NFT crafting/combining: Where to store burnRef?

[arjanjohan]

Discord user ID

arjanjohan

Describe your question in detail.

tldr: How to burn NFT's? Any code examples of NFT's that are used (to combine/craft into new NFT's) in Aptos Move?

We are working on an NFT platform where users can combine 2 NFT's and create a new NFT. The old NFT's should get burned. We started with the nft_launchpad.move file as template. But we are having issues with burning the old NFT's. Where should we store the BurnRef?

Currently I added this. When minting an NFT, the BurnRef is stored here.

    /// Unique per collection
    struct CollectionBurnRefs has key, store, drop {
        burn_ref_map: SimpleMap<address, token::BurnRef>,
    }

But when I actually want to burn the NFT's, I cannot dereference the BurnRefand use it. See my

        // Retrieve the burn_refs

        let main_nft_addr = object::object_address(&main_nft);
        let other_nft_addr = object::object_address(&other_nft);

        let collection_burn_refs = borrow_global<CollectionBurnRefs>(object::object_address(&main_collection_obj));
        let main_nft_burn_ref = simple_map::borrow(&collection_burn_refs.burn_ref_map, &main_nft_addr);
        let other_nft_burn_ref = simple_map::borrow(&collection_burn_refs.burn_ref_map, &other_nft_addr);

        // Burn NFTs using the retrieved burn_refs
        token::burn(*main_nft_burn_ref);
        token::burn(*other_nft_burn_ref);

Source code:
https://github.com/gmove-hackers/gmove-nft/blob/move-modules/move/sources/launchpad.move

What error, if any, are you getting?

error[E05001]: ability constraint not satisfied
┌─ /home/arjan/git/hack/olympus/gmove/move/sources/launchpad.move:401:21
│
116 │ burn_ref_map: SimpleMap<address, token::BurnRef>,
│ -------------- The type '(aptos_token_objects=0x4)::token::BurnRef' does not have the ability 'copy'
·
401 │ token::burn(*other_nft_burn_ref);
│ ^^^^^^^^^^^^^^^^^^^ Invalid dereference. Dereference requires the 'copy' ability
│
┌─ /home/arjan/.move/https___github_com_aptos-labs_aptos-core_git_mainnet/aptos-move/framework/aptos-token-objects/sources/token.move:93:12
│
93 │ struct BurnRef has drop, store {
│ ------- To satisfy the constraint, the 'copy' ability would need to be added here

What have you tried or looked at? Or how can we reproduce the error?

No response

Which operating system are you using?

Linux (Ubuntu, Fedora, Windows WSL, etc.)

Which SDK or tool are you using? (if any)

N/A

Describe your environment or tooling in detail

No response

[gregnazario]

The easiest way is to put the BurnRef in the token's object itself with a move_to with the object's address. You will then need to do a move_from the object's address to recover the BurnRef, which must be consumed by burn. You cannot use a reference of the BurnRef

Here's an example https://github.com/aptos-labs/daily-move/blob/main/snippets/modifying-nfts/sources/modify_nfts.move#L201-L228

[arjanjohan]

Great, that is exactly what I needed. I adjusted the code according to the example you gave and it compiles. Thanks for unblocking us!
Also the other daily-move examples seem like they will be very useful for us!