Why Am I Getting an \'Unresolved External Symbol\' Error (LNK2001) for Static Objects?

Resolving Unresolved External Symbol Error (LNK2001) for Static Objects

In an attempt to assign a value to a static field in one class from the main method, you have encountered an unexplained error, "unresolved external symbol." This error indicates that the linker was unable to locate a definition for the static member variable during the linking process.

According to the C reference, a declaration of a static data member inside a class definition is not a definition. This means that you cannot define the static member variable within the class itself. To resolve the error, you must define the static member outside the class in a namespace scope.

In your case, you should define the static member variable B::a outside the B class, as follows:

<code class="cpp">A* B::a;</code>

This ensures that the linker can successfully locate the definition of the static member variable. By following the One Definition Rule (ODR), which ensures that there is only one definition for each entity in the program, the compiler can resolve the external symbol and successfully link the code.

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