Why Do C Templates Cause 'Unresolved External Symbol' Linker Errors?

Why Do Templates Cause "Unresolved External Symbol" Errors?

When developing C code with templates, it's common to encounter "unresolved external symbol" errors during linking. This issue arises because templated classes and functions are not instantiated until they are utilized, usually in a separate source file (.cpp).

When a template is employed, the compiler demands the complete code for that function to construct the correct function with the appropriate type. However, the function's code is contained in the template's source file and therefore unavailable.

Consequently, the compiler presumes the function is specified elsewhere and only includes its call. When the template's source file is compiled, the specific template type from the program source is not utilized, and thus it fails to generate the essential function code. This results in the unresolved external symbol error.

How to Resolve the Issue:

1. Include the Member Function Definition in the Template's Header: Define the member function in the template's header file instead of creating a separate source file. This provides the compiler with access to the complete code for the function during the compilation of the typed function in the program source.
2. Inline All Member Functions in the Template's Source: This is no longer recommended for modern versions of Visual Studio (2017 ).
3. Use the "export" Keyword in the Template's Source: Define the member functions in the template's source using the "export" keyword. However, this approach is not widely supported by compilers and has been deprecated in C 11.

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