# Indexing argument buffers Assign resource indices within an argument buffer. ## Discussion You can index an argument buffer similarly to buffers, textures, and samplers. However, you index individual argument buffer resources with a generic `[[id(n)]]` attribute instead of the specific type `[[buffer(n)]]`, `[[texture(n)]]`, and `[[sampler(n)]]` attributes. > Note: > Because argument buffers are represented by ``doc://com.apple.metal/documentation/Metal/MTLBuffer`` objects, they can be explicitly distinguished from regular buffers in the Metal shading language if they contain any resources indexed with the `[[id(n)]]` attribute. If any member of a Metal shading language structure has an `[[id(n)]]` attribute, the whole structure is treated as an argument buffer. Manually assigned argument buffer resource indices don’t need to be contiguous, but they need to be unique and arranged in an increasing order. The following example shows manual and automatic index assignment: ```metal struct My_Indexed_AB { texture2d texA [[id(1)]]; texture2d texB [[id(3)]]; }; struct My_Aggregate_AB { My_Indexed_AB abX; // abX = id(0); texA = id(1); texB = id(3) My_Indexed_AB abY; // abY = id(4); texA = id(5); texB = id(7) }; ``` ### Automatically assigned index IDs If the `[[id(n)]]` attribute is omitted for any argument buffer resource, an index ID is automatically assigned according to preset rules: **Structure Members** IDs are assigned to structure members in order, starting at 0, by adding 1 to the highest ID used by the previous structure member. The following example shows automatically assigned index IDs for structure members: ```metal struct MaterialTexture { texture2d tex; // Assigned to index 0 float4 uvScaleOffset; // Assigned to index 1 }; ``` **Array Elements** IDs are assigned to array elements in order, starting at 0, by adding 1 to the highest ID used by the previous array elements. The following example shows automatically assigned index IDs for array elements: ```metal struct Material { float4 diffuse; // Assigned to index 0 array, 3> texSet1; // Assigned to indices 1-3 texture2d texSet2[3]; // Assigned to indices 4-6 MaterialTexture materials[3]; // Assigned to indices 7-12 int constants[4] [[id(20)]]; // Assigned to indices 20-23 }; ``` **Nested Structs and Arrays** If a structure member or array element is itself a structure or array, its own structure members or array elements are assigned indices according to the previous rules. If an ID is provided for a top-level structure or array, this ID becomes the starting index for nested structure members or array elements. The following example shows automatically assigned index IDs for nested structures and arrays: ```metal struct Material { MaterialTexture diffuse; // Assigned to indices 0-1 MaterialTexture normal [[id(4)]]; // Assigned to indices 4-5 MaterialTexture specular; // Assigned to indices 6-7 } ``` **Combined Argument Buffer Resources and Regular Resources** Argument buffer resources are assigned generic indices according to the previous rules. Regular resources are assigned type indices in their respective resource argument tables. The following example shows automatically assigned index IDs for combined argument buffer resources and regular resources: ```metal fragment float4 my_fragment( constant texture2d & texturesAB1 [[buffer(0)]], // Assigned to generic index 0 and buffer index 0 constant texture2d & texturesAB2[10] [[buffer(1)]], // Assigned to generic indices 0-9 and buffer index 1 array, 10> texturesArray [[texture(0)]] // Assigned to texture indices 0-9 ) {...} ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)