Template Performance: Inline vs. Separate Files

When it comes to template performance, the location of the template definition (inline in the header or in a separate implementation file) doesn't directly affect runtime performance. However, it can impact compile times and code organization.

Inline template definitions in headers can lead to longer compile times, especially for large projects. This is because the template code is compiled in every translation unit that includes the header. On the other hand, separate implementation files can potentially reduce compile times if you use explicit instantiation.

1// In a .cpp file
2template class MyTemplate<int>;
3template class MyTemplate<double>;

This approach can reduce overall compilation time in large projects by limiting the number of instantiations.

At runtime, there's typically no performance difference. Modern compilers are good at optimizing template code, regardless of where it's defined. The generated machine code should be identical in both cases.

Separate implementation files can improve code organization, especially for complex templates. It keeps header files cleaner and easier to read. However, it requires careful management to avoid linker errors.

Remember that member functions defined within the class template are implicitly inline:

1template <typename T>
2class MyClass {
3 public:
4  // Implicitly inline
5  void foo() {
6    // ...
7  }
8};

This can lead to better performance through inlining, regardless of whether the template is in a header or separate file.

In summary, the choice between inline and separate implementations mainly affects compile times and code organization. For most projects, inline definitions in headers are simpler and less error-prone. Only consider separate implementations if you're facing significant compile-time issues in large projects.