Most important things about C++ templates… lesson learned

Question

What are most important things you know about templates: hidden features, common mistakes, best and most useful practices, tips...common mistakes/oversight/assumptions

Answer 1

From "Exceptional C++ style", Item 7: function overload resolution happens before templates specialization. Do not mix overloaded function and specializations of template functions, or you are in for a nasty surprise at which function actually gets called.

template void f(T t) { ... }   // (a)
template void f(T *t) { ... }  // (b)
template<> void f(int *t) { ... } // (c)
...
int *pi; f(pi); // (b) is called, not (c)!

On top of Item 7:

Worse yet, if you omit the type in template specialization, a different function template might get specialized depending on the order of definition and as a result a specialized function may or may not be called.

Case 1:

template void f(T t) { ... }  // (a)
template void f(T *t) { ... } // (b)
template<> void f(int *t) { ... }      // (c) - specializes (b)
...
int *pi; f(pi); // (c) is called

Case 2:

template void f(T t) { ... }  // (a)
template<> void f(int *t) { ... }      // (c) - specializes (a)
template void f(T *t) { ... } // (b)
...
int *pi; f(pi); // (b) is called