Exe size -- Sean usually includes a macro that lets you include the implementation only once (so you can use it as a pure header in multiple places, then as a definition just once).
Compile times -- if you use that macro, the implementation will be skipped by the preprocesser which is really fast.
Even if you build the whole code multiple times in your project, C compilers are also very very fast these days.
Many people follow Sean's style but use C++. That can be much slower to compile if you use fancy features. But if you avoid templates it's usually fine.
It's interesting that many big C++ projects are also header-only, but for totally the opposite reason -- they use templates everywhere so nothing can be compiled separately. That approach definitely does slow down compilation and increase your code size.
Yes, but unfortunately for C++ libraries that make heavy use of templates, it's the only solution. Because templates need to be specialized to generate any code, they can't be compiled ahead of time into shared or static libraries. So in C++ you have this stupid non-sense.
Instantiating the templates at link time would probably make more sense. With LTO we're already deferring a significant portion of the compilation process to link time so this is an obvious extension.
This issue was also on the mind of the C++ standards committee when they began work on the new module system. I'm not sure how the current spec behaves with respect to template instantiation, though.
C++98 contained some provisions for that ("export template"), but anecdotally there is exactly one compiler that supports it and the whole mechanisms was found to cause more problems than it solves. See: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n142...
Problem with template instantiation is that the mechanism is too general to be amendable to some kind of meaningful precomputation.
With Lisp you also need to have the macro definition ready when compiling code that depends on it. This is essentially the problem that all the various "system definition facilities" try to solve in at least somewhat usable manner.
You need the package definition for the source-code-read time of whatever you're compiling, and the macros for its macro-expansion time. Practically speaking, these are tied together.
It is not equally powerful, but Java's approach is easure. This means that at compile time the language can differentiate List<String> and List<Buffer>, and will type check them correctly, but at runtime the generic type gets "erased" and all the code turns into List<Object> (where in Java all types derive from Object).
As a result, there do not need to be multiple specializations of the List type for strings and buffers. No matter how many different parameterizations there are of a given generic class, there is only one implementation. I think this is probably a good thing most of the time, since on average the performance gains of this form of specialization are insignificant compared to the costs of code size. You wouldn't want 100 or more different implementations of List clogging memory and needing to be JIT-compiled.
One of Java's constraints is also that all generic types are reference types. So one reason why it wouldn't be too useful to instantiate separate List<String> and List<Buffer> is because these are basically just List<ReferenceToObject> anyway. C++ templates can provide both value and reference semantics, and can work on primitive types such as `int` which may reside on the stack, whereas Java generics only operate on full-fledged garbage collected objects that are dynamically allocated on the heap.
The disadvantage of this approach is that it precludes some constructs that are possible in other languages. For example, in C++ a template method could take some type `T` and construct a new instance of it. In C++ you could do this, but in Java it's not legal:
<T> T broken() { return new T(); }
In C++ you could also create a new local variable of type `T` that will be allocated on the stack. This `T` can represent anything from a primitive `int` to a complex struct or class, or a point or reference to a dyanmically allocated object. The C++ code could then dynamically allocate a new instance of `T`.
This makes it more difficult to use implement generic programming patterns analogous to things like std::allocator in C++, and to deliberately specify different implementations of parameterized types and algorithms. Fortunately these are not too significant of practical downsides in Java.
All modern compilers support precompiled headers. If it's a library external to your project, you just compile it once into .pch file and when you change your own code, the .pch is just loaded without having to compile the headers again.
I tend to use a single compilation unit, and use plain C. For a 50k line project I have build times of 1-1.2sec. On top of that I'm also linking to opengl, windows.h and a couple of headers in the standard lib.
Exactly :) I come from a .net background, where compile times are kinda bad compared to what I get in C. I had also heard of big C++ projects that could take more than 30 mins to build. So I was always scared to move to native code. Until I learned the single compilation unit trick! And if compile times were to get slow again, there are many more ways to bring it down even further,
Since when are .Net compile times worse than c? Csc compiles extremely fast, most long compile times are caused by msbuild doing... not much at all really.
I work on a much bigger project and we use that method (unity builds) - it helps for full builds and on small projects, but it makes incremental changes very slow on bigger projects.
Exe size -- Sean usually includes a macro that lets you include the implementation only once (so you can use it as a pure header in multiple places, then as a definition just once).
Compile times -- if you use that macro, the implementation will be skipped by the preprocesser which is really fast.
Even if you build the whole code multiple times in your project, C compilers are also very very fast these days.
Many people follow Sean's style but use C++. That can be much slower to compile if you use fancy features. But if you avoid templates it's usually fine.
It's interesting that many big C++ projects are also header-only, but for totally the opposite reason -- they use templates everywhere so nothing can be compiled separately. That approach definitely does slow down compilation and increase your code size.