I have my library module:
========================================================
module mylib.vector;

// alias Vector!(float, 4) Vector4f;

struct Vector(T, uint size)
{
	T[size] array = 0;
	...
}
========================================================

And I have client module:
========================================================
import mylib.vector;

alias Vector!(float, 4) Vector4f;
	
void main()
{
	auto x = Vector4f([1.0f, 2.0f, 3.0f, 4.0f]);
}
========================================================

If alias would be in vector module (commented there) I will have to compile
both modules (otherwise I'll get link errors for some vector functions), but I want to simply import vector module and immediately use predefined aliases for vector template struct. How can I do this?

Ivan Agafonov:

> (otherwise I'll get link errors for some vector functions),<

What errors and in what cases?

Bye,
bearophile

On 9/5/2012 7:46 AM, Ivan Agafonov wrote:
> I have my library module:
> ========================================================
> module mylib.vector;
>
> // alias Vector!(float, 4) Vector4f;
>
> struct Vector(T, uint size)
> {
>      T[size] array = 0;
>      ...
> }
> ========================================================
>
> And I have client module:
> ========================================================
> import mylib.vector;
>
> alias Vector!(float, 4) Vector4f;
>
> void main()
> {
>      auto x = Vector4f([1.0f, 2.0f, 3.0f, 4.0f]);
> }
> ========================================================
>
> If alias would be in vector module (commented there) I will have to compile
> both modules (otherwise I'll get link errors for some vector functions),
> but I want to simply import vector module and immediately use predefined
> aliases for vector template struct. How can I do this?

If I understand your issue correctly, it isn't a "problem". It's the way D is designed to work. Simply importing a module does not mean that you do not also need to compile and link it in your program.

What importing does is make the symbols (declarations, definitions and so on) in the module available at compile time so that the compiler can know what is and isn't visible to the module it is currently compiling. But if there are any symbols in the imported module that need to be present at link time (such as variable declarations), then the imported module needs to be compiled and linked as well. In other words, it doesn't work like Java, where importing a class file causes the related class to be loaded a runtime.

Take the following example:

=============================
// foo.d
module foo;

struct Bar
{
    int x;
}

// baz.d
module baz;

import foo;

void main()
{
    Bar bar;
    bar.x = 1;
}
=============================

You can compile this example like so:

dmd baz.d

And you wind up with a working program. The struct definition in foo.d is just a definition. The compiler uses it to know what is and isn't callable on Bar, how much memory an instance of it needs, and so on. So you can declare and instantiate instances of it in other modules without compiling and linking the foo module. Now let's change it up a bit:

============================
// foo.d
module foo;

struct Bar
{
    int x;
}

// Declare an instance of Bar here in the foo module
Bar foobar;

// baz.d
module baz;

import foo;

void main()
{
    Bar bar;
    bar.x = 1;

    foobar.x = 2;
}
============================

Now compile like so:

dmd baz.d

And this happens:

============================
OPTLINK (R) for Win32  Release 8.00.12
Copyright (C) Digital Mars 1989-2010  All rights reserved.
http://www.digitalmars.com/ctg/optlink.html
baz.obj(baz)
 Error 42: Symbol Undefined _D3foo6foobarS3foo3Bar
--- errorlevel 1
============================

The reason is because when the compile processes baz.d, it sees that it uses the variable foobar. That variable is not found in the baz module, but it is publicly visible in the foo module which baz imports. So the compiler says, ok, this is no problem. It goes about its job and creates baz.obj and passes it off to the linker. The linker looks through baz.obj, finds a reference to foobar, which, given the name mangling (_D3foo6foobarS3foo3Bar), should exist in an object file called foo (the linker knows nothing about D modules). So the linker wants to take foo.obj and link it together with bar.obj to create the final executable. Only, there is no foo.obj because you didn't compile it.

dmd baz.d foo.d

Now both object files can be created and linked together to make the final executable.

A template alias actually instantiates a template. A template instantiation must be available to the linker. So if you instantiate in the vector module, you *must* compile and link it. Generally, you should either pass all the modules you import along to the compiler, or link with a library in which those modules have been compiled. This way, you don't have to worry about what is declared or instantiated where.

On 9/5/2012 10:50 AM, Mike Parker wrote:

>
>
> compiler says, ok, this is no problem. It goes about its job and creates
> baz.obj and passes it off to the linker. The linker looks through
> baz.obj, finds a reference to foobar, which, given the name mangling
> (_D3foo6foobarS3foo3Bar), should exist in an object file called foo (the
> linker knows nothing about D modules). So the linker wants to take
> foo.obj and link it together with bar.obj to create the final
> executable. Only, there is no foo.obj because you didn't compile it.

And, I should probably say, this isn't actually accurate. The linker knows nothing about D name mangling either. But the point is, if a symbol the linker needs isn't available in any of the object files you pass to it, it will complain.

On Wednesday, 5 September 2012 at 01:49:50 UTC, Mike Parker wrote:
> On 9/5/2012 7:46 AM, Ivan Agafonov wrote:
>> I have my library module:
>> ========================================================
>> module mylib.vector;
>>
>> // alias Vector!(float, 4) Vector4f;
>>
>> struct Vector(T, uint size)
>> {
>>     T[size] array = 0;
>>     ...
>> }
>> ========================================================
>>
>> And I have client module:
>> ========================================================
>> import mylib.vector;
>>
>> alias Vector!(float, 4) Vector4f;
>>
>> void main()
>> {
>>     auto x = Vector4f([1.0f, 2.0f, 3.0f, 4.0f]);
>> }
>> ========================================================
>>
>> If alias would be in vector module (commented there) I will have to compile
>> both modules (otherwise I'll get link errors for some vector functions),
>> but I want to simply import vector module and immediately use predefined
>> aliases for vector template struct. How can I do this?
>
> If I understand your issue correctly, it isn't a "problem". It's the way D is designed to work. Simply importing a module does not mean that you do not also need to compile and link it in your program.
>
> What importing does is make the symbols (declarations, definitions and so on) in the module available at compile time so that the compiler can know what is and isn't visible to the module it is currently compiling. But if there are any symbols in the imported module that need to be present at link time (such as variable declarations), then the imported module needs to be compiled and linked as well. In other words, it doesn't work like Java, where importing a class file causes the related class to be loaded a runtime.
>
> Take the following example:
>
> =============================
> // foo.d
> module foo;
>
> struct Bar
> {
>     int x;
> }
>
> // baz.d
> module baz;
>
> import foo;
>
> void main()
> {
>     Bar bar;
>     bar.x = 1;
> }
> =============================
>
> You can compile this example like so:
>
> dmd baz.d
>
> And you wind up with a working program. The struct definition in foo.d is just a definition. The compiler uses it to know what is and isn't callable on Bar, how much memory an instance of it needs, and so on. So you can declare and instantiate instances of it in other modules without compiling and linking the foo module. Now let's change it up a bit:
>
> ============================
> // foo.d
> module foo;
>
> struct Bar
> {
>     int x;
> }
>
> // Declare an instance of Bar here in the foo module
> Bar foobar;
>
> // baz.d
> module baz;
>
> import foo;
>
> void main()
> {
>     Bar bar;
>     bar.x = 1;
>
>     foobar.x = 2;
> }
> ============================
>
> Now compile like so:
>
> dmd baz.d
>
> And this happens:
>
> ============================
> OPTLINK (R) for Win32  Release 8.00.12
> Copyright (C) Digital Mars 1989-2010  All rights reserved.
> http://www.digitalmars.com/ctg/optlink.html
> baz.obj(baz)
>  Error 42: Symbol Undefined _D3foo6foobarS3foo3Bar
> --- errorlevel 1
> ============================
>
> The reason is because when the compile processes baz.d, it sees that it uses the variable foobar. That variable is not found in the baz module, but it is publicly visible in the foo module which baz imports. So the compiler says, ok, this is no problem. It goes about its job and creates baz.obj and passes it off to the linker. The linker looks through baz.obj, finds a reference to foobar, which, given the name mangling (_D3foo6foobarS3foo3Bar), should exist in an object file called foo (the linker knows nothing about D modules). So the linker wants to take foo.obj and link it together with bar.obj to create the final executable. Only, there is no foo.obj because you didn't compile it.
>
> dmd baz.d foo.d
>
> Now both object files can be created and linked together to make the final executable.
>
> A template alias actually instantiates a template. A template instantiation must be available to the linker. So if you instantiate in the vector module, you *must* compile and link it. Generally, you should either pass all the modules you import along to the compiler, or link with a library in which those modules have been compiled. This way, you don't have to worry about what is declared or instantiated where.

Thank you, I anderstand this now. But this should not be true in some cases:
> A template alias actually instantiates a template.
This only aliases, and if I create many specializations for future purposes, it should not instantiate each of them, sinse in code i can use any number of them, or not use at all. Real instantiation must be in the module, in which I use this aliases. I believe that this is the case, but with one factor - When I use this alias in main module compiller instantiatiates it in the module, in which it defined. I can't see the reasons by which compiller do so. Because if i put this aliases in main module or specialise in place (auto someVector = Vector!(float, 4)(0.5, ...);) compiller will not do so. And I think there will take place only one instance for each specialisation in every module in which they are used. So in which .obj file realisation appears? How many questions...
PS: Sorry for my english.

On Tuesday, 4 September 2012 at 23:51:49 UTC, bearophile wrote:
> Ivan Agafonov:
>
>> (otherwise I'll get link errors for some vector functions),<
>
> What errors and in what cases?
>

 Error 42: Symbol Undefined _D4time4math6vector16__T6VectorTfVi4Z6Vector8toStringMFZAya
hello.obj(hello)
 Error 42: Symbol Undefined _D4time4math6vector16__T6VectorTfVi4Z6Vector6__initZ
hello.obj(hello)
 Error 42: Symbol Undefined _D4time4math6vector16__T6VectorTfVi4Z6Vector6__ctorMFfZS4time4math6vector16__T6VectorTfVi4Z6Vector
hello.obj(hello)
 Error 42: Symbol Undefined _D4time4math6vector16__T6VectorTfVi4Z6Vector6__ctorMFG4fZS4time4math6vector16__T6VectorTfVi4Z6Vector

On Tuesday, 4 September 2012 at 22:46:00 UTC, Ivan Agafonov wrote:
> I have my library module:
> ========================================================
> module mylib.vector;
>
> // alias Vector!(float, 4) Vector4f;
>
> struct Vector(T, uint size)
> {
> 	T[size] array = 0;
> 	...
> }
> ========================================================
>
> And I have client module:
> ========================================================
> import mylib.vector;
>
> alias Vector!(float, 4) Vector4f;
> 	
> void main()
> {
> 	auto x = Vector4f([1.0f, 2.0f, 3.0f, 4.0f]);
> }
> ========================================================
>
> If alias would be in vector module (commented there) I will have to compile
> both modules (otherwise I'll get link errors for some vector functions), but I want to simply import vector module and immediately use predefined aliases for vector template struct. How can I do this?


I've encountered the exact same problem.

I create an alias to define a specific type out of a template in the same library module that the template resides in. I can build the library OK, but when I build an executable and link in the library, the linker chokes out with an undefined symbol names.

If I comment out the alias, and instead define the same alias in each module where it is being used, I can rebuild my library and the executable OK.

This looks like a bug to me. The mystery is why almost no one is complaining about it, so it must be that just about nobody defines template aliases in this way.

I'll wait a while to see if there are any more comments before filing a bug report

--rt

One more thing about this. If I leave in the alias in the same module where the template is defined, and also re-define the same alias in the other modules that make use out of it, the libraries will compile OK, but I'll get linking errors when I try to build an executable and link in the libs.

For each re-defined alias, I was able to confirm that the name mangling is identical in each module, so there's something going wrong specifically when the alias is defined in the same module where the template resides.

--rt

In my case, the problem had to do with the order in which I was linking my static libs, simply changing the order resolved the undefined references. Turns out it's a common problem when working with static libs and it's unrelated to D.

--rt

On Sat, Mar 02, 2013 at 04:17:10AM +0100, Rob T wrote:
> In my case, the problem had to do with the order in which I was linking my static libs, simply changing the order resolved the undefined references. Turns out it's a common problem when working with static libs and it's unrelated to D.
[...]

It's the linker's fault, not any compiler's. Symbols are resolved according to the order of modules you specify on the command-line, so order does matter, unfortunately.

IMNSHO this is a silly design -- in this day and age, one would've expected better algorithms than that for linking -- but that's how it is right now.


T

-- 
People say I'm indecisive, but I'm not sure about that. -- YHL, CONLANG