Understanding template instantiation
As mentioned before, templates are only blueprints from which the compiler creates actual code when it encounters their use. The act of creating a definition for a function, a class, or a variable from the template declaration is called template instantiation. This can be either explicit, when you tell the compiler when it should generate a definition, or implicit, when the compiler generates a new definition as needed. We will look at these two forms in detail in the next sections.
Implicit instantiation
Implicit instantiation occurs when the compiler generates definitions based on the use of templates and when no explicit instantiation is present. Implicitly instantiated templates are defined in the same namespace as the template. However, the way compilers create definitions from templates may differ. This is something we will see in the following example. Let's consider this code:
template <typename T>
struct foo
{
...