#ifdef vs #if defined()

Both #ifdef and #if defined() are used for conditional compilation in C++, but they have a slight difference in syntax.

The #ifdef MACRO checks if MACRO is defined. If it is, the code block following the directive is included in the compilation.

1#include <iostream>
2#define DEBUG
3
4int main() {
5#ifdef DEBUG  
6  std::cout << "Debug mode is on\n";
7#endif
8}

1Debug mode is on

#if defined(MACRO) also checks if MACRO is defined, but it allows for more complex expressions.

1#include <iostream>
2#define DEBUG
3
4int main() {
5#if defined(DEBUG) && !defined(RELEASE)  
6  std::cout << "Debug mode is on,"
7    " Release mode is off\n";
8#endif
9}

1Debug mode is on, Release mode is off

In most cases, #ifdef and #if defined() can be used interchangeably. However, #if defined() provides more flexibility when combining multiple conditions using logical operators like && and ||.