empty-class | 易学教程

Construct an empty object without the default constructor

阅读更多关于 Construct an empty object without the default constructor

问题 Suppose I have a type F . I know that F is empty, but F has no default constructor, so I can't use F() to construct it. Is there a way to obtain a valid object of type F anyway? I seem to recall a mention that there was such a way with arcane usage of unions. Ideally, it would be constexpr friendly. This can be useful because captureless lambdas only gained a default constructor in C++20. In C++17, if I want to "pass a lambda to a template" and call that lambda without having an instance of

Construct an empty object without the default constructor

阅读更多关于 Construct an empty object without the default constructor

Handles Comparison: empty classes vs. undefined classes vs. void*

阅读更多关于 Handles Comparison: empty classes vs. undefined classes vs. void*

问题 Microsoft's GDI+ defines many empty classes to be treated as handles internally. For example, (source GdiPlusGpStubs.h ) //Approach 1 class GpGraphics {}; class GpBrush {}; class GpTexture : public GpBrush {}; class GpSolidFill : public GpBrush {}; class GpLineGradient : public GpBrush {}; class GpPathGradient : public GpBrush {}; class GpHatch : public GpBrush {}; class GpPen {}; class GpCustomLineCap {}; There are other two ways to define handles. They're, //Approach 2 class BOOK; //no need

Why sizeof(Derived4) is 8 byte? I think it should be 5 bytes

阅读更多关于 Why sizeof(Derived4) is 8 byte? I think it should be 5 bytes

问题 This is the output of the given program: sizeof(Empty) 1 sizeof(Derived1) 1 sizeof(Derived2) 4 sizeof(Derived3) 1 sizeof(Derived4) 8 sizeof(Dummy) 1 This is the program: #include <iostream> using namespace std; class Empty {}; class Derived1 : public Empty {}; class Derived2 : virtual public Empty {}; class Derived3 : public Empty { char c; }; class Derived4 : virtual public Empty { char c; }; class Dummy { char c; }; int main() { cout << "sizeof(Empty) " << sizeof(Empty) << endl; cout <<

why a const object of an empty class cant be created

阅读更多关于 why a const object of an empty class cant be created

问题 #include <iostream> class A { public: void foo() const { std::cout << "const version of foo" << std::endl; } void foo() { std::cout << "none const version of foo" << std::endl; } }; int main() { A a; const A ac; a.foo(); ac.foo(); } The above code can't be compiled, could anyone of you tell me why? 回答1: You need to initialize it. This is a known problem with the spec. 回答2: Initialize it as: const A ac = A(); Working code : http://www.ideone.com/SYPO9 BTW, this is not initializaiton : const A

Construct an empty object without the default constructor

阅读更多关于 Construct an empty object without the default constructor

Suppose I have a type F . I know that F is empty, but F has no default constructor, so I can't use F() to construct it. Is there a way to obtain a valid object of type F anyway? I seem to recall a mention that there was such a way with arcane usage of unions. Ideally, it would be constexpr friendly. This can be useful because captureless lambdas only gained a default constructor in C++20. In C++17, if I want to "pass a lambda to a template" and call that lambda without having an instance of it, I need to be able to reconstruct it from the type. auto const f = [](int x) { return x; }; using F =

what is the size of empty class in C++,java?

阅读更多关于 what is the size of empty class in C++,java?

What is the size of an empty class in C++ and Java? Why is it not zero? sizeof(); returns 1 in the case of C++. Martin York Short Answer: The standard explicitly says that a class can not have zero size. Long Answer: Because each object needs to have a unique address (also defined in the standard) you can't really have zero sized objects. Imagine an array of zero sized objects. Because they have zero size they would all line up on the same address location. So it is easier to say that objects can not have zero size. Note: Even though an object has a non zero size, if it actually takes up zero

Handles Comparison: empty classes vs. undefined classes vs. void*

阅读更多关于 Handles Comparison: empty classes vs. undefined classes vs. void*

Microsoft's GDI+ defines many empty classes to be treated as handles internally. For example, (source GdiPlusGpStubs.h ) //Approach 1 class GpGraphics {}; class GpBrush {}; class GpTexture : public GpBrush {}; class GpSolidFill : public GpBrush {}; class GpLineGradient : public GpBrush {}; class GpPathGradient : public GpBrush {}; class GpHatch : public GpBrush {}; class GpPen {}; class GpCustomLineCap {}; There are other two ways to define handles. They're, //Approach 2 class BOOK; //no need to define it! typedef BOOK *PBOOK; typedef PBOOK HBOOK; //handle to be used internally //Approach 3

python empty class object

阅读更多关于 python empty class object

问题 I'm teaching a python class on Object Oriented Programming and as I'm brushing up on how to explain Classes, I saw an empty class definition: class Employee: pass the example then goes on to define a name and other attributes for an object of this class: john = Employee() john.full_name = "john doe" interesting! I'm wondering if there's a way to dynamically define a function for an instance of a class like this? something like: john.greet() = print 'hello world!' this doesn't work in my

Why empty base class optimization is not working?

阅读更多关于 Why empty base class optimization is not working?

Why is the empty base class optimization (EBO) not being fully applied in Visual C++? If I have a lot of base classes, is there any way for me to help the compiler make this optimization? #include <iostream> struct T1 { }; struct T2 { }; struct T3 { }; struct T4 { }; struct T5 { }; struct T6 { }; struct Test : T1, T2, T3, T4, T5, T6 { }; int main() { std::cout << sizeof(Test); } // Prints 5 This is a longstanding bug in the Visual C++ compiler. When a class derives from multiple empty base classes, only the initial empty base class will be optimized using the empty base optimization (EBO).