Differences between struct in C and C++

Question

I am trying to convert a C++ struct to C but keep getting "undeclared identifier"? Does C++ have a different syntax for referring to structs?

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Answer 1

You need to refer to KEY_STATE with struct KEY_STATE. In C++ you can leave the struct out, but not in plain C.

Another solution is to do a type alias:

typedef struct KEY_STATE KEY_STATE

Now KEY_STATE means the same thing as struct KEY_STATE

Answer 2

In C, the name of the type is struct KEY_STATE.

So you have to declare the second struct as

typedef struct _DEVICE_EXTENSION
{
    WDFDEVICE WdfDevice;
    struct KEY_STATE kState;
} DEVICE_EXTENSION, *PDEVICE_EXTENSION;

If you do not want to write struct all the time, you can use a typedef declare KEY_STATE similar to DEVICE_EXTENSION:

typedef struct _KEY_STATE
{
    /* ... */
} KEY_STATE;

Answer 3

You could/should typedef the struct so that you don't need the struct keyword every time you will declare a variable of that type.

typedef struct _KEY_STATE 
{
    bool kSHIFT; //if the shift key is pressed 
    bool kCAPSLOCK; //if the caps lock key is pressed down
    bool kCTRL; //if the control key is pressed down
    bool kALT; //if the alt key is pressed down
} KEY_STATE;

Now you can do:

KEY_STATE kState;

or (as in the example you have) :

struct KEY_STATE kState;

Answer 4

You have to qualify a struct variable with the 'struct' keyword:

typedef struct _DEVICE_EXTENSION
{
    WDFDEVICE WdfDevice;
    struct KEY_STATE kState;
} DEVICE_EXTENSION, *PDEVICE_EXTENSION;

When you use DEVICE_EXTENSION you don't have to do use 'struct' because you are doing a struct definition and a typedef in a single compound statement. So you could do the same for KEY_STATE if you wanted to use it in a similar fasion:

typedef struct _KEY_STATE_t
{
    bool kSHIFT; //if the shift key is pressed 
    bool kCAPSLOCK; //if the caps lock key is pressed down
    bool kCTRL; //if the control key is pressed down
    bool kALT; //if the alt key is pressed down
} KEY_STATE;

Answer 5

There is no bool type in C prior to C99.

Also, there is no type called KEY_STATE when you do struct KEY_STATE.

Try this instead:

typedef struct _KEY_STATE 
{
    unsigned kSHIFT : 1; //if the shift key is pressed 
    unsigned kCAPSLOCK : 1; //if the caps lock key is pressed down
    unsigned kCTRL : 1; //if the control key is pressed down
    unsigned kALT : 1; //if the alt key is pressed down
} KEY_STATE;