非模板类与模板类的多个定义

Question

为什么编译器抱怨在多个.cpp文件中定义的非模板类，但是对于模板类来说很好，其定义在各种.cpp文件中重复（通过包含类的.inl文件），即使class是否在多个.cpp文件中显式实例化？

Answer 1

The non-template case is because in that scenario your program violates the one definition rule so the linker (not compiler) will complain about multiple definitions.

For templates on the other hand, the language specifies that this must work and the linker sorts out what to do. I'm not 100% sure if an explicit template instantiation should be treated the same as a non-template function though.

Answer 2

模板函数是内联的，只要每个定义相同，就允许在多个编译单元中定义内联函数。

Answer 3

Do all compilers complain, always? I've never seen one which did, and the standard doesn't allow it: you're allowed to define a class or a template once in each translation unit, as long as all of the definitions are identical. In fact, you're required to define the class in every translation unit which uses it in a way that requires it to be a complete type. C++ does not have any mechanism for exporting class definitions to other translation units.

Are you sure you're not confusing classes with functions. You're not allowed to define a function more than once, unless it is inline. You still have to define a function template in each translation unit which uses it, and the same rules apply for function templates as for classes and class templates.

Note that if you violate these rules, by defining the function in more than one translation unit, or by the definitions of the classes or templates not being tokenwise identical (after preprocessing, and including name binding), then you have undefined behavior. The compiler (actually the linker) may complain about it, but it's not required: most complain about multiple definitions of a function, but I don't know of any which complain when the class or template definitions differ between translation units.