> For the complete documentation index, see [llms.txt](https://docs.alephzero.org/aleph-zero/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.alephzero.org/aleph-zero/build/cross-contract-calls.md).

# Cross contract calls

{% hint style="info" %}
Before we start, you may want to check out our [Bulletin Board Example repository](https://github.com/Cardinal-Cryptography/bulletin-board-example): all the code you will find in this tutorial is there in its full form. The examples in there are not limited to cross-contract calls and cover almost every aspect of smart contract development, so we encourage you to study it in addition to reading this guide.
{% endhint %}

The ability of smart contracts to call methods of another contract is arguably one of the most powerful concepts offered by Ink!. For example, a DEX will need to call the `transfer_from` method of a PSP22 token. However, this is also a feature that requires a lot of care during implementation.

### Basics

Ink! offers two ways of calling another contract, each with a different set of trade-offs:

* **importing a reference** to another contract and using it almost like any other method:
  * very convenient
  * the compiler is able to check whether a method you want to call exists and if the parameter types match
  * it doesn't allow you to dynamically construct calls
  * it doesn't allow you to send tokens with the call
* **dynamically building a call** (the `CallBuilder` method):
  * a lot of control over the execution
  * it allows for sending tokens along with the call
  * it allows for dynamically choosing a method to call
  * the compiler is not able to assist you in creating a valid call so you need to be extra careful  

{% hint style="info" %}
While the choice of a particular method may be a matter of personal preference, we suggest using the references by default and choosing the `CallBuilder` only if you really need it.
{% endhint %}

### The examples

In the two subsequent sections, we will use the `BulletinBoard` contract as an example. The contract allows users to post pieces of text ('bulletins') to the board and highlight selected posts. The highlighted posts are tracked by a separate contract: `HighlightedPosts`.

The split between the two contracts is pretty arbitrary and not really necessary for this logic to work. However, it serves a higher purpose of demonstrating cross-contract calls in an easy-to-understand example.