### `try/catch` (1) * Handle external call failures * React on failed external calls and contract creation. * Cannot be used for internal function calls * Call public function with <span class="highlightRed">`this`</span> to make it external. * Eventual `out of gas` error is caught by the low-level `catch` clause. * `out of gas` error is not caught if transaction executing code runs out of gas.

### `try/catch`(2) * following revert inside the try success block will revert the transaction: ``` function execute() public { ry externalContract.someFunction() { // this will revert the execution of the transaction // even if the external call succeeded revert(); } catch { // ... } } ```

### `try/catch`(3) * Constructor calls ``` contract TryCatcher { // ... function execute() public { try new CalledContract() returns(CalledContract returnedInstance) { // returnedInstance can be used to obtain the // address of the newly deployed contract emit SuccessEvent(); } catch { emit CatchEvent(); } } } ```

### `try/catch`(4) * External calls ``` contract CalledContract { function getTwo() public returns (uint256) { return 2; } } contract TryCatcher { CalledContract public externalContract; function execute() public returns (uint256, bool) { try externalContract.getTwo() returns (uint256 v) { uint256 newValue = v + 2; return (newValue, true); } catch { emit CatchEvent(); } } } ```

### `try/catch`(5) * Use returned data inside catch statement ``` contract TryCatcher { event ReturnDataEvent(bytes someData); // ... function execute() public returns (uint256, bool) { try externalContract.someFunction() { // ... } catch (bytes memory returnData) { emit ReturnDataEvent(returnData); } } } ```

### `try/catch`(6) * revert-style exception (`require` or `revert`) error signature matches <span class="highlightRed">`catch Error`</span> block. <pre class="hljs"> contract TryCatcher { event ReturnDataEvent(bytes someData); event CatchStringEvent(string someString); event SuccessEvent(); // ... function execute() public { try externalContract.someFunction() { emit SuccessEvent(); } <span class="highlightRed">catch Error(string memory revertReason) { emit CatchStringEvent(revertReason); }</span> catch (bytes memory returnData) { emit ReturnDataEvent(returnData); } } } </pre>

### `try/catch`(7) * assert-style exception (`assert`) error signature matches <span class="highlightRed">`catch`</span> block <pre class="hljs"> contract TryCatcher { event ReturnDataEvent(bytes someData); event CatchStringEvent(string someString); event SuccessEvent(); // ... function execute() public { try externalContract.someFunction() { emit SuccessEvent(); } catch Error(string memory revertReason) { emit CatchStringEvent(revertReason); } <span class="highlightRed">catch (bytes memory returnData) { emit ReturnDataEvent(returnData); }</span> } } </pre>

### `try/catch`(8) * error like overflow/underflow matches <span class="highlightRed">`catch Panic()`</span> block <pre class="hljs"> contract TryCatcher { event PanicErrorEvent(uint errorCode); event CatchStringEvent(string someString); event SuccessEvent(); // ... function execute() public { try externalContract.someFunction() { emit SuccessEvent(); } catch Error(string memory revertReason) { emit CatchStringEvent(revertReason); } <span class="highlightRed">catch Panic(uint errorCode) { emit PanicErrorEvent(errorCode); }</span> } } </pre>

### `try/catch`(Panic codes) 1. **`0x01`**: assert evaluates to false. 2. **`0x11`**: underflow/overflow. 3. **`0x12`**: e.g. `5/0` or `23%0`. 4. **`0x21`**: big value conversion, negative to enum type. 5. **`0x31`**: Call `.pop()` on an empty array. 6. **`0x32`**: e.g. `x[i]` where i >= x.length or i < 0 7. **`0x41`**: too much memory or create large array. 8. **`0x51`**: zero-initialized variable of internal function type.