Defining Constants with #define

While #define can be used to define constants in C++, it's generally recommended to use const or constexpr variables instead. Here's why:

Type Safety

With #define, the preprocessor performs a simple text replacement. The constant PI doesn't have a type, which can lead to unexpected behavior or type mismatches.

1#define PI 3.14159
2
3double area = PI * radius * radius;

Scoping and Namespaces

Using const or constexpr, the constant PI is a typed variable with a specific scope. It respects namespaces and follows the usual scoping rules, preventing name clashes.

1const double PI = 3.14159;
2
3double area = PI * radius * radius;

Debugging

When using #define, the preprocessor replaces MAX_SIZE with 100 before compilation. If you encounter an issue related to MAX_SIZE, the debugger will show 100 instead of MAX_SIZE, making it harder to understand the code.

1#define MAX_SIZE 100
2
3int array[MAX_SIZE];

Consistency with Variables

By using const or constexpr, you define constants in the same way as variables, making the code more consistent and readable.

1constexpr int MAX_SIZE = 100;
2
3int array[MAX_SIZE];

In modern C++, const and constexpr provide a type-safe and more maintainable way to define constants. They offer better type checking, scoping, and debugging capabilities compared to #define. Therefore, it's recommended to prefer const and constexpr over #define for defining constants in C++.