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What is the role of N() in defining tables using multi_index. The primary key function does not return string. Why so? e.g. string primary_key() const {return Some_String_Object}. This throws an error.

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The primary key for a multi_index table can not be a string. In general it should be an int.

The N() macro converts the contents within the brackets into a uint64_t.

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As per the functionality of multi index table in EOS, primary index must be uint64_t type and unique.

N() refers to the macro defined as:

#define N(p) :: eosio :: string_to_name(#p)

N() is used to generate a compile time uint64_t from base32 encoded string interpretation of p.

For more technical insights, refer: https://github.com/EOSIO/eos/blob/master/contracts/eosiolib/types.hpp

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