variadic-functions

问题 I am used to declaring variadic functions like this: int f(int n, ...); When reading The C++ Programming Language I found that the declarations in the book omit the comma: int f(int n...); // the comma has been omitted It seems like this syntax is C++ specific as I get this error when I try to compile it using a C compiler: test.c:1:12: error: expected ‘;’, ‘,’ or ‘)’ before ‘...’ token int f(int n...); Is there any difference between writing int f(int n, ...) and int f(int n... )? Why was

问题 I tried this and get weird behavior from JAVA, can someone explain this for me? boolean testNull(String... string) { if(string == null) { return true; } else { System.out.println(string.getClass()); return false; } } boolean callTestNull(String s) { return testNull(s); } Then I have test case: @Test public void test_cases() { assertTrue(instance.testNull(null)); // NULL assertFalse(instance.testNull()); // NOT NULL assertFalse(instance.callTestNull(null)); // NOT NULL } The question is if I

问题 This question already has answers here : what is the use of … in c++ (2 answers) Closed 5 years ago . I was wondering if the below function prototype is valid. It compiled fine, but the three period is kinda throwing me off and I couldn't find anything similar on Google. void foo(int, ...); Thanks! 回答1: Yes, it's valid. In this example, ... creates a variadic function using the va_list mechanism. This is how variadic functions are implemented in C, and to some degree in C++ (though C++11's

问题 How exactly do variadic functions treat numeric constants? e.g. consider the following code: myfunc(5, 0, 1, 2, 3, 4); The function looks like this: void myfunc(int count, ...) { } Now, in order to iterate over the single arguments with va_arg , I need to know their sizes, e.g. int , short , char , float , etc. But what size should I assume for numeric constants like I use in the code above? Tests have shown that just assuming int for them seems to work fine so the compiler seems to push them

问题 I am writing a heavy multi threaded [>170 threads] c++11 program. Each thread is logging information into one file used by all threads. For performance reasons I want to create a log thread which is writing the information via fprintf() into the global file. I have no idea how to organize the structure into which the worker threads are writing the information which can be then read by the log thread. Why do I not call sprintf() in each worker thread and then just provide the output buffer to

问题 I'm using java generics and varargs. If I use the following code, I'll get a ClassCastException , even though I'm not using casts at all. Stranger yet, if I run this on Android (dalvik) no stack trace is included with the exception, and if I change the interface to abstract class, the exception variable e is empty. The code: public class GenericsTest { public class Task<T> { public void doStuff(T param, Callback<T> callback) { // This gets called, param is String "importantStuff" // Working

问题 I have declared the following method: private void mockInvokeDBHandler(Map<String, Object>... rows) { List<Map<String, Object>> allRows = Arrays.asList(rows)); // rest of method omitted } It is invoked by clients using something like Map<String, Object> row1 = new HashMap<String, Object>(); Map<String, Object> row2 = new HashMap<String, Object>(); mockInvokeDBHandler(row1, row2); However, the last line shown above generates a warning Type safety : A generic array of Map is created for a

问题 I have declared the following method: private void mockInvokeDBHandler(Map<String, Object>... rows) { List<Map<String, Object>> allRows = Arrays.asList(rows)); // rest of method omitted } It is invoked by clients using something like Map<String, Object> row1 = new HashMap<String, Object>(); Map<String, Object> row2 = new HashMap<String, Object>(); mockInvokeDBHandler(row1, row2); However, the last line shown above generates a warning Type safety : A generic array of Map is created for a

问题 I want to implement a has_no_duplicates<...> type trait that evaluates to std::true_type if the passed variadic type list has no duplicate types. static_assert(has_no_duplicates<int, float>{}, ""); static_assert(!has_no_duplicates<float, float>{}, ""); Let's assume, for the scope of this question, that I want to do that using multiple inheritance. When a class inherits from the same type more than once, an error occurs. template<class T> struct type { }; template<class... Ts> struct dup

问题 Is it possible to have a TLD map to the following function: public static <T> T[] toArray(T... stuff) { return stuff; } So that I can do: <c:forEach items="${my:toArray('a', 'b', 'c')}"... I tried the following <function-signature> s java.lang.Object toArray( java.lang.Object... ) java.lang.Object[] toArray( java.lang.Object[] ) And others but nothing seems to work. 回答1: Unfortunately that's not possible. The EL resolver immediately interprets the commas in the function as separate arguments