Reinstantiate 'Distance' type for template expansion.

[ascherer]

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)
• No

What this PR does

It fixes the example program miles_span.cpp for compilation.

Motivation

Recently I upgraded the PROTOTYPES changefiles for The Stanford GraphBase (see https://github.com/ascherer/sgb/releases/tag/2025-12-28; with the grace of Donald Knuth, see https://www-cs-faculty.stanford.edu/~knuth/sgb.html) and now I tried to compile the four SGB example codes girth.cpp, miles_span.cpp, roget_components.cpp, and topo-sort-with-sgb.cpp with g++ 13.3.0 on Kubuntu 24.04LTS.

Testing

andreas@TravelMate .../Extern/graph/example develop
$ g++  miles_span.cpp -I /usr/include/sgb -L /usr/lib/x86_64-linux-gnu/sgb -lgb
andreas@TravelMate .../Extern/graph/example develop ?1
$ ./a.out 
The graph miles(100,0,0,0,0,10,0) has 405 edges,
  and its minimum spanning tree has length 14467.
andreas@TravelMate .../Extern/graph/example develop ?1
$ cat miles_span.expected 
The graph miles(100,0,0,0,0,10,0) has 405 edges,
  and its minimum spanning tree has length 14467

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 33382344232 vs develop run 33382344232 (4015effa5e).

Header-inclusion weights (graph files pulling each direct dependency in):

No header-inclusion-weight changes.

Transitive Boost modules: 66 → 66 (0)

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 33382344207 vs develop run 33382344207 (4015effa5e).

Job	Baseline	After	Delta
macos (clang, 14)	647	647	0
macos (clang, 17)	610	610	0
macos (clang, 20)	610	610	0
ubuntu (clang-19, 14)	647	647	0
ubuntu (clang-19, 17)	610	610	0
ubuntu (clang-19, 20)	610	610	0
ubuntu (clang-19, 23)	610	610	0
ubuntu (gcc-14, 14)	795	795	0
ubuntu (gcc-14, 17)	931	931	0
ubuntu (gcc-14, 20)	931	931	0
ubuntu (gcc-14, 23)	931	931	0
windows_msvc_14_3 (msvc-14.3)	952	952	0

[codecov]

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[Becheler]

Thank you @ascherer !

Looks like an old commit in 2016 (f4d50aa) was trying to modernize examples with auto but this introduced the bug you solved. Given that /examples folder is not build in CI (and we could probably not easily build this one given its dep on SGB) it's not too surprising the bug remained for so long. Thanks for fixing it !

I am still not sure what to do with the examples folder. @ascherer would you mind telling us how you ended up discovering the file, how you would have prefered to discover it ?

Just for context so we do a better job with those examples! 😄

[ascherer]

In 1994 I bought Knuth's book "The Stanford GraphBase" and earned my first "bug check" for porting the SGB software to the Commodore Amiga.

In the early 2000s I met Jeremy Siek at the Net.ObjectDays conference in Erfurt and we talked in depth about the SGB interface of the Boost Graph Library. This resulted in the improved #include structure:

graph/include/boost/graph/stanford_graph.hpp

Line 18 in bb97feb

// Thanks to Andreas Scherer for numerous suggestions and fixes!

And from time to time (every decade or so), while still working on my PROTOTYPES changefiles of SGB, I remember to check the four BGL examples.

Recently I managed to upgrade my SGB codebase to genuinely compile with C++23 compilers, so today I checked that the SGB header files included in stanford_graph.hpp are still valid.

[ascherer]

I must have met Jeremy at the very first Net.ObjectDays conference in Oktober 2000, where he gave a talk at the C++ template workshop, because of his commit in November 2000: fac92e1

[Becheler]

Just a small feedback, not mandatory though 👍🏽

[Becheler]

If necessary I do prefer using an explicit type alias to limit complexity of signatures.

using Distance = typename property_map< Graph*, z_property< long > >::type;
total_length_visitor< Distance > length_vis(sp_length, d);

[Becheler]

That's amazing context thank you ! 😄
So you have been around since the early days of BGL ! Impressive !
What is your take about the way BGL could document the SGB interface ? I am not too familiar with its usage so I am unsure what users would need to read in the documentation ?
Anyway thank you for your contributions ! 🚀

[ascherer]

What is your take about the way BGL could document the SGB interface ?

In older versions of boost::graph there was a dedicated page for the sgb/bgl interface: https://www.boost.org/doc/libs/1_34_0/libs/graph/doc/stanford_graph.html

Later versions don't seem to have this.

In section Installing the SGB I would suggest to point to https://github.com/ascherer/sgb/releases/tag/2025-12-28 and the "local" branch with the latest modifications of the Stanford GraphBase.

[Becheler]

The CI failing in unrelated to you changes (well, examples are not even in CI, I should fix that at some point). I will merge it, and I will open a documentation PR to reinstate the previous state 😄
thanks again!