Item*> ItemMap; typedef map<string,ItemSet*> ItemSetMap; class Library { public: // general functions void addKeywordForItem(const Item* const item, const string& keyword); const ItemSet* itemsForKeyword(const string& keyword) const; void printItem(ostream& out, const Item* const item) const; // book-related functions const Item* addBook(const string& title, const string& author, int const nPages); const ItemSet* booksByAuthor(const string& author) const; const ItemSet* books() const; // music-related functions const Item* addMusicCD(const string& title, const string& band, const int nSongs); void addBandMember(const Item* const musicCD, const string& member); const ItemSet* musicByBand(const string& band) const; const ItemSet* musicByMusician(const string& musician) const; const ItemSet* musicCDs() const; // movie-related functions const Item* addMovieDVD(const string& title, const string& director, const int nScenes); void addCastMember(const Item* const movie, const string& member); const ItemSet* moviesByDirector(const string& director) const; const ItemSet* moviesByActor(const string& actor) const; const ItemSet* movies() const; ~Library(); void Purge(ItemSet &set); void Purge(ItemSetMap &map); }; here is some functions for adding info using new in library. Keep in mind i am cutting out alot of code to keep this post short. library.cpp #include "Library.h" #include "book.h" #include "cd.h" #include "dvd.h" #include <iostream> // general functions ItemSet allBooks; ItemSet allCDS; ItemSet allDVDs; ItemSetMap allBooksByAuthor; ItemSetMap allmoviesByDirector; ItemSetMap allmoviesByActor; ItemSetMap allMusicByBand; ItemSetMap allMusicByMusician; const ItemSet* Library::itemsForKeyword(const string& keyword) const { const StringSet* kw; ItemSet* obj = new ItemSet(); return obj; const Item* Library::addBook(const string& title, const string& author, const int nPages) { ItemSet* obj = new ItemSet(); Book* item = new Book(title,author,nPages); allBooks.insert(item); // add to set of all books obj->insert(item); return item; const Item* Library::addMusicCD(const string& title, const string& band, const int nSongs) { ItemSet* obj = new ItemSet(); CD* item = new CD(title,band,nSongs); return item; void Library::addBandMember(const Item* musicCD, const string& member) { ItemSet* obj = new ItemSet(); (((CD*) musicCD)->addBandMember(member)); obj->insert((CD*) musicCD); here is the library destructor..... Library::~Library() { Purge(allBooks); Purge(allCDS); Purge(allDVDs); Purge(allBooksByAuthor); Purge(allmoviesByDirector); Purge(allmoviesByActor); Purge(allMusicByBand); Purge(allMusicByMusician); } void Library::Purge(ItemSet &set) { for (ItemSet::iterator it = set.begin(); it != set.end(); ++it) delete *it; set.clear(); } void Library::Purge(ItemSetMap &map) { for (ItemSetMap::iterator it = map.begin(); it != map.end(); ++it) delete it->second; map.clear(); } so, basically item, cd, dvd class all have a set like this: typedef set<string> StringSet; class CD : public Item StringSet* music; and i am deleting it like: but those superclasses are not being called.. Item destructor is. CD::~CD() { delete music; } Do, i need a copy constructor? and how do i delete those objects i am creating in the library class? and how can i get the cd,dvd, destructor called? would the addbandmember function located in the library.cpp cause me to have a copy constructor? Any real help you can provide me to help me clean up this mess instead of telling me not to use pointers in my sets i would really appreciate. How can i delete the memory i am creating in those functions? I cannot delete them in the function!!

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  • Beware of const members

    - by nmarun
    I happened to learn a new thing about const today and how one needs to be careful with its usage. Let’s say I have a third-party assembly ‘ConstVsReadonlyLib’ with a class named ConstSideEffect.cs: 1: public class ConstSideEffect 2: { 3: public static readonly int StartValue = 10; 4: public const int EndValue = 20; 5: } In my project, I reference the above assembly as follows: 1: static void Main(string[] args) 2: { 3: for (int i = ConstSideEffect.StartValue; i < ConstSideEffect.EndValue; i++) 4: { 5: Console.WriteLine(i); 6: } 7: Console.ReadLine(); 8: } You’ll see values 10 through 19 as expected. Now, let’s say I receive a new version of the ConstVsReadonlyLib. 1: public class ConstSideEffect 2: { 3: public static readonly int StartValue = 5; 4: public const int EndValue = 30; 5: } If I just drop this new assembly in the bin folder and run the application, without rebuilding my console application, my thinking was that the output would be from 5 to 29. Of course I was wrong… if not you’d not be reading this blog. The actual output is from 5 through 19. The reason is due to the behavior of const and readonly members. To begin with, const is the compile-time constant and readonly is a runtime constant. Next, when you compile the code, a compile-time constant member is replaced with the value of the constant in the code. But, the IL generated when you reference a read-only constant, references the readonly variable, not its value. So, the IL version of the Main method, after compilation actually looks something like: 1: static void Main(string[] args) 2: { 3: for (int i = ConstSideEffect.StartValue; i < 20; i++) 4: { 5: Console.WriteLine(i); 6: } 7: Console.ReadLine(); 8: } I’m no expert with this IL thingi, but when I look at the disassembled code of the exe file (using IL Disassembler), I see the following: I see our readonly member still being referenced by the variable name (ConstVsReadonlyLib.ConstSideEffect::StartValue) in line 0001. Then there’s the Console.WriteLine in line 000b and finally, see the value of 20 in line 0017. This, I’m pretty sure is our const member being replaced by its value which marks the upper bound of the ‘for’ loop. Now you know why the output was from 5 through 19. This definitely is a side-effect of having const members and one needs to be aware of it. While we’re here, I’d like to add a few other points about const and readonly members: const is slightly faster, but is less flexible readonly cannot be declared within a method scope const can be used only on primitive types (numbers and strings) Just wanted to share this before going to bed!

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  • excplicitly casting constness in

    - by jimifiki
    With the following code void TestF(const double ** testv){;} void callTest(){ double** test; TestF(test); } I get this: error C2664: 'TestF' : cannot convert parameter 1 from 'double **' to 'const double **' I cannot understand why. Why test cannot be silently casted to const double**? Why should I do it explicitly? I know that TestF(const_cast<const double**>(test)) makes my code correct, but I feel this should be unnecessary. Are there some key concepts about const that I'm missing?

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  • Difference between static const char* and const char*.

    - by Will MacDonagh
    Could someone please explain the difference in how the 2 snippets of code are handled below? They definitely compile to different assembly code, but I'm trying to understand how the code might act differently. I understand that string literals are thrown into read only memory and are effectively static, but how does that differ from the explicit static below? struct Obj1 { void Foo() { const char* str( "hello" ); } }; and struct Obj2 { void Bar() { static const char* str( "hello" ); } };

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  • Cast then check or check then cast?

    - by jamesrom
    Which method is regarded as best practice? Cast first? public string Describe(ICola cola) { var coke = cola as CocaCola; if (coke != null) { string result; // some unique coca-cola only code here. return result; } var pepsi = cola as Pepsi; if (pepsi != null) { string result; // some unique pepsi only code here. return result; } } Or should I check first, cast later? public string Describe(ICola cola) { if (cola is CocaCola) { coke = (CocaCola) cola; string result; // some unique coca-cola only code here. return result; } if (cola is Pepsi) { pepsi = (Pepsi) cola; string result; // some unique pepsi only code here. return result; } } Can you see any other way to do this?

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  • pure/const functions in C++

    - by Albert
    Hi, I'm thinking of using pure/const functions more heavily in my C++ code. (pure/const attribute in GCC) However, I am curious how strict I should be about it and what could possibly break. The most obvious case are debug outputs (in whatever form, could be on cout, in some file or in some custom debug class). I probably will have a lot of functions, which don't have any side effects despite this sort of debug output. No matter if the debug output is made or not, this will absolutely have no effect on the rest of my application. Or another case I'm thinking of is the use of my own SmartPointer class. In debug mode, my SmartPointer class has some global register where it does some extra checks. If I use such an object in a pure/const function, it does have some slight side effects (in the sense that some memory probably will be different) which should not have any real side effects though (in the sense that the behaviour is in any way different). Similar also for mutexes and other stuff. I can think of many complex cases where it has some side effects (in the sense of that some memory will be different, maybe even some threads are created, some filesystem manipulation is made, etc) but has no computational difference (all those side effects could very well be left out and I would even prefer that). How does it work out in practice? If I mark such functions as pure/const, could it break anything (considering that the code is all correct)?

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  • C++ const-reference semantics?

    - by Kristoffer
    Consider the sample application below. It demonstrates what I would call a flawed class design. #include <iostream> using namespace std; struct B { B() : m_value(1) {} long m_value; }; struct A { const B& GetB() const { return m_B; } void Foo(const B &b) { // assert(this != &b); m_B.m_value += b.m_value; m_B.m_value += b.m_value; } protected: B m_B; }; int main(int argc, char* argv[]) { A a; cout << "Original value: " << a.GetB().m_value << endl; cout << "Expected value: 3" << endl; a.Foo(a.GetB()); cout << "Actual value: " << a.GetB().m_value << endl; return 0; } Output: Original value: 1 Expected value: 3 Actual value: 4 Obviously, the programmer is fooled by the constness of b. By mistake b points to this, which yields the undesired behavior. My question: What const-rules should you follow when designing getters/setters? My suggestion: Never return a reference to a member variable if it can be set by reference through a member function. Hence, either return by value or pass parameters by value. (Modern compilers will optimize away the extra copy anyway.)

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  • Delphi interface cast using TValue

    - by conciliator
    I've recently experimented extensively with interfaces and D2010 RTTI. I don't know at runtime the actual type of the interface; although I will have access to it's qualified name using a string. Consider the following: program rtti_sb_1; {$APPTYPE CONSOLE} uses SysUtils, Rtti, TypInfo, mynamespace in 'mynamespace.pas'; var ctx: TRttiContext; InterfaceType: TRttiType; Method: TRttiMethod; ActualParentInstance: IParent; ChildInterfaceValue: TValue; ParentInterfaceValue: TValue; begin ctx := TRttiContext.Create; // Instantiation ActualParentInstance := TChild.Create as IParent; {$define WORKAROUND} {$ifdef WORKAROUND} InterfaceType := ctx.GetType(TypeInfo(IParent)); InterfaceType := ctx.GetType(TypeInfo(IChild)); {$endif} // Fetch interface type InterfaceType := ctx.FindType('mynamespace.IParent'); // This cast is OK and ChildMethod is executed (ActualParentInstance as IChild).ChildMethod(100); // Create a TValue holding the interface TValue.Make(@ActualParentInstance, InterfaceType.Handle, ParentInterfaceValue); InterfaceType := ctx.FindType('mynamespace.IChild'); // This cast doesn't work if ParentInterfaceValue.TryCast(InterfaceType.Handle, ChildInterfaceValue) then begin Method := InterfaceType.GetMethod('ChildMethod'); if (Method <> nil) then begin Method.Invoke(ChildInterfaceValue, [100]); end; end; ReadLn; end. The contents of mynamespace.pas is as follows: {$M+} IParent = interface ['{2375F59E-D432-4D7D-8D62-768F4225FFD1}'] procedure ParentMethod(const Id: integer); end; {$M-} IChild = interface(IParent) ['{6F89487E-5BB7-42FC-A760-38DA2329E0C5}'] procedure ChildMethod(const Id: integer); end; TParent = class(TInterfacedObject, IParent) public procedure ParentMethod(const Id: integer); end; TChild = class(TParent, IChild) public procedure ChildMethod(const Id: integer); end; For completeness, the implementation goes as procedure TParent.ParentMethod(const Id: integer); begin WriteLn('ParentMethod executed. Id is ' + IntToStr(Id)); end; procedure TChild.ChildMethod(const Id: integer); begin WriteLn('ChildMethod executed. Id is ' + IntToStr(Id)); end; The reason for {$define WORKAROUND} may be found in this post. Question: is there any way for me to make the desired type cast using RTTI? In other words: is there a way for me to invoke IChild.ChildMethod from knowing 1) the qualified name of IChild as a string, and 2) a reference to the TChild instance as a IParent interface? (After all, the hard-coded cast works fine. Is this even possible?) Thanks!

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  • C++ find method is not const?

    - by Rachel
    I've written a method that I'd like to declare as const, but the compiler complains. I traced through and found that this part of the method was causing the difficulty: bool ClassA::MethodA(int x) { bool y = false; if(find(myList.begin(), myList.end(), x) != myList.end()) { y = true; } return y; } There is more happening in the method than that, but with everything else stripped away, this was the part that didn't allow the method to be const. Why does the stl find algorithm prevent the method from being const? Does it change the list in any way?

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  • STL Static-Const Member Definitions

    - by javery
    How does the following work? #include <limits> int main() { const int* const foo = &std::numeric_limits<int> ::digits; } I was under the impression that in order to take an address of a static const-ant member we had to physically define it in some translation unit in order to please the linker. That said, after looking at the preprocessed code for this TU, I couldn't find an external definition for the digits member (or any other relevant members). I tested this on two compilers (VC++ 10 and g++ 4.2.4) and got identical results (i.e., it works). Does the linker auto-magically link against an object file where this stuff is defined, or am I missing something obvious here?

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  • boost::shared_ptr<const T> to boost::shared_ptr<T>

    - by Flevine
    I want to cast the const-ness out of a boost::shared_ptr, but I boost::const_pointer_cast is not the answer. boost::const_pointer_cast wants a const boost::shared_ptr, not a boost::shared_ptr. Let's forego the obligitory 'you shouldn't be doing that'. I know... but I need to do it... so what's the best/easiest way to do it? For clarity sake: boost::shared_ptr<const T> orig_ptr( new T() ); boost::shared_ptr<T> new_ptr = magic_incantation(orig_ptr); I need to know the magic_incantation() Thanks!

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  • Modifying reference member from const member function in C++

    - by Philipp
    I am working on const-correctness of my code and just wondered why this code compiles: class X { int x; int& y; public: X(int& _y):y(_y) { } void f(int& newY) const { //x = 3; would not work, that's fine y = newY; //does compile. Why? } }; int main(int argc, char **argv) { int i1=0, i2=0; X myX(i1); myX.f(i2); ... } As far as I understand, f() is changing the object myX, although it says to be const. How can I ensure my compiler complains when I do assign to y? (Visual C++ 2008) Thank a lot!

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