declaration

undefined C struct forward declaration

由 送分小仙女□ 提交于 2019-11-27 20:35:32
I have a header file port.h, port.c, and my main.c I get the following error: 'ports' uses undefined struct 'port_t' I thought as I have declared the struct in my .h file and having the actual structure in the .c file was ok. I need to have the forward declaration as I want to hide some data in my port.c file. In my port.h I have the following: /* port.h */ struct port_t; port.c: /* port.c */ #include "port.h" struct port_t { unsigned int port_id; char name; }; main.c: /* main.c */ #include <stdio.h> #include "port.h" int main(void) { struct port_t ports; return 0; } Many thanks for any

Why does initializing an extern variable inside a function give an error?

由 ◇◆丶佛笑我妖孽 提交于 2019-11-27 20:29:24
This code compiles fine: extern int i = 10; void test() { std::cout << "Hi" << i << std::endl; } While this code gives an error: void test() { extern int i = 10; std::cout << "Hi" << i << std::endl; } error: 'i' has both 'extern' and initializer I read this in C++ Primer : Any declaration that includes an explicit initializer is a definition. We can provide an initializer on a variable defined as extern, but doing so overrides the extern. An extern that has an initializer is a definition. It is an error to provide an initializer on an extern inside a function . Can someone provide an

C++ “fatal error LNK1120” unresolved static class members

由 谁说胖子不能爱 提交于 2019-11-27 19:37:00
问题 I get the following error message (someone feel free to edit the unnecessary bits): 1>FIXDecoder.obj : error LNK2001: unresolved external symbol "private: static class std::unordered_map,class std::allocator >,class std::basic_string,class std::allocator >,struct std::hash,class std::allocator > >,struct std::equal_to,class std::allocator > >,class std::allocator,class std::allocator > const ,class std::basic_string,class std::allocator > > > > FD::FixValueMappingsDict" (?FixValueMappingsDict

Declaration or Definition in C

由 大憨熊 提交于 2019-11-27 19:23:45
From External Variables Wiki : If neither the extern keyword nor an initialization value are present, the statement can be either a declaration or a definition. It is up to the compiler to analyse the modules of the program and decide. I was not able to fully grasp the meaning of this statement with respect to C. For example, does it imply that: int i; is not necessarily a declaration (as I have been assuming until now), but could be a definition as well (by definition of Definition & Declaration on the same webpage, no puns intended)? In a nutshell, is the above statement: a. just a

Does #ifdef (or other Preprocessor Directives) Work for Function Declarations (to Test for Function Existence)?

由 半腔热情 提交于 2019-11-27 18:53:06
问题 Why doesn’t the following code work as expected? void foobar(int); #ifndef foobar printf("foobar exists"); #endif It always prints the message; it obviously cannot detect the existence of a function as an entity. (Is it an over-loading issue?) Why can’t #ifdef (or its variants) detect function declarations? Declarations should be available at pre-processing, so it should work, shouldn’t it? If not, is there an alternative or work-around? 回答1: Declarations should be available at pre-processing

Why do Java array declarations use curly brackets?

由 最后都变了- 提交于 2019-11-27 18:33:00
问题 I would like to know why Java array declarations use curly brackets as opposed to the standard parenthesis. As illustrated here. I imagine this may take further understanding of curly brackets in general, but this specifically is on my agenda right now. Object[] tableHeaders = {"Cars","Trucks","Tacos"}; This is correct, as opposed to. Object[] tableHeaders = ("Cars","Trucks","Tacos"); 回答1: Curly brackets usually denotes sets and ensembles while parenthesis usually denotes parameters in C-like

Template class member specialization without declaration in header

由 为君一笑 提交于 2019-11-27 17:46:49
问题 I have a template class that I declare in a header with one method and no definition of that method in the header. In a .cc file, I define specializations of that method without ever declaring them in the header . In a different .cc file, I call the method for different template parameters for which specializations exist. It looks like this: foo.h: template<typename T> class Foo { public: static int bar(); }; foo.cc: #include "foo.h" template<> int Foo<int>::bar() { return 1; } template<> int

More elegant way of declaring multiple variables at the same time

由 安稳与你 提交于 2019-11-27 16:45:45
To declare multiple variables at the "same time" I would do: a, b = True, False But if I had to declare much more variables, it turns less and less elegant: a, b, c, d, e, f, g, h, i, j = True, True, True, True, True, False, True ,True , True, True Is there a better / elegant / convenient way to do this? This must be very basic, but if I do used a list or a tuple for storing the variables, how would I have to approach so that I would be helpful since: aList = [a,b] Is not valid, I would have to do: a, b = True, True Or what am I missing? As others have suggested, it's unlikely that using 10

How to know in C# code which type a variable was declared with

由 不问归期 提交于 2019-11-27 16:36:31
问题 I want to have some function that would return "Base" if a variable of class Base was passed to it, "Derived" if it was declared as Derived , etc. Not depending on runtime type of a value it was assigned to. 回答1: See code below for example. The key is to use Generics , extension method was used just for nice syntax. using System; static class Program { public static Type GetDeclaredType<T>(this T obj) { return typeof(T); } // Demonstrate how GetDeclaredType works static void Main(string[]

popen implicitly declared even though #include <stdio.h> is added

由 喜你入骨 提交于 2019-11-27 16:02:23
问题 This is tiny snippet of my code. #include <stdio.h> #include <unistd.h> #include <stdlib.h> #include <time.h> #include <sys/stat.h> #include <sys/wait.h> #include <sys/types.h> #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> ... FILE * pipe; ... pipe = popen ("ls /tmp -1", "r"); ... pclose(pipe); blarg.c:106: warning: implicit declaration of function ‘popen’ blarg.c:106: warning: assignment makes pointer from integer without a
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