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How can I avoid recompiling a specific template function each time I compile my project?

May 22, 2023

Say I have a file foo.hpp with a template

template<int a>
void foo()
{
    // Complex function
}

which I use in my main.cpp:

#include "foo.hpp"

int main()
{
    // quickly compiled code
    // ...

    foo<3>();

    // more quickly compiled code
}

Now each time I compile my project after changing main.cpp, foo<3>() needs to be compiled again, even though it doesn’t change.
The vast majority of the compile time is spent compiling foo<3>() in my case, so avoiding this is critical.

I would like to compile my project in two steps:

Compile foo<3>(). This takes long but I only do it once.
Compile main.cpp. This is now fast because foo<3>() is already compiled.

How can I achieve this behavior? I tried doing explicit instantiation in a different file and compiling this first, but main.cpp still takes the same time to compile.

Thanks for any help!

EDIT:
Clarification of what I tried using explicit instantiation:

Create a new file precompiled.cpp:

#include "foo.hpp"

template void foo<3>();

Then tried compiling this first with g++ -c precompiled.cpp and afterwards, compile main.cpp using g++ precompiled.o main.cpp.
But this instantiates foo<3>() again in step 2, which I want to avoid.