How do I get the number of members in a structure?

Question

I want to count the number of members in a structure. For example:

typedef struct
{
    char    MrChar;
    int MrInt;
    long    MrLong;
} Bg_Typedef;
Bg_Typedef FooStr;

I create a function prototype that should return number of members in the structure

int NumberOfMem(Bg_Typedef *psStructure); 

=> NumberOfMem(&FooStr) should return 3

Answer 1

It can be done with X_MACRO 's.

Do something like this:

#define X_BG_MEMBERS \
    X(char, MrChar) \
    X(int, MrInt) \
    X(long, MrLong)

typedef struct {
#define X(type, member) type member;
    X_BG_MEMBERS
#undef X
} Bg_Typedef;

Bg_Typedef FooStr;

Define a function which will count the members. Can also just be a variable, but make the variable static const so that it is not overwritten

static int
bg_members_count() {
    #define X(_, __) +1
    static int COUNT = 0
    X_BG_MEMBERS;
    #undef X
    
    return COUNT;
}

Now you can do something like this in main:

#include <stdio.h>
...

int main() {
    printf("The number of members defined in Bg_Typedef is %d\n", bg_members_count());
}

You should get something like:

The number of members defined in Bg_Typedef is 3

You might also just want a constant, so you can do the following

#define X(_, __) +1
static const int COUNT = X_BG_MEMBERS;
#undef X

---

Alternative Pattern

In order to avoid having lots of #define X... followed by #undef X , it may be beneficial to do something like this instead:

#define X_BG_MEMBERS(X) \
    X(char, MrChar) \
    X(int, MrInt) \
    X(long, MrLong)

#define BG_STRUCT_FIELD(type, field) type field;
#define BG_COUNT_MEMBER(_, __) +1

typedef struct {
  X_BG_MEMBERS(BG_STRUCT_FIELD)
} Bg_Typedefarguably;

static int
bg_members_count() {
    static int COUNT = X_BG_MEMBERS(BG_COUNT_MEMBER);    
    return COUNT;
}

// OR constant
// static const int COUNT = X_BG_MEMBERS(BG_COUNT_MEMBER);

It works the same as the above, but should be noticeably more readable. See ref .

Answer 2

There is no way to do this that is inbuilt into the C language AFAIK. If you want to do this you would need to remember the number of members or hard code the number as return value of your function. C can tell you the size in bytes of your structs but not the number of members they contain. Alternatively you could use a member function of your struct to return the hard coded number of members.

Answer 3

C only allows you to determine the number of bytes a structure requires (including padding bytes) using the sizeof operator. As long as the struct members all have the same type, you can use sizeof(struct foo)/sizeof(membertype) to compute the number of members. In the general case, with differently sized member types, this is impossible from within the C language (you could post process the source automatically and fill in the result, but that's ugly). C simply does not allow what is called Introspection in other languages (like eg perl).

But then, you (and the compiler) know the number of members at compile time . Why do you want to compute a known number at runtime ? Maybe you can state the actual problem you are trying to solve and we can point to a solution not involving member counts...

Answer 4

This cannot be done in C.

If you really need this, you should try a more high level language which supports reflection. (Java, Python ).

http://en.wikipedia.org/wiki/Reflection_%28computer_programming%29