global-variables

问题 What is the major difference between JavaScript declared and undeclared variables, since the delete operator doesn't work on declared variables? var y = 43; // declares a new variable x = 42; delete x; // returns true (x is a property of the global object and can be deleted) delete y; // returns false (delete doesn't affect variable names) Why does this happen? Variables declared globally are also the properties of the window object, so why can't it be deleted? 回答1: Declared and undeclared

问题 This question already has answers here : How to avoid having class data shared among instances? (7 answers) Closed last year . I started coding in python a week ago, it is my mistake i started coding using oops,classes and objects that soon. I assumed my C++ proficiency will help.... I got bit by the following code class A: var=0 list=[] def __init__(self): pass Here to my surprise, var and list are kinda global variable, it is shared across all instances it seems.... What I thought was it

问题 In recent conversations with fellow students, I have been advocating for avoiding globals except to store constants. This is a sort of typical applied statistics-type program where everyone writes their own code and project sizes are on the small side, so it can be hard for people to see the trouble caused by sloppy habits. In talking about avoidance of globals, I'm focusing on the following reasons why globals might cause trouble , but I'd like to have some examples in R and/or Stata to go

问题 What I know about C++ is that the order of the constructions (and destructions) of global instances should not be assumed. While I'm writing code with a global instance which uses std::cout in the constructor & destructor, I got a question. std::cout is also a global instance of iostream. Is std::cout guaranteed to be initialized before any other global instances? I wrote a simple test code and it works perfectly, but still I don't know why. #include <iostream> struct test { test() { std:

问题 How do I share a global variable with thread? My Python code example is: from threading import Thread import time a = 0 #global variable def thread1(threadname): #read variable "a" modify by thread 2 def thread2(threadname): while 1: a += 1 time.sleep(1) thread1 = Thread( target=thread1, args=("Thread-1", ) ) thread2 = Thread( target=thread2, args=("Thread-2", ) ) thread1.join() thread2.join() I don't know how to get the two threads to share one variable. 回答1: You just need to declare a as a

问题 Apparently there's a lot of variety in opinions out there, ranging from, " Never! Always encapsulate (even if it's with a mere macro!) " to " It's no big deal - use them when it's more convenient than not. " So. Specific, concrete reasons (preferably with an example) Why global variables are dangerous When global variables should be used in place of alternatives What alternatives exist for those that are tempted to use global variables inappropriately While this is subjective, I will pick one

问题 Apparently there's a lot of variety in opinions out there, ranging from, " Never! Always encapsulate (even if it's with a mere macro!) " to " It's no big deal - use them when it's more convenient than not. " So. Specific, concrete reasons (preferably with an example) Why global variables are dangerous When global variables should be used in place of alternatives What alternatives exist for those that are tempted to use global variables inappropriately While this is subjective, I will pick one

问题 How can I declare a global variable in JavaScript? 回答1: If you have to generate global variables in production code (which should be avoided) always declare them explicitly : window.globalVar = "This is global!"; While it is possible to define a global variable by just omitting var (assuming there is no local variable of the same name), doing so generates an implicit global, which is a bad thing to do and would generate an error in strict mode . 回答2: If this is the only application where you

问题 I've heard from a variety of places that global variables are inherently nasty and evil, but when doing some non-object oriented Javascript, I can't see how to avoid them. Say I have a function which generates a number using a complex algorithm using random numbers and stuff, but I need to keep using that particular number in some other function which is a callback or something and so can't be part of the same function. If the originally generated number is a local variable, it won't be

问题 I know the variable name, like $var and its value. If I check for isset($GLOBALS[$var_name]) && $GLOBALS[$var_name] == $var I may get a positive result even if the variable is not really global, but happens to have the same name and value as an existing global variable :( I need this for a debug function that I'm building, to display more info about the type of the variable that was passed. Could I also find out if a variable is static? :) ps: reflection doesn't help me.. It can only get info