help on typedefs - basic c/c++

Question

hi, i have been going through some code and came across a statement that somehow disturbed me.

typedef GLfloat vec2_t[2];   

typedef GLfloat vec3_t[3];

From my perspective, a statement such as

typedef unsigned long ulong;

Means that ulong is taken to mean unsigned long
Now, can the statement below mean that vec2_t[2] is equivalent to GLfloat??

typedef GLfloat vec2_t[2];

Most likely, Probably its not the intended meaning. I would appreciate it if someone clears this up for me. Thanks

Answer 1

Basically a typedef has exactly the same format as a normal C declaration, but it introduces another name for the type instead of a variable of that type.

In your example, without the typedef, vec2_t would be an array of two GLfloats. With the typedef it means the vec2_t is a new name for the type "array of two GLfloats".

typedef GLfloat vec2_t[2];

This means that these two declarations are equivalent:

vec2_t x;

GLfloat x[2];

Answer 2

In your case, vec2_t is an array of 2 GLfloats and vec3_t is an array of 3 GLfloats. You can then do stuff like:

vec2_t x;
// do stuff with x[0] and x[1]

Answer 3

It means that vec2_t is an array of 2 GLfloat and that vec3_t is an array of 3.

More info

Answer 4

The intent would be clearer if C syntax allowed it to be written as

typedef GLfloat[2] vec2_t;   

typedef GLfloat[3] vec3_t;

But that's not valid syntax.

Answer 5

When you want to create a typedef-name vec2_t for an array type GLfloat[2], the proper syntax would be not

typedef GLfloat[2] vec2_t;

(as a beginner might expect), but rather

typedef GLfloat vec2_t[2];

I.e. the general structure of the syntax here, as it has already been said, is the same an in a variable declaration.

Answer 6

C declaration syntax can be confusing, but you only need one simple rule: read from the inside-out. Once you do that, just understand typedef creates another name (an alias) for a type.

The inside is the declared-identifier (or where it would go if missing). Examples:

T a[2]; // array (length 2) of T
T* a[2]; // array (length 2) of pointer to T ([] before *)
T (*p)[2]; // pointer to array (length 2) of T (parens group)
T f(); // function returning T
T f(int, char*); // function of (int, pointer to char) returning T
T (*p)(int); // pointer to function of (int) returning T

T (*f(char, T(*)[2]))(int);
// f is a function of (char,
//                     pointer to array (length 2) of T)
//                 returning a pointer to a function of (int)
//                                                   returning T

typedef T (*F(char, T(*)[2]))(int);
// F is the type:
//        function of (char,
//                     pointer to array (length 2) of T)
//                 returning a pointer to a function of (int)
//                                                   returning T
// (yes, F is a function type, not a pointer-to-function)
F* p1 = 0; // pointer to F
T (*(*p2)(char, T(*)[2]))(int) = 0; // identical to p1 from the previous line