object-slicing

问题 My question is about how to implement the classic ICloneable interface in such a way that it won't lead to inadvertent object-slicing when a future programmer isn't paying close attention. Here's an example of the kind of programming error I'd like to detect (preferably at compile-time): #include <stdio.h> class ICloneable { public: virtual ICloneable * clone() const = 0; }; class A : public ICloneable { public: A() {} A(const A & rhs) {} virtual ICloneable * clone() const {return new A(*this

问题 So I am refreshing on C++, and honestly it's been awhile. I made a console pong game as a sort of refresher task and got some input on using polymorphism for my classes to derive from a base "GameObject" (that has some base methods for drawing objects to the screen). One of the pieces of input was (and I had subsequently asked about) was how memory worked when deriving from base classes. Since I hadn't really done much advanced C++. For instance lets say we have a base class, for now it just

问题 This question already has answers here : Closed 7 years ago . Possible Duplicate: Virtual Functions Object Slicing let's consider: #include <vector> #include <iostream> using namespace std; struct A { virtual void do_it() { cout << "A" << endl; } }; struct B : public A { virtual void do_it() { cout << "B" << endl; } }; int main() { vector<A> v; v.push_back(B()); v[0].do_it(); // output is A } which function will be called? Basically is it possible to use polymorphism without pointers if no

问题 I'm trying to understand how polymorphism, object slicing and pointers work in this block of code. I'm working in Visual Studio. #include <iostream> class Man { public: virtual void speak() { std::cout << "I'm a man." << "\n"; } private: }; class Soldier : public Man { public: virtual void speak() { std::cout << "I'm a soldier." << "\n"; } private: }; int main() { Man man1; Soldier soldier1; man1 = soldier1; std::cout << "Man1: "; man1.speak(); Man *man2 = new Man; Man *soldier2 = new Soldier

问题 I did read on numpy indexing but I didn't find what I was looking for. I have a 288*384 image, where each pixel can have a labelling in [0,15]. It is stored in a 3d (288,384,16)-shaped numpy array im . With im[:,:,1] , I can for example get the image where all pixels have the label 1. I have another 2d array labelling , (288*384)-shaped, containing a label for each pixel. How do I get the image where each pixel has the corresponding pixel using some clever slicing? Using loops, that would be:

问题 I have a pandas dataframe with 10 keys. If I try to access a column that is not present, even then it returns a NaN for this. I was expecting a KeyError. How is pandas not able to identify the missing column ? In the example below, vendor_id is a valid column in dataframe. The other column is absent from the dataset. final_feature.ix[:,['vendor_id','this column is absent']] Out[1017]: vendor_id this column is absent 0 434236 NaN type(final_feature) Out[1016]: pandas.core.frame.DataFrame EDIT

问题 I'm having a devil of a time understanding references. Consider the following code: class Animal { public: virtual void makeSound() {cout << "rawr" << endl;} }; class Dog : public Animal { public: virtual void makeSound() {cout << "bark" << endl;} }; Animal* pFunc() { return new Dog(); } Animal& rFunc() { return *(new Dog()); } Animal vFunc() { return Dog(); } int main() { Animal* p = pFunc(); p->makeSound(); Animal& r1 = rFunc(); r1.makeSound(); Animal r2 = rFunc(); r2.makeSound(); Animal v

问题 The following is my code, #include<iostream> #include<string> using namespace std; class TestClass { public: virtual void test(string st1, string st2); }; class ExtendedTest: public TestClass { public: virtual void test(string st1, string st2); }; void TestClass::test(string st1, string st2="st2") { cout << st1 << endl; cout << st2 << endl; } void ExtendedTest::test(string st1, string st2="st2") { cout << "Extended: " << st1 << endl; cout << "Extended: " << st2 << endl; } void pass(TestClass

问题 I think I understand the concept of virtual methods and vtables, but I don't understand why there is a difference between passing the object as a pointer(or reference) and passing it by value (which kind of scraps the vtable or something?) Why would something like this work: Material* m = new Texture; poly->setMaterial(m); // methods from Texture are called if I keep carrying the pointer around And not this?: Material m = Texture(); poly->setMaterial(m); // methods from Material are called if

问题 How do I: Select last 3 columns in a dataframe and create a new dataframe? I tried: y = dataframe.iloc[:,-3:] Exclude last 3 columns and create a new dataframe? I tried: X = dataframe.iloc[:,:-3] Is this correct? I am getting array dimensional errors further in my code and want to make sure this step is correct. Thank you 回答1: just do: y = dataframe[dataframe.columns[-3:]] This slices the columns so you can sub-select from the df Example: In [221]: df = pd.DataFrame(columns=np.arange(10)) df