How to Use #define in C++: Syntax and Examples

In C++, #define is used to create macros or constants by defining a name that the preprocessor replaces with a value or code before compilation. It works by simple text substitution and does not respect C++ types or scopes.

Syntax

The #define directive has two main forms:

• Constant definition: #define NAME value replaces all occurrences of NAME with value.
• Macro definition: #define NAME(args) code defines a macro that replaces NAME(args) with code, where args are parameters.

This happens before the compiler runs, during preprocessing.

cpp

#define PI 3.14
#define SQUARE(x) ((x) * (x))

Example

This example shows how to use #define to create a constant and a macro function to calculate the area of a circle and square a number.

cpp

#include <iostream>

#define PI 3.14159
#define SQUARE(x) ((x) * (x))

int main() {
    double radius = 5.0;
    double area = PI * SQUARE(radius);
    std::cout << "Area of circle with radius " << radius << " is " << area << std::endl;
    return 0;
}

Output

Area of circle with radius 5 is 78.53975

Common Pitfalls

Common mistakes when using #define include:

• Not using parentheses around macro parameters and expressions, which can cause unexpected results due to operator precedence.
• Using #define for constants instead of const or constexpr, which are safer and type-aware.
• Macros do not respect scopes, so they can cause name clashes or unexpected substitutions.

cpp

#define BAD_SQUARE(x) x * x  // Wrong: missing parentheses

// Correct way:
#define GOOD_SQUARE(x) ((x) * (x))

Quick Reference

Usage	Description	Example
Constant	Defines a constant value	#define MAX 100
Macro with args	Defines a macro function	#define MUL(a,b) ((a)*(b))
Undefine	Removes a macro definition	#undef MAX
Conditional	Defines macros conditionally	#ifdef DEBUG ... #endif

Key Takeaways

#define creates text substitutions before compilation in C++.

Always use parentheses in macros to avoid operator precedence bugs.

Prefer const or constexpr for constants instead of #define.

Macros do not respect scopes and can cause unexpected issues.

Use #undef to remove macro definitions if needed.