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digitalmars.D.learn - Missing Symbol Accessing Templated Function Through Interface

reply Peter <peter_a_wood yahoo.com> writes:
Hi,

Can anyone explain to me what's causing the following code to 
generate a missing symbol error...

import std.stdio;

interface IProblem {
    void writeln(T...)(T arguments);
}

class Problem : IProblem {
    void writeln(T...)(T arguments) {
       // This is just here to have code in an implementation.
       stdout.writeln("The implementation function was called.");
    }
}

IProblem getProblem() {
    return(new Problem());
}

void main() {
    auto problem = getProblem();

    problem.writeln("Some text and a number - ", 1234);
}

Now obviously this codes a bit contrived but its my attempt to 
reduce an issue I'm seeing to its more concise form. Note that if 
I cast the return value from the call to getProblem() to a 
Problem instance this rectifies the issue but I'm unclear as to 
why this might be. The error I get when I compile this is...

    Undefined symbols for architecture x86_64:
      "_D3app8IProblem18__T7writelnTAyaTiZ7writelnMFAyaiZv", 
referenced from:
          __Dmain in template_issue.o
    ld: symbol(s) not found for architecture x86_64
    clang: error: linker command failed with exit code 1 (use -v 
to see invocation)
    --- errorlevel 1
    dmd failed with exit code 1.

Apologies is the cause is blatantly obvious to more experienced D 
coders.

Regards,

Peter
Mar 08 2016
next sibling parent Nicholas Wilson <iamthewilsonator hotmail.com> writes:
On Tuesday, 8 March 2016 at 11:50:32 UTC, Peter wrote:

 Hi,

 Can anyone explain to me what's causing the following code to 
 generate a missing symbol error...

 import std.stdio;

 interface IProblem {
    void writeln(T...)(T arguments);
 }

 class Problem : IProblem {
    void writeln(T...)(T arguments) {
       // This is just here to have code in an implementation.
       stdout.writeln("The implementation function was called.");
    }
 }

 IProblem getProblem() {
    return(new Problem());
 }

 void main() {
    auto problem = getProblem();

    problem.writeln("Some text and a number - ", 1234);
 }

 Now obviously this codes a bit contrived but its my attempt to 
 reduce an issue I'm seeing to its more concise form. Note that 
 if I cast the return value from the call to getProblem() to a 
 Problem instance this rectifies the issue but I'm unclear as to 
 why this might be. The error I get when I compile this is...

    Undefined symbols for architecture x86_64:
      "_D3app8IProblem18__T7writelnTAyaTiZ7writelnMFAyaiZv", 
 referenced from:
          __Dmain in template_issue.o
    ld: symbol(s) not found for architecture x86_64
    clang: error: linker command failed with exit code 1 (use -v 
 to see invocation)
    --- errorlevel 1
    dmd failed with exit code 1.

 Apologies is the cause is blatantly obvious to more experienced 
 D coders.

 Regards,

 Peter


Probably because the idea of a template function in an interface 
is
wrong, and I'm surprised that dmd accepts that. The reason is that
declaring a (non final)template function in an interface is like 
declaring a
function pointer to an uninstansiated(sp?) template function. Also
how many copies should appear in the vtbl for different template
argument combinations?
  Calling a templates on a class is fine, which explains why 
casting
solves the issue.

Solutions: if the function in the interface must take template 
args
parameterise the interface not the function (not sure if this 
works)
OR
if the function must take a variable number of args just use 
regular
runtime varadic arguments.

Nic
Mar 08 2016
prev sibling parent reply crimaniak <crimaniak gmail.com> writes:
On Tuesday, 8 March 2016 at 11:50:32 UTC, Peter wrote:

 Hi,

 Can anyone explain to me what's causing the following code to 
 generate a missing symbol error...


Relevant comment: https://issues.dlang.org/show_bug.cgi?id=8553#c1
Mar 08 2016
parent Peter <peter_a_wood yahoo.com> writes:
On Tuesday, 8 March 2016 at 12:24:06 UTC, crimaniak wrote:

 On Tuesday, 8 March 2016 at 11:50:32 UTC, Peter wrote:

 Hi,

 Can anyone explain to me what's causing the following code to 
 generate a missing symbol error...


 Relevant comment: 
 https://issues.dlang.org/show_bug.cgi?id=8553#c1


Thanks for link, makes sense.
Mar 08 2016