Are there any compile-time optimizations I can use with separated template implementations?

Question

Ryan McCombe · Accepted Answer

Yes, there are several compile-time optimizations you can employ when working with separated template implementations.

These optimizations can help reduce compilation times and improve overall build efficiency. Let's explore some key strategies:

1. Explicit Template Instantiation

One of the most effective optimizations is explicit template instantiation:

1// MyTemplate.cpp
2#include "MyTemplate.h"
3
4template <typename T>
5void MyTemplate<T>::foo() {
6  // Implementation
7}
8
9// Explicit instantiations
10template class MyTemplate<int>;
11template class MyTemplate<double>;

This technique reduces the number of instantiations across translation units, potentially speeding up compilation.

2. Extern Templates

Extern templates, introduced in C++11, can significantly reduce compilation times:

1// MyTemplate.h
2template <typename T>
3class MyTemplate {
4  // Class definition
5};
6
7extern template class MyTemplate<int>;
8
9// MyTemplate.cpp
10template class MyTemplate<int>;

This tells the compiler not to instantiate the template in other translation units, reducing redundant work.

3. Precompiled Headers

For large projects, using precompiled headers can dramatically reduce compilation times:

1// stdafx.h
2#include <vector>
3#include <string>
4#include <iostream>
5
6// Other commonly used headers
7// ...
8
9// Use this precompiled header in your .cpp files
10#include "stdafx.h"

4. Forward Declarations

Use forward declarations where possible to reduce header dependencies:

1// Forward declare instead of including the full header
2template <typename T>
3class MyTemplate;
4
5void someFunction(MyTemplate<int>& obj);

5. Template Metaprogramming Techniques

Utilize template metaprogramming to perform computations at compile-time:

1template <unsigned N>
2struct Factorial {
3  static constexpr unsigned value =
4    N * Factorial<N - 1>::value;
5};
6
7template <>
8struct Factorial<0> {
9  static constexpr unsigned value = 1;
10};
11
12// Usage
13constexpr unsigned result = Factorial<5>::value;

6. Concepts and Constraints (C++20)

Using concepts can lead to faster compilation and better error messages:

1#include <concepts>
2
3template <std::integral T>
4class MyIntegralTemplate {
5  // Implementation
6};

7. Modules (C++20)

Although not directly related to templates, using modules can significantly improve compilation times:

1// MyModule.ixx
2export module MyModule;
3
4export template <typename T>
5class MyTemplate {
6  // Implementation
7};

1// Usage
2import MyModule;

8. Lazy Instantiation

Design your templates to delay instantiation of member functions until they're actually used:

1template <typename T>
2class MyTemplate {
3 public:
4  void commonFunction() {
5    // Implement here, always instantiated
6  }
7
8  void rarelyUsedFunction();
9};

1// In a separate .cpp file
2template <typename T>
3void MyTemplate<T>::rarelyUsedFunction() {
4  // Implementation here, only instantiated when used
5}

Remember, the effectiveness of these optimizations can vary depending on your specific project structure and compiler. Always measure the impact on your build times to ensure these techniques are beneficial for your use case.

Templates and Header Files

Compile-Time Optimizations for Separate Templates