What are the differences and usage scenarios between typedef and #define in C language?

What are the differences and usage scenarios between typedef and #define in C language?

Core Differences:

1. Processing Stage

   • #define: Preprocessor stage, text replacement
   • typedef: Compiler stage, type alias

2. Scope

   • #define: Global scope, from definition point to end of file
   • typedef: Follows normal scoping rules

3. Type Checking

   • #define: No type checking, pure text replacement
   • typedef: Has type checking, compiler validates

4. Debugging Support

   • #define: Shows original code during debugging
   • typedef: Shows alias type during debugging

Usage Scenario Comparison:

1. Type Aliases

   c
   // typedef - Recommended
   typedef unsigned int uint32_t;
   uint32_t value = 100;
   
   // #define - Not recommended
   #define uint32_t unsigned int
   uint32_t value = 100;

2. Function Pointers

   c
   // typedef - Clear and readable
   typedef int (*CompareFunc)(const void*, const void*);
   CompareFunc compare = my_compare;
   
   // #define - Hard to understand
   #define CompareFunc int (*)(const void*, const void*)
   CompareFunc compare = my_compare;

3. Structures

   c
   // typedef - Concise
   typedef struct {
       int x;
       int y;
   } Point;
   
   Point p1 = {10, 20};
   
   // #define - Not applicable

4. Array Types

   c
   // typedef - Type safe
   typedef int Array10[10];
   Array10 arr1, arr2;
   
   // #define - May cause unexpected behavior
   #define Array10 int[10]
   Array10 arr1, arr2;  // Only arr2 is an array

#define Advantage Scenarios:

1. Constant Definitions

   c
   #define MAX_SIZE 100
   #define PI 3.14159
   #define VERSION "1.0.0"

2. Conditional Compilation

   c
   #ifdef DEBUG
       #define LOG(x) printf x
   #else
       #define LOG(x)
   #endif

3. Macro Functions

   c
   #define SQUARE(x) ((x) * (x))
   #define MIN(a, b) ((a) < (b) ? (a) : (b))
   #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))

typedef Advantage Scenarios:

1. Improve Code Readability

   c
   typedef unsigned long long ull;
   typedef struct Node* NodePtr;

2. Cross-platform Compatibility

   c
   #ifdef _WIN64
       typedef __int64 intptr_t;
   #else
       typedef long intptr_t;
   #endif

3. Callback Function Types

   c
   typedef void (*Callback)(int result);
   void register_callback(Callback cb);

Common Pitfalls:

1. Macro Side Effects

   c
   #define SQUARE(x) ((x) * (x))
   int i = 2;
   int result = SQUARE(i++);  // Undefined behavior

2. Macro Parentheses Issues

   c
   #define MUL(a, b) a * b
   int result = MUL(2 + 3, 4);  // Result is 14, not 20

3. typedef Cannot Initialize Arrays

   c
   typedef int IntArray[10];
   IntArray arr = {1, 2, 3};  // Error
   int arr[10] = {1, 2, 3};   // Correct