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I once read somewhere that C++ essentially enumerates all of the possible template types based on usage at compile time so the concept of template doesn't exist during runtime. It also seems that's what the accepted answer is from Template Compilation

My question is, if this is the case, how does STL handle custom types, when everything is compiled and done already? This could more broadly apply to any custom library that's compiled with templates.

(I could have asked this in the answer's comment but I don't have enough points)

XYZ
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    they are not compiled... – user1810087 May 24 '19 at 07:40
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    STL is not a thing for quite some time now. If you mean the standard C++ library, then large chunks of it, in particular the parts that were derived from STL, exist in header files and are only compiled togetger with the user program. – n. m. could be an AI May 24 '19 at 07:48

1 Answers1

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If a library wants to provide templates which its clients can instantiate with arbitrary types, it must provide the templates' complete definitions in header files. This is why a lot of C++ libraries, including most of Boost, are header-only. The compiler then has access to the template's definition and can instantiate it with any types/values the client provides as template arguments.

For a detailed treatment of the topic, please refer to the Stack Overflow question Why can templates only be implemented in the header file?.

Note that this only applies if, as I said, the templates are intended for use with arbitrary types. If the set of instantiations is limited and can be determined at the time the shared library is built, the library can create explicit instantiations of all the templates it wants all combinations of template arguments it wants to support. Then, exposing the definitions of the templates is not necessary, but of course, it will not be possible to instantiate the templates with different types in client code.

As an example of this, there are some geometric libraries which provide their definitions as templates so that they can work with both float and double to represent floating-point numbers, but do not expose the template definitions; they simply pre-instantiate all their code with float and double. Clients can then use these instantiations, but cannot use them with for exmaple long double or MyCustomFloat.

Angew is no longer proud of SO
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