typedef in C/C++

The typedef declaration provides a way to declare an identifier as a type alias in C/C++. It is used to replace a possibly complex type name. We can also use #define for defining alias.

typedef char char_t, *char_p, (*fP)(void);

Above example declares

• char_t to be an alias for char 
• char_p to be an alias for char*
• fP to be an alias for char (*)(void) i.e. function pointer which returns char

Usage in struct

typedef char char_t, *char_p, (*fP)(void); 

char func(void){
  return 'c';
}

// Equivalent to 
// typedef struct Point Point;
typedef struct Point{
   int a;
   int b;
   Point *next;
} Point;

// Common idiom to avoid having to write "struct S"
typedef struct {
    int a;
    int b;
} S, *pS;

//Following two objects have the same type
pS ps1;
S* ps2;

// A is int[]
typedef int A[]; 

// type of a is int[2] and type of b is int[3]
A a = {1, 2}, b = {3,4,5};   

// Example showing discussed so far
int main() {
   char_t c;
   char_p p = &c;
   Point point;
   fP f = &func;
   return 0;
}

 typedef means you no longer have to write struct all over the place as shown in the above example. struct defines a tag name, which is not a type name. So below will through error

struct Point{
   int a;
   int b;
   Point *next;
};

Point point; // compile error!
struct Point point; // this is correct

Usage in Function Pointer

Below example shows the application of the typedef in declaring and using function pointer.

float doMultiplication (float num1, float num2 ) {
  return num1 * num2; 
}

// Defining a pointer to a function which returns a float and takes two parameters, each of type float.
typedef float(*pt2Func)(float, float);

pt2Func *myFnPtr = &doMultiplication;

// Invoking doMultiplication
float result = (*myFnPtr)(2.0, 5.1);