Ultimate Guide: Fixing C++ Error - Redefinition of Class (Step-by-Step)

C++ is a powerful programming language, but like any other language, it has its quirks. One common issue that developers encounter is the "redefinition of class" error. This error occurs when the compiler encounters multiple declarations of the same class in a program. In this guide, we'll walk you through the steps to fix this error and explain why it occurs.

Table of Contents

  1. Understanding the Redefinition of Class Error
  2. Step-by-Step Guide to Fixing the Error
  3. Step 1: Locate the Redefinition
  4. Step 2: Check for Multiple Inclusions
  5. Step 3: Use Include Guards
  6. Step 4: Use #pragma once
  7. FAQ
  8. Related Links

Understanding the Redefinition of Class Error

The "redefinition of class" error occurs when the compiler encounters two or more declarations of the same class. This can happen for several reasons, such as:

  • Copy-pasting code without modifying class names.
  • Including header files multiple times in different parts of the program.
  • Circular dependencies between header files.

In C++, each class should be defined only once to avoid ambiguity in the program. The compiler needs to know the exact structure of a class and its member functions to generate correct and efficient code.

Step-by-Step Guide to Fixing the Error

Step 1: Locate the Redefinition

First, you need to find the locations in your code where the class is being redefined. The error message from the compiler will usually provide the file names and line numbers of the redefinition.

For example, the error message might look like this:

error: redefinition of 'class MyClass'
In file included from main.cpp:2,
                 from main.cpp:5:
my_class.h:4:7: note: previous definition of 'class MyClass'

In this case, the redefinition occurs in my_class.h at line 4.

Step 2: Check for Multiple Inclusions

Check if the same header file is being included multiple times. This can happen when the header file is included in multiple source files or when it's included indirectly through other header files.

To identify multiple inclusions, search your codebase for #include "my_class.h" (replace my_class.h with the name of the header file containing the class definition). If you find it in multiple places, you might be dealing with a multiple inclusion issue.

Step 3: Use Include Guards

To prevent the same header file from being included multiple times, use include guards. Include guards are preprocessor directives that ensure a header file is included only once in a compilation unit.

Here's how to add include guards to a header file:

#ifndef MY_CLASS_H
#define MY_CLASS_H

// Your class definition goes here

#endif // MY_CLASS_H

Replace MY_CLASS_H with a unique identifier for your header file. This will prevent the header file from being included more than once and should fix the "redefinition of class" error if it's caused by multiple inclusions.

Step 4: Use #pragma once

Another way to prevent multiple inclusions is to use the #pragma once directive. This directive is supported by most modern compilers and can be a more concise alternative to include guards. Simply add #pragma once at the beginning of your header file:

#pragma once

// Your class definition goes here

Keep in mind that #pragma once is not part of the C++ standard, so it might not be supported by all compilers. However, it's widely supported and is considered a safe and efficient option for preventing multiple inclusions.


1. What is the difference between declaration and definition in C++?

In C++, a declaration introduces a name into the program and specifies its type, while a definition provides a complete description of an object or function. For example, a class declaration might look like this:

class MyClass;

A class definition includes the class's member variables and functions:

class MyClass {
    int my_var;
    void my_function();

2. Can I declare a class multiple times?

Yes, you can declare a class multiple times, as long as the declarations are identical. However, you can define a class only once in your program.

3. What is a compilation unit?

A compilation unit is a single source file (along with all the header files it includes) that is compiled into an object file by the compiler. Each compilation unit is compiled separately.

4. What are circular dependencies in header files, and how do they cause the "redefinition of class" error?

Circular dependencies occur when two or more header files depend on each other, either directly or indirectly. This can lead to multiple inclusions of the same header file, causing the "redefinition of class" error. To avoid circular dependencies, use forward declarations and include only the necessary header files.

5. Can I use both include guards and #pragma once in the same header file?

Yes, you can use both include guards and #pragma once in the same header file. However, it's usually unnecessary because most modern compilers support #pragma once. If you're using a widely-supported compiler, you can choose either include guards or #pragma once.

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