Why Do I Get 'Unresolved External Symbol' Errors When Using Templated C Code?

Unresolved External Symbols in Template-Based C Code

When working with templated classes and functions in C , splitting the code between a source (.cpp) and header (.h) file can lead to "unresolved external symbol" errors during linking. This can be perplexing, especially when the object file has been built and included.

Understanding the Issue

Templates are not instantiated until they are used. This means that when the compiler encounters a template, it does not generate code for it immediately. Instead, it assumes that the function definition exists elsewhere and inserts a placeholder.

However, if the function definition is placed in a separate source file, the compiler may not have access to it when compiling the template's source file. As a result, it fails to generate the specific function code, leading to the unresolved external symbol error.

Solutions

To resolve this issue, you can consider the following solutions:

1. Inline Member Functions: Declare all member functions within the template's header file using the inline keyword. This ensures that the compiler has immediate access to the function code. However, note that this approach is deprecated and may not work on all compilers.
2. Explicit Definition in Header File: Define the full function implementation within the template's header file instead of creating a separate source file. This allows the compiler to access the function code during both the program and template compilation.
3. Export Member Functions (Deprecated): Define member functions within the template's source file using the export keyword. This instructs the compiler to make the functions available outside the source file. However, this feature has been deprecated since C 11 and should not be used.

By leveraging either of these solutions, you can ensure that the compiler has access to the complete function definition during both program and template compilation, effectively preventing the occurrence of unresolved external symbols.

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