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Preprocessor directives

Preprocessor directives are instructions that tell the compiler how to process the source code before actual compilation begins. They do not produce executable code but instead control how the compiler treats parts of the code.

Think of them as the control-flow of the compilation and not of the actual code.

Directive Description
#define Defines a symbol (macro in C++).
#undef Undefines a previously defined symbol.
#if, #elif, #else, #endif Conditionally includes/excludes code.
#region, #endregion (C# only) Organizes code into collapsible sections in IDEs like Visual Studio.
#error Forces a compilation error with a custom message.
#warning Issues a compiler warning with a custom message. (C# only)
#pragma Sends special instructions to the compiler (e.g., disable warnings).
#line Modifies line numbers for compiler/debugger.

Conceptual Differences

  • C# Preprocessor Directives: Mostly for conditional compilation, regions, warnings, etc., integrated with Visual Studio tooling.
  • C++ Preprocessor Directives: More low-level and powerful, often used for header guards, macros, platform-specific code, and more.

Example

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#include <iostream>

#define DEBUG_MODE

void Initialize()
{
    std::cout << "Initializing..." << std::endl;
}

void Cleanup()
{
    std::cout << "Cleaning up..." << std::endl;
}

int main()
{
#ifdef DEBUG_MODE
    std::cout << "Debug mode active" << std::endl;
#endif

    Initialize();
    Cleanup();

    return 0;
}

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#define DEBUG

class Program
{
    #region Initialization
    static void Initialize()
    {
        Console.WriteLine("Initializing...");
    }
    #endregion

    #region Cleanup
    static void Cleanup()
    {
        Console.WriteLine("Cleaning up...");
    }
    #endregion

    static void Main()
    {
#if DEBUG
        Console.WriteLine("Debug mode active");
#endif
        Initialize();
        Cleanup();
    }
}

C/C++: Header Guards

C++ often uses preprocessor directives to avoid duplicate definitions across header files.

Example

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// MyHeader.h
#ifndef MYHEADER_H
#define MYHEADER_H

void SayHello();

#endif // MYHEADER_H

Note

  • #region / #endregion is not supported in standard C++, it's a Visual Studio extension primarily for C#.
  • C++ uses #pragma more frequently for compiler-specific behavior (e.g., packing structs or disabling warnings).
  • #error is useful in both C# and C++ for enforcing platform or configuration requirements.