operators

问题 I am confused about the evaluation time of sizeof operator. When does the sizeof operator get evaluated? Does its evaluation time (compile-time or runtime) depend on the language (C? C++?)? Can we use sizeof in case of objects created at runtime in C++? 回答1: In almost all cases, sizeof is evaluated based on static type information (at compile-time, basically). One exception (the only one, I think) is in the case of C99's variable-length arrays (VLAs). 回答2: Almost always compile time. But the

问题 This question already has answers here : How do chained comparisons in Python actually work? (1 answer) Python comparison operators chaining/grouping left to right? (1 answer) Closed 22 mins ago . Shouldn't it be parsed as (3 in [3]) == True since in and == have the same precedence? It's definitely not being parsed as 3 in ([3] == True) since that raises a TypeError . How else can it be parsed? p.s. I'm aware that PEP 8 says "Don't compare boolean values to True or False using == ." This is

问题 The code below is part of an rss feed parser using WordPress's simplepie fetch_feed()... Code is: if ($enclosure = $item->get_enclosure(0)) { $image_thumb = $item->get_enclosure()->get_link().'screenshot.jpg'; $image = $item->get_enclosure()->get_link().'screenshot-full.jpg'; } $link = esc_url( strip_tags( $item->get_link() ) ); $content = $item->get_content(); Upon trying to activate the theme in which this code appears, I'm getting the following error: Parse error: syntax error, unexpected

问题 As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance. Closed 6 years ago . leisure/curiosity question as implied in the title. I personally prefer the new operators as to make code more readable in my opinion.

问题 In the "Introduction" section of K&R C (2E) there is this paragraph: C, like any other language, has its blemishes. Some of the operators have the wrong precedence; ... Which operators are these? How are their precedence wrong? Is this one of these cases? 回答1: There is a clear rule of precedence that is incontrovertible. The rule is so clear that for a strongly typed system (think Pascal) the wrong precedence would give clear unambiguous syntax errors at compile time. The problem with C is

问题 public class Operators { public static void main(String[] args) { int a = 12; System.out.println("Bitwise AND:"+(12&12)); System.out.println("Bitwise inclusive OR:"+(12|12)); System.out.println("Bitwise exclusive OR:"+(12^12)); } } OUTPUT: Bitwise AND:12 Bitwise inclusive OR:12 Bitwise exclusive OR:0 I understand first two, but not the third. 回答1: XOR tells whether each bit is different. 1 XOR 1 = 0 1 XOR 0 = 1 0 XOR 1 = 1 0 XOR 0 = 0 In other words "either but not both" 0011 XOR 0101 = 0110

问题 In PHP, is if(!$foo) equivalent with if($foo != true) or with if($foo !== true) or is it even something completly different of both? 回答1: if(!$foo) is the equivalent to if($foo != true) so $foo = null; if(!$foo){ echo "asd"; } will ouptut "asd" 回答2: Note that, == OR != compares the values of variables for equality, type casting as necessary. === OR !== checks if the two variables are of the same type AND have the same value. This answer will give you better explanation of this concept: https:

问题 I am trying to get generic type-bound operators to work using gfortran 4.9, however I get errors. I have tried using Type(Vector) and Class (Vector) without success. Type :: Vector Real :: x, y, z Contains Procedure :: set => vector_set Procedure :: write => vector_write generic :: Operator (+) => vector_add End Type Vector Function vector_add & ( & u, v & ) & Result (w) !!$ Input Type (Vector), Intent(in) :: u, v !!$ Output Type (Vector) :: w w% x = u% x + v% x w% y = u% y + v% y w% z = u% z

问题 I have seen other questions on SO regarding this, but none that explains it in full. What is the right ways for compilers to handle the two situations below? I have tried it with gcc 4.7.1 (with -std=c++0x), VS2010 and VS2012 an get different results on all: Example 1: struct BB { // generic cast template<typename T> operator T() const { return 0; } // string cast operator std::string() const { return string("hello"); } }; int main() { BB b; string s = b; } Output: gcc 4.7.1: Ok VS2010: Ok

问题 This question already has answers here : What does !! (double exclamation point) mean? (3 answers) Closed 6 years ago . Okay so I came across a code which looks like @documents_names = sort { !!$deleted_documents_names{$a} == !!$deleted_documents_names{$b} ? uc($a) cmp uc($b) : !!$deleted_documents_names{$a} cmp !!$deleted_documents_names{$b} } @documents_names; It's the first time I'm seeing the use of double negation. What's the use of it? When would a person use it? 回答1: It converts non