pure-virtual

shared_ptr<Shape> circle(new Circle(Vec2f(0, 0), 0.1, Vec3f(1, 0, 0))); shared_ptr<Shape> rect(new Rect2f(Vec2f(0, 0), 5.0f, 5.0f, 0, Vec3f(1.0f, 1.0f, 0)) ); I'm trying to understand why the above won't compile. For Whatever reason, when I try to create an instance of Rect2f (which DOES inherit from the Shape class specified the shared_ptr template argument, just like Circle ), I get the following errors : error: expected type-specifier before 'Rect2f' error: expected ')' before 'Rect2f' Everything about the Circle shared_ptr is perfectly fine. There are no problems with it; it's only the

Whilst compiling with avr-gcc I have encountered linker errors such as the following: undefined reference to `__cxa_pure_virtual' I've found this document which states: The __cxa_pure_virtual function is an error handler that is invoked when a pure virtual function is called. If you are writing a C++ application that has pure virtual functions you must supply your own __cxa_pure_virtual error handler function. For example: extern "C" void __cxa_pure_virtual() { while (1); } Defining this function as suggested fixes the errors but I'd like to know: what the purpose of this function is, why I

In C++, it is legal to give an implementation of a pure virtual function: class C { public: virtual int f() = 0; }; int C::f() { return 0; } Why would you ever want to do this? Related question: The C++ faq lite contains an example: class Funct { public: virtual int doit(int x) = 0; virtual ~Funct() = 0; }; inline Funct::~Funct() { } // defined even though it's pure virtual; it's faster this way; trust me I don't understand why the destructor is declared pure virtual and then implemented; and I don't understand the comment why this should be faster. Declared destructors must always be

I want to go to there. Seriously though, how does one implement a pure virtual method in an "Apple" way? Do you use a Protocol with your base class and throw exceptions on those methods? When you program in Objective-C you need to purge your mind of such things as virtual methods. You don't call methods on Objective-C objects, you send messages to them. Objects either respond to messages or they don't, but due to the dynamic binding, you can't tell this until run time. Thus, you can declare a method on a base object and not not provide an implementation, no problem (except for the compiler

Pure virtual functions (when we set = 0 ) can also have a function body. What is the use to provide a function body for pure virtual functions, if they are not going get called at all? Your assumption that pure virtual function cannot be called is absolutely incorrect. When a function is declared pure virtual, it simply means that this function cannot get called dynamically , through a virtual dispatch mechanism. Yet, this very same function can easily be called statically , non-virtually , directly (without virtual dispatch). In C++ language a non-virtual call to a virtual function is