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Comment by iamNumber4 | Hacker News Reader

You should probably read up on ABI Compatibility, and, Compiling vs linking as well.

Let's be clear a compiler builds a object files that have to be linked to create a executable binary. IE compiler makes .o files. Linker takes output object files from the compiler an puts them (.o,.so/.dll,.a) together, to make .exe files.

This all works because you are linking to the C/C++ library binaries (the .o, .so, .a, .dll) files generated by a C/C++ compiling/linking. In most cases this is shared object library/dll provided by the operating system or pre-built library you install on the system. There is no need to compile it because it is pre-built. if you do compile the library yourself, it is a one time thing.

In D you create a .d file that describes what is in a C++ library .o/.so/.a/.dll file. So that upon linking, you take your D code's .o files + the C/C++ .o files to put together a full executable binary.

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That's not how C++ templates work. You cannot deliver them as binaries.

Templates are delivered as C++ include files (i.e. source code). When you use them, the compiler fills in the types you want and compiles the result.

For example, there's the std::vector<> template. You can fill in any type that meets the specific requirements of the template and use it like std::vector<int> or std::vector<YourC++ClassType>.

However, you probably won't be able to do std::vector<YourDClassType>. ABI compatibility is not enough to do this as you would be effectively mixing C++ and D source code here.

> However, you probably won't be able to do std::vector<YourDClassType>. ABI compatibility is not enough to do this as you would be effectively mixing C++ and D source code here.

I think this should be working. You don't have to 'mix source code', you can implement YourDClassType in D and use it in C++. You simply have to write a C++ header describing the class. Then write a D file describing the external C++ sdt::vector<YourDClassType> file and link everything together.

Not sure if anybody actively tested this though ;-)

As mentioned before, C++ compatibility is not 100% yet. My comments are purely what works now; which by describing to D what is in a c/c++ .o file so you can link it in. In your .d file that describes the C/C++ you want to use, sometimes you will have to describe the template to D by creating D template of the C++ template.

Also if your "library" is all templates, it's not a library, it's a framework that uses other libraries. There still has to be at some point tangible code that is compiled to an object file to be linked. You will have to find all of the dependancies and make sure that they are also described in a .d file.

This might mean you have a bunch of work that you need to do in order use the c++ code you want to bring in into D. But then again, if it's a public/open source library, and you go down that rabbit hole and make it work, you just made D even better.

please see https://dlang.org/spec/cpp_interface.html#cpp-templates for more details.

> if your "library" is all templates, it's not a library

TIL STL is not a library.

I think you misunderstood your parent. A lot of modern C++ 'libraries' are header-only, because they heavily use templates. There is no object code or library to link against until you instantiate templates.

That's not really relevant to their question. Many templates are implemented via inlining/instantiation in client code by default, and as such they won't be present in the output of the C++ compiler.

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