Few unusual C/C++ declarations

Question

I came across this Enum and Struct declarations in a project supposedly done by an expert. The declarations / definitions are little different than what im used to so far.

enum EnumKeys {
  KEY_MENU ,
  KEY_EXIT ,
  KEY_DOWN ,
  KEY_UP  ,
  KEY_RIGHT ,
.
.
.

};

But nowhere in the code the actual Enum had been defined by a line of code like

 EnumKeys test; 

for example. But still the KEY_MENU, KEY_EXIT are freely available just after this declaration. For example, if I type cout << KEY_EXIT, it prints 1.

Other instance is, this structure declaration / definition.

typedef struct t_ExpoData {
  int8_t  expNorm;
  int8_t  expDr;
  int8_t  drSw;
}ExpoData;

typedef struct t_ModelData {
  ...
  ...
  ExpoData  expoData[4]; // 3*4
}ModelData;

So the way I read this is, there is a new structure with the name ExpoData and one instance of it is ExpoData. Instead of using that instance right away, why the second declaration ExpoData expoData[4] ?

If anyone understands this, as of what clang specification is this, and how to interpret this correctly, that is much appreciated.

Answer 1

But nowhere in the code the actual Enum had been defined by something like EnumKeys test;

For the enumeration, there may never be any declaration of a variable of that type, but it may be used for creating other values. Often in C, enum types are combined into ints or longs. For example, a developer could write:

int bitmask = KEY_UP | KEY_DOWN;

and then use that value in a comparison to check whether the up or down arrow key was pressed.

So the way I read this is, there is a new structure with the name ExpoData and one instance of it is ExpoData.

No, that's not what this means. The typedef in this style:

typedef struct <someName> {
    ... fields
} <somePossiblyOtherName>;

is a typical C type definition. They take the form:

typedef <existing type> <new type>;

Furthermore, C struct definitions can be done without the typedef like this:

struct <typeName> {
    ...fields
};

You can then use that type by typing struct <typeName> before a variable declaration, function return type or function argument. Like this:

struct <typeName> functionName(struct <typeName> argument)

where typeName is the name of the type.

So what the type definition in your code is saying is that they are declaring a new type that is a struct t_ExpoData and they are giving the new type the name ExpoData. It saves users of the type from having to type struct t_ExpoData every time they want to use it. Instead they can type just ExpoData. It could have been written as 2 different lines like this:

struct t_ExpoData {
    ... fields...
};
typedef struct t_ExpoData ExpoData;

Instead of using that instance right away, why the second declaration ExpoData expoData[4] ?

The author of the code is saying that they want an array of 4 instances of ExpoData in their ModelData structure, and they want that field to be called expoData. You would access it like this:

ModelData someModel;
someModel.expoData[0].expNorm = 5;