Problem

Ruby 3.2+ emits deprecation warnings when using the legacy Data_Wrap_Struct API:

warning: undefining the allocator of T_DATA class GSL::Vector
warning: undefining the allocator of T_DATA class GSL::Matrix
warning: undefining the allocator of T_DATA class GSL::Rng
...

These warnings appear on every require 'gsl' and when instantiating GSL objects.

Root Cause

The codebase uses the legacy Ruby C API:

• Data_Wrap_Struct / Data_Make_Struct
• Data_Get_Struct
• rb_undef_alloc_func

Ruby recommends migrating to the TypedData API which provides better GC integration, type safety, and memory profiling.

Scope

~3,300 occurrences across the codebase

Required Changes

For each class, the migration involves:

1. Define a rb_data_type_t struct

static const rb_data_type_t gsl_vector_type = {
    .wrap_struct_name = "GSL::Vector",
    .function = {
        .dmark = NULL,  // or mark function if needed
        .dfree = gsl_vector_free,
        .dsize = NULL,  // optional: memory size function
    },
    .flags = RUBY_TYPED_FREE_IMMEDIATELY,
};

2. Replace Data_Wrap_Struct

// BEFORE
Data_Wrap_Struct(klass, mark_func, free_func, ptr);

// AFTER
TypedData_Wrap_Struct(klass, &gsl_vector_type, ptr);

3. Replace Data_Get_Struct

// BEFORE
Data_Get_Struct(obj, gsl_vector, v);

// AFTER
TypedData_Get_Struct(obj, gsl_vector, &gsl_vector_type, v);

4. Remove rb_undef_alloc_func calls

TypedData handles allocation differently; these calls become unnecessary.

Workaround

Users can suppress warnings until migration is complete:

Warning[:deprecated] = false
require 'gsl'
Warning[:deprecated] = true

Or via environment:

RUBYOPT='-W:no-deprecated' ruby script.rb

References

• Ruby TypedData documentation
• Ruby 3.2 deprecation notes