# 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 <a href="#basics" id="basics"></a>

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

* **Importing a Reference to Another Contract:**

  * **Convenience:** Using a reference to another contract is very convenient.
  * **Compiler Assistance:** The compiler can check whether the method you want to call exists and if the parameter types match.
  * **Limitations:** It doesn’t allow you to dynamically construct calls or send tokens with the call.

  **Dynamically Building a Call (using the `CallBuilder` method):**

  * **Control:** Offers a lot of control over the execution.
  * **Token Transfer:** Allows for sending tokens along with the call.
  * **Dynamic Method Selection:** Enables dynamically choosing a method to call.
  * **Compiler Limitations:** The compiler cannot assist 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 <a href="#the-examples" id="the-examples"></a>

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.


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