multiple definition of first defined here

Art Scpae

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25-10-2024

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Understanding the "Multiple Definition of First Defined Here" Error in C++

When you encounter the cryptic error message "multiple definition of first defined here" in your C++ code, it usually means you've stumbled upon a common compiler error related to symbol duplication. This error indicates that your compiler has found the same variable, function, or class defined multiple times in different parts of your code. Let's break down why this happens and how to fix it.

Causes of the Error:

1. Multiple Source Files: A primary reason is that you're working with multiple source files (e.g., .cpp files) that each define the same symbol.
2. Header File Inclusion: You might be including the same header file multiple times, each containing the definition of the symbol.
3. Duplicate Definitions: The most obvious cause: you literally have the same variable or function definition written in multiple places within your code.

How to Troubleshoot and Resolve:

1. Identifying the Source: The compiler usually provides a line number where the error occurs. This is your starting point. Look at the indicated line and check the surrounding code.
2. Check Header File Inclusion:
   • Are you including the same header file multiple times in your code? If so, use header guards:

     #ifndef MY_HEADER_H
     #define MY_HEADER_H
     // Code within the header file 
     #endif
     

   • Is the header file being included indirectly through other header files? This can create a cascade of unintended inclusions.
3. Separate Definitions:
   • Classes: If you're defining a class in a header file, make sure you only declare the class in the header. The actual implementation (member function definitions) should go in a separate source file.
   • Functions: Similar to classes, you can declare functions in headers and define them in separate source files.
   • Variables: For global variables, you can declare them in a header file and define them in only one source file.

Example:

Let's look at an example scenario where you might encounter the error:

Header File (my_header.h):

#ifndef MY_HEADER_H
#define MY_HEADER_H

int global_var; 

#endif 

Source File 1 (main.cpp):

#include "my_header.h"

int main() {
  global_var = 10; 
  return 0;
}

Source File 2 (other_file.cpp):

#include "my_header.h"

int main() {
  global_var = 20; 
  return 0;
}

In this case, you've defined global_var in the header file and included it in both source files. When the compiler tries to build the final program, it will find the same definition twice, resulting in the "multiple definition of first defined here" error.

Solution:

You could fix this by moving the definition of global_var to a single source file and declaring it in the header:

Header File (my_header.h):

#ifndef MY_HEADER_H
#define MY_HEADER_H

extern int global_var; // Declaration

#endif 

Source File 1 (main.cpp):

#include "my_header.h"

int global_var = 10;  // Definition

int main() {
  return 0;
}

Source File 2 (other_file.cpp):

#include "my_header.h"

// No need to redefine global_var here

In Summary:

The "multiple definition of first defined here" error can be tricky, but understanding its causes and following the principles of header file inclusion and separation of declarations and definitions can help you troubleshoot and resolve it effectively.