How Can I Resolve Multiple Definition Errors Caused by Global Variables in C ?

Resolving Multiple Definition Errors: Understanding Variable Scope in C

In C , multiple definitions of a variable can lead to compilation errors. To understand this issue, let's delve into the given scenario.

Consider the provided four files: FileA.cpp, FileA.h, FileB.cpp, and FileB.h. In FileB.h, a global variable named wat is declared as int wat;. However, during compilation, the compiler detects multiple definitions of wat.

This error arises because the variable wat is defined in both FileB.h and FileB.cpp. By default, C places global variables in the global scope, which is shared across all compilation units. When multiple definitions of the same variable exist, the compiler cannot determine which definition to use.

To resolve this issue, one can leverage the concept of forward declaration. This technique allows us to declare the existence of a variable without providing its definition. In this case, modify FileB.h as follows:

extern int wat;

In FileB.cpp, define the variable as usual:

int wat = 0;

By using the extern keyword, we are instructing the compiler that wat exists elsewhere in the program and that its definition will be provided later. This ensures that wat is declared and defined only once.

In summary, multiple variable definitions occur when a variable is defined more than once without proper scoping mechanisms. Forward declarations help resolve this by indicating the existence of a variable while deferring its definition to a specific source file.

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