Why Am I Getting \"Error LNK2005: \"int k\" Already Defined\" in My Win32 Console Application?

Error LNK2005: "int k" Already Defined

When linking a Win32 console application with multiple C files, an error "error LNK2005: "int k" (?a@@3HA) already defined in A.obj" may arise. This error occurs when a variable with the same name is defined in multiple files.

In the given example, both A.cpp and B.cpp define a variable k. According to the one definition rule, each global variable or function must have a single definition. Having multiple definitions leads to ambiguity and linking errors.

Solutions:

To resolve this error, you can use the following approaches:

Use Nameless Namespace (Anonymous Namespace):

If the variable k is intended to be private to each file, use a nameless namespace to prevent the symbol name collision.

<code class="cpp">namespace 
{
    int k;
}</code>

This isolates the symbol k within each file, preventing other files from accessing or redefining it.

Declare and Define Variable in Separate Files:

If you need to share the variable k across multiple files, use extern to declare it in the header file and define it in a separate compilation unit.

A.h

<code class="cpp">extern int k;</code>

A.cpp

<code class="cpp">#include "A.h"
int k = 0;</code>

B.cpp

<code class="cpp">#include "A.h"

// Use `k` anywhere in the file</code>

By declaring k as extern in the header file, other files can access and use it without redefining it.

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