The C++ standard says nothing about how implementations should manage template i nstantiation, so every implementation handles instantiation in its own particular way. While we cannot say exactly how your compiler will handle instantiation, there are two important points to keep in mind: The first is that for C++ implementations that follow the traditional edit-compile-link model, instantiation often happens not at compile time, but at link time. It is not until the templates are instantiated that the implementation can verify that the template code can be used with the types that were specified. Hence, it is possible to get what seem like compile-time errors at link time.
The second point matters if you write your own templates: Most current implementations require that in order to instantiate a template, the definition of the template, not just the declaration, has to be accessible to the implementation. Generally, this requirement implies access to the source files that define the template, as well as the header file. How the implementation locates the source file differs from one implementation to another. Many implementations expect the header file for the template to include the source file, either directly or via a #include. The most certain way to know what your implementation expects is to check its documentation.
8.1.2 Template instantiation （Accelerated C++）,布布扣,bubuko.com