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Inconsistent usage for Property Map misuse produces different, unhelpful errors

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
38/100
Issue type
Feature
Clarity
Needs clarification
Activity status
Quiet
Tech stack
cpp
Domain
compilers

Research direction

Start with boost/property_map/property_map.hpp around line 311 and compare the Dijkstra and Bellman examples linked in the issue. Review how those algorithms validate property-map categories and use the Compiler Explorer cases to assess the diagnostics. Done means agreeing on a scoped diagnostic approach and demonstrating clearer errors for the affected misuse cases.

Written by the indexing model from the issue text.

Description

api

Not sure this is a very helpful issue, as I am still catching up on Boost Concepts, but I will drop it for the record and discussion

Problem 1: Algorithms that perform no check at all

When a user passes a property map with the wrong category to an algorithm, the compiler error is deeply nested and unhelpful.

For example, passing a read-only distance property map to dijkstra fails to compile with a long and rather cryptic error, ending with

app/boost/include/boost/property_map/property_map.hpp:311:40: error: lvalue required as left operand of assignment
  311 |     static_cast<const PropertyMap&>(pa)[k] = v;

See on Compiler Explorer

Problem 2: Algorithms that check boost concepts

Some algorithms like Bellman use Boost Concepts to check the property map category, but the output is actually worse than no check.
See on compiler explorer

Proposal

I guess the use of Boost Concepts is justified by pre-C++11 static_assert. Also for documentation purpose. But maybe we should open a conversation on making compilation error messages more clear.

Maybe add static_assert checks for property map categories in algorithms that currently lack them:

static_assert(
    std::is_convertible<
        typename property_traits<DistanceMap>::category,
        read_write_property_map_tag>::value,
    "distance_map must be a ReadWrite property map (supports both get and put)");

This produces a single line error:

error: static_assert failed "distance_map must be a ReadWrite property map 
(supports both get and put)"

See on Compiler Explorer

Dominant language
C++
Stars
396
Forks
244
Avg merge
2d 6h
Merged PRs (30d)
29

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