variadic-functions | 易学教程

Passing an ellipsis to another variadic function [duplicate]

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问题 This question already has an answer here: Forward an invocation of a variadic function in C 10 answers I have approximately 30 variadic functions. Each one accepts a path as the final argument, e.g.: bool do_foo(struct *f, int q, const char *fmt, ...) In each function, I have to check that the expanded format is less then or equal to a certain size. So, I find myself copy / pasting the same chunk of code to check for how many characters vsnprintf() didn't print, set errno accordingly and bail

How to handle java variable length arguments in clojure?

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I'am wrapping a java lib into clojure, but i have problems dealing with variable length arguments. Say, TestClass.aStaticFunction(Integer... intList){/*....*/} How could i call this function in clojure? Since Java varargs are actually arrays , you can call vararg functions in Clojure by passing an array. You could convert a Clojure seq (maybe by using Clojure's variety of variable argument functions) into an array: (TestClass/aStaticFunction (into-array Integer [(int 1),(int 2)])) or (defn a-static-function-wrapper [& args] (TestClass/aStaticFunction (into-array Integer args)) Or make an array

C++11: Number of Variadic Template Function Parameters?

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问题 How can I get a count of the number of arguments to a variadic template function? ie: template<typename... T> void f(const T&... t) { int n = number_of_args(t); ... } What is the best way to implement number_of_args in the above? 回答1: Just write this: const std::size_t n = sizeof...(T); //you may use `constexpr` instead of `const` Note that n is a constant expression (i.e known at compile-time), which means you may use it where constant expression is needed, such as: std::array<int, n> a; /

Calling Java varargs method with single null argument?

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If I have a vararg Java method foo(Object ...arg) and I call foo(null, null) , I have both arg[0] and arg[1] as null s. But if I call foo(null) , arg itself is null. Why is this happening? How should I call foo such that foo.length == 1 && foo[0] == null is true ? The issue is that when you use the literal null, Java doesn't know what type it is supposed to be. It could be a null Object, or it could be a null Object array. For a single argument it assumes the latter. You have two choices. Cast the null explicitly to Object or call the method using a strongly typed variable. See the example

Spread Syntax vs Rest Parameter in ES2015 / ES6

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问题 I am confused about the spread syntax and rest parameter in ES2015. Can anybody explain the difference between them with proper examples? 回答1: When using spread, you are expanding a single variable into more: var abc = ['a', 'b', 'c']; var def = ['d', 'e', 'f']; var alpha = [ ...abc, ...def ]; console.log(alpha)// alpha == ['a', 'b', 'c', 'd', 'e', 'f']; When using rest arguments, you are collapsing all remaining arguments of a function into one array: function sum( first, ...others ) { for (

Populating a va_list

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问题 Is there a way to create a va_list from scratch? I'm trying to call a function that takes a va_list as a parameter: func(void **entry, int num_args, va_list args, char *key); ...from a function that doesn't take a variable number of arguments. The only way I can think of is to create an intermediary function that takes varargs and then passing along its va_list, which is pretty stupid: void stupid_func(void **entry, char *key, int num_args, ...) { va_list args; va_start(args, num_args); func

Why doesn't autoboxing overrule varargs when using method overloading in Java 7?

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问题 We have a class LogManager in our Java project which looks like this: public class LogManager { public void log(Level logLevel, Object... args) { // do something } public void log(Level logLevel, int value, Object... args) { // do something else } } When compiling the project with OpenJDK 6 under Debian everyting works fine. When using OpenJDK 7 the build (done with ant) produces the following errors and the build fails: [javac] /…/LogManager.java:123: error: reference to log is ambiguous,

Possible heap pollution via varargs parameter

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I understand this occurs with Java 7 when using varargs with a generic type; But my question is.. What exactly does Eclipse mean when it says "its use could potentially pollute the heap?" And How does the new @SafeVarargs annotation prevent this? Heap pollution is a technical term. It refers to references which have a type that is not a supertype of the object they point to. List<A> listOfAs = new ArrayList<>(); List<B> listOfBs = (List<B>)(Object)listOfAs; // points to a list of As This can lead to "unexplainable" ClassCastException s. // if the heap never gets polluted, this should never

How to pass Scala array into Scala vararg method?

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问题 Consider the code below: private def test(some:String*){ } private def call () { val some = Array(\"asd\", \"zxc\") test(some) } It prints expect String, found Array[String] Why? Are Scala varargs not arrays? Note I found several questions on Stack Overflow about Scala varargs, but all of them are about calling Java varargs methods or about converting Scala lists to arrays. 回答1: Append :_* to the parameter in test like this test(some:_*) And it should work as you expect. If you wonder what

How to properly match varargs in Mockito

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I've been trying to get to mock a method with vararg parameters using Mockito: interface A { B b(int x, int y, C... c); } A a = mock(A.class); B b = mock(B.class); when(a.b(anyInt(), anyInt(), any(C[].class))).thenReturn(b); assertEquals(b, a.b(1, 2)); This doesn't work, however if I do this instead: when(a.b(anyInt(), anyInt())).thenReturn(b); assertEquals(b, a.b(1, 2)); This works, despite that I have completely omitted the varargs argument when stubbing the method. Any clues? topchef Mockito 1.8.1 introduced anyVararg() matcher : when(a.b(anyInt(), anyInt(), Matchers.<String>anyVararg()))