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Templates, on the other hand, are not code per se, but
instructions for code generation. Only template instantiations are real code.
When a compiler has seen a complete template definition during a compilation
and then encounters a point of instantiation for that template in the same
translation unit, it must deal with the fact that an equivalent point of
instantiation may be present in another translation unit. The most common
approach consists of generating the code for the instantiation in every
translation unit and letting the linker weed out duplicates. That particular
approach also works well with inline functions that cannot be inlined and with
virtual function tables, which is one of the reasons for its popularity. Nonetheless,
several compilers prefer instead to rely on more complex schemes to avoid
generating a particular instantiation more than once. Either way, it is the
responsibility of the C++ translation system to avoid errors due to multiple
equivalent points of instantiation.
A drawback of this approach is that all template source code
is visible to the client, so there is little opportunity for library vendors to
hide their implementation strategies. Another disadvantage of the inclusion
model is that header files are much larger than they would be if function
bodies were compiled separately. This can increase compile times dramatically
over traditional compilation models.
To help reduce the large headers required by the inclusion
model, C++ offers two (non-exclusive) alternative code organization mechanisms:
you can manually instantiate each specialization using explicit instantiation
or you can use exported templates, which support a large degree of
separate compilation.
| Thinking in C++ Vol 2 - Practical Programming |
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