Since there are two ways of writing enums, structs, unions or types where one uses typedef, or alternatively doesn't, I was wondering what would be the benefits and disadvantages of each approach.
E.g. 1: typedef enum { ENUM_A, ENUM_B } ENUM_OBJECT;
E.g. 2: typedef unsigned char uint8
Personally I like encapsulation provided by the second variant, but when I write code for all enums, structs and unions I would always avoid writing the first example with typedef, but I would use this approach:
E.g. 1: enum ENUM_TAG { ENUM_A, ENUM_B };
enum ENUM_TAG some_variable;
Yes, I know that it can take a bit more horizontal space, but to me it better details idea of what is the type than something like this:
typedef int matrix_buffer_t[2][5];
matrix_buffer_t some_variable;
Can someone outline facts (not personal opinions, since these are discussible) about differences between usage of typedef and no typedef? How does this influence compiled code, program memory size, etc?
I've tried looking at the assembly diff when I compile the following code with no typedef:
struct TestStruct
{
int field;
};
int main(void)
{
struct TestStruct test;
printf ("%d\n", test.field);
return 1;
}
vs code with typedef:
typedef struct
{
int field;
} TestStruct;
int main(void)
{
TestStruct test;
printf ("%d\n", test.field);
return 1;
}
Assembly is definitely not the same. I'm giving a side to side comparison:
___________________________________________________________________________________________________________
.LC0: .LC0:
.string "%d\n" .string "%d\n"
main: main:
pushq %rbp push rbp
movq %rsp, %rbp mov rbp, rsp
subq $16, %rsp sub rsp, 16
movl -4(%rbp), %eax mov eax, DWORD PTR [rbp-4]
movl %eax, %esi mov esi, eax
movl $.LC0, %edi mov edi, OFFSET FLAT:.LC0
movl $0, %eax mov eax, 0
call printf call printf
movl $1, %eax mov eax, 1
leave leave
ret ret
It was compiled using godbolt and gcc compiler. For sure I can see the difference I am just wondering when it is better to use which approach depending on benefits/flaws.
Note: I've tried compiling to a .map file which gives addresses of each variable, size, types, etc. and when typedef is used, .map file becomes more compex.
Typedef is the only feature that makes C language grammar a context-sensitive grammar.
Its effect is to change the environment of the parser, to convert some symbols into special symbols.
Without typedef a parser for the C language would not need an environmental structure to keep track of the definitions of type symbols.
Note that in ISO/IEC 9899:1999 5.1.1.2 Translation phases, typedef acts into the 7th step (the syntax analyser step):