Why Do I Get 'Unresolved External Symbol' Errors with C Templates?

Understanding "Unresolved External Symbol" Errors in Templates

When using templates in C where code is split between header (H) and source (CPP) files, you may encounter "unresolved external symbol" errors during linking. This arises because templates are instantiated only at the time of usage. When used, the compiler requires the complete function definition to generate the correct code.

In this scenario, the function's code is located in the template's source file, which is not accessible during the program source compilation. The compiler assumes the function is defined elsewhere and only inserts the function call. However, the template's source file does not include the specific template type used in the program source, resulting in unresolved external symbols.

To resolve this issue, consider the following options:

1. Inline Template Functions: Define all template member functions as inline in the source file. This forces the compiler to include the function code even when the specific template type is not used.
2. Header-only Template Class: Include the full definition of template member functions in the header file, eliminating the need for a separate source file and making the code available to the compiler during program source compilation.

By providing the compiler with access to the full template function code during program source compilation, these solutions effectively address the "unresolved external symbol" errors.

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