在头文件"unknown type"中使用结构是指:错误 [英] Using a struct in a header file "unknown type" error

问题描述

我正在Kubuntu中使用Kdevelop. 我已经在我的datasetup.h文件中声明了一个结构:

I am using Kdevelop in Kubuntu. I have declared a structure in my datasetup.h file:

#ifndef A_H
#define A_H

struct georeg_val {

    int p;
    double h;
    double hfov;
    double vfov;
};

#endif

现在当我在main.c文件中使用它时

Now when I use it in my main.c file

int main()
{
    georeg_val gval;

    read_data(gval); //this is in a .cpp file

}

我收到以下错误:

georeg_chain.c:7:3:错误:未知类型名称'georeg_val'

georeg_chain.c:7:3: error: unknown type name 'georeg_val'

(在georeg_val gval;行中)

如果有人能帮助我解决此错误，我将不胜感激.

I would appreciate if anyone could help me resolve this error.

推荐答案

在C语言中，有两种方法可以声明结构:

In C one has two possibilities to declare structure:

struct STRUCT_NAME {} ;

或

typedef struct {} STRUCT_ALIAS;

如果使用第一种方法(为结构命名)，则必须通过将变量明确标记为a struct来定义变量:

If you use first method (give struct a name) - you must define variable by marking it explicitly being a struct:

struct STRUCT_NAME myStruct;

但是，如果您使用第二种方法(给别名构造一个别名)，则可以省略struct标识符-编译器可以推断出仅属于alias的变量类型:

However if you use second method (give struct an alias) then you can omit struct identifier - compiler can deduce type of variable given only it's alias :

STRUCT_ALIAS myStruct;

奖励积分: 您可以使用其名称和别名声明struct:

Bonus points: You can declare struct with both it's name and alias:

typedef struct STRUCT_TAG {} STRUCT_TAG;
// here STRUCT_NAME == STRUCT_ALIAS

然后在变量定义中，您可以使用第一种或第二种方法.为什么两个世界都好?使用结构别名可以使结构变量定义更短-有时候这是一件好事.但是结构名称让您创建forward declarations.在某些情况下，这是必不可少的工具-考虑到您在结构之间具有循环引用:

Then in variable definition you can use either first or second method. Why both of two worlds is good ? Struct alias lets you to make struct variable definitions shorter - which is a good thing sometimes. But struct name let's you to make forward declarations. Which is indispensable tool in some cases - consider you have circular references between structs:

struct A {
  struct B * b;
}
struct B {
  struct A * a;
}

除了该体系结构可能存在缺陷之外-当以第一种方式(使用名称)声明结构并通过将结构指针标记为struct显式引用结构指针时，将编译此循环定义.

Besides that this architecture may be flawed - this circular definition will compile when structs are declared in the first way (with names) AND struct pointers are referenced explicitly by marking them as struct.

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