Why Am I Getting an \"Unresolved External Symbol\" Error in Visual Studio?

Unresolved External Symbol: A Common Error in Visual Studio

While coding in Visual Studio, encountering an unresolved external symbol error can be frustrating. It indicates that the compiler cannot find the definition of a function or variable referenced in your code. Here's how to approach this issue:

Causes of Unresolved External Symbols

This error typically occurs when:

• A function or variable is declared without a corresponding definition in your project.
• The definition is present but cannot be found by the compiler due to missing include directives or library dependencies.

Identifying the Problem

To identify the missing definition, examine the error message carefully. It will specify the unresolved symbol and the referring function. This can point you to the source file containing the declaration but not the definition.

Fixing the Issue

1. Verify Function Definitions: Ensure that the function or variable in question is properly defined in the corresponding source file (.cpp). Look for declarations ending in a semicolon (;) but definitions missing the body (brackets {}) or the class scope (A::).
2. Check Include Directives: Verify that the header file (.h) containing the declaration is included in the source file (.cpp) where the symbol is used.
3. Link to External Libraries: If the definition is not found in your project, check if it is defined in an external library. Add the necessary library dependencies and link to the library in your project's settings.
4. Rebuild the Project: Once the missing definition is addressed, rebuild the project to ensure that the compiler can find all required symbols.

Example

Suppose you get the following error:

error LNK2019: unresolved external symbol "void myClass::myFunction()"

This indicates that the function myFunction is declared in the class myClass but not defined. To resolve it, ensure that you have a definition like this:

void myClass::myFunction()
{
  // Function body
}

in the corresponding source file and that you have included the necessary header file in the source file where you use myFunction.

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