initialization

问题 I need a bit of help to understand how python initialising works. I have a class (Bar) with another class (Foo) as a field/variable. When I try to initialise this variable directly in Bar (not in the class __init__) all instances of Bar will point to the same Foo. But if I have an __init__ method, as in Bar2, each Bar2 instance will have a unique Foo instance. What is happening here? class Foo(): number = 0 class Bar(): foo = Foo() class Bar2(): foo = None def __init__(self): self.foo = Foo()

问题 I am trying to create an object using placement new (I know to use smart pointers, this is just to learn). My code is as follows: #include <vector> #include <iostream> #include <memory> using namespace std; // please excuse this // if you change like 19 to arr1 (or any other var name) instead of arr and line 40 to arr1 then it works struct A { int in = 999; A() {cout << "A ctor\n";} ~A() {cout << "A dtor\n";} }; char arr[sizeof(A)]; class B { public: static char arr[sizeof(A)]; const static A

问题 I am trying to create an object using placement new (I know to use smart pointers, this is just to learn). My code is as follows: #include <vector> #include <iostream> #include <memory> using namespace std; // please excuse this // if you change like 19 to arr1 (or any other var name) instead of arr and line 40 to arr1 then it works struct A { int in = 999; A() {cout << "A ctor\n";} ~A() {cout << "A dtor\n";} }; char arr[sizeof(A)]; class B { public: static char arr[sizeof(A)]; const static A

问题 I'm trying to make a gui which has a SimpleWindow class that contains a textPanel class: class textPanel{ private: std::string text_m; public: textPanel(std::string str):text_m(str){} ~textPanel(); }; class SimpleWindow{ public: SimpleWindow(); ~SimpleWindow(); textPanel text_panel_m; }; SimpleWindow::SimpleWindow(): text_panel_m(std::string temp("default value")) { } I want to be able to initialize the text_panel_m using a const char* that gets converted to a std::string without needing to

问题 I'm trying to make a gui which has a SimpleWindow class that contains a textPanel class: class textPanel{ private: std::string text_m; public: textPanel(std::string str):text_m(str){} ~textPanel(); }; class SimpleWindow{ public: SimpleWindow(); ~SimpleWindow(); textPanel text_panel_m; }; SimpleWindow::SimpleWindow(): text_panel_m(std::string temp("default value")) { } I want to be able to initialize the text_panel_m using a const char* that gets converted to a std::string without needing to

问题 (I'm sure this question has already been answered, I'm just not sure on the right words to use to ask it. If someone would tell me what the correct terminology is that would be awesome!) I'm implementing a HashSet in C++ for a data structures class, and I have a question about C++ syntax regarding structs. Here is my code: struct HashNode { T value; HashNode* next = nullptr; }; Will this code correctly initialize the next pointer to nullptr when new HashNode is called? If not, what is the

问题 gcc 8 and clang 7 do not accept the following code, which should default-construct a temporary of type unsigned int : unsigned int ui = unsigned int{}; clang 7 reports an error such as <source>:6:22: error: expected primary-expression before 'unsigned' Visual C++ 2015 and 2017 accept this. Obviously, this works with a type such as int , or any default-constructible class type. Is this correct C++14 code (and in that case a bug of clang and gcc)? If not, why not? Which types other than

问题 gcc 8 and clang 7 do not accept the following code, which should default-construct a temporary of type unsigned int : unsigned int ui = unsigned int{}; clang 7 reports an error such as <source>:6:22: error: expected primary-expression before 'unsigned' Visual C++ 2015 and 2017 accept this. Obviously, this works with a type such as int , or any default-constructible class type. Is this correct C++14 code (and in that case a bug of clang and gcc)? If not, why not? Which types other than

问题 I simply want to create an array of two dimensional arrays to store coordinate points. So I want an array where each index returns a two dimensional array which I would use as x and y . Here's what I've tried: waypoints = new int[4][,] { {{0},{6, 0}}, {{1},{1, 1}}, {{2},{1, 5}}, {{3},{6, 5}} }; I realize this probably looks stupid, but I've tried looking it up on Google, and I have not gotten any good results. It gives an error: "error CS0623: Array initializers can only be used in a variable

问题 I simply want to create an array of two dimensional arrays to store coordinate points. So I want an array where each index returns a two dimensional array which I would use as x and y . Here's what I've tried: waypoints = new int[4][,] { {{0},{6, 0}}, {{1},{1, 1}}, {{2},{1, 5}}, {{3},{6, 5}} }; I realize this probably looks stupid, but I've tried looking it up on Google, and I have not gotten any good results. It gives an error: "error CS0623: Array initializers can only be used in a variable