Nested Template Classes in Separate Files

Handling nested template classes when separating declarations and definitions can be a bit complex, but it's manageable with the right approach. Let's break it down step by step.

First, declare your nested template classes in the header file:

1// OuterTemplate.h
2#pragma once
3
4template <typename T>
5class OuterTemplate {
6 public:
7  template <typename U>
8  class InnerTemplate {
9   public:
10    void foo();
11  };
12
13  void bar();
14};

Now, let's implement these in a separate .cpp file:

1// OuterTemplate.cpp
2#include <iostream>
3#include "OuterTemplate.h"
4
5// Outer template method implementation
6template <typename T>
7void OuterTemplate<T>::bar() {
8  std::cout << "OuterTemplate::bar()\n";
9}
10
11// Inner template method implementation
12template <typename T>
13template <typename U>
14void OuterTemplate<T>::InnerTemplate<U>::foo() {
15  std::cout << "OuterTemplate::InnerTemplate::foo()";
16}
17
18// Explicit instantiations
19template class OuterTemplate<int>;
20template class OuterTemplate<int>
21  ::InnerTemplate<double>;

Note the syntax for implementing the nested template method and the explicit instantiations at the end.

Here's how you might use these nested templates:

1// main.cpp
2#include "OuterTemplate.h"
3
4int main() {
5  OuterTemplate<int> outer;
6  outer.bar();
7
8  OuterTemplate<int>::InnerTemplate<double> inner;
9  inner.foo();
10}

1OuterTemplate::bar()
2OuterTemplate::InnerTemplate::foo()

1. Syntax for nested template implementations can be confusing at first.
2. You need to explicitly instantiate both outer and inner templates for each combination you intend to use.
3. Compile times can increase with complex nested templates.

• Keep nested templates simple and avoid deep nesting if possible.
• If you find yourself with many levels of nesting, consider refactoring your design.
• Consider using type aliases to simplify complex nested template types:

1template <typename T>
2class OuterTemplate {
3 public:
4  template <typename U>
5  class InnerTemplate {
6    /* ... */
7  };
8
9  using IntInner = InnerTemplate<int>;
10};

Remember, while separating nested template declarations and definitions is possible, it often introduces more complexity than it solves. For many projects, keeping everything in the header file might be simpler and less error-prone.

Only separate if you have a specific need, such as reducing compilation times in very large projects.