Solidity Minimalist Introduction | Lecture 30: Try Catch

2022-10-17 22:00

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原文标题：《 Solidity 极简入门: 30. Try Catch 》

Original author: 0xAA

I've been relearning solidity recently to consolidate the details and also write a "Solidity Minimality Primer" for kids to use (programming geeks can find another tutorial) and update 1-3 sessions every week.

All open source code and tutorial github:github.com/AmazingAng/WTFSolidity

try-catch is a standard way of handling exceptions in almost every modern programming language, and it was added to solidity0.6. In this lecture, we will introduce how to use try-catch to handle exceptions in smart contracts.

try-catch

In solidity, try-catch can only be used for external functions or calls to constructor (treated as external functions) when contracts are created. The basic syntax is as follows:

externalContract.f() is a function call of an external contract. The try module runs if the call succeeds, while the catch module runs if the call fails.

If the called function has a return value, then returns(returnType val) must be declared after the try, and the returned variable can be used in the try module; If a contract is created, the return value is the newly created contract variable.

In addition, the catch module supports catching special causes of exceptions:

Try-catch practical & have spent

OnlyEven

We create an external contract OnlyEven and use try-catch to handle exceptions:

The OnlyEven contract contains a constructor and an onlyEven function.

The constructor takes a parameter, a, and require throws an exception if a=0. assert throws an exception when a=1. All other conditions were normal. The onlyEven function takes an argument b, and require throws an exception if b is odd.

Handle external function call exceptions

First, define some event and state variables in the TryCatch contract:

SuccessEvent is the event that is released after a successful call. CatchEvent and CatchByte are the events that are released when an exception is thrown, corresponding to require/revert and assert exceptions, respectively. even is a state variable of the OnlyEven contract type.

We then use try-catch in execute to handle exceptions in the call to the external function onlyEven:

When execute(0) is run, no exception is thrown because 0 is even, and the call succeeds and releases the SuccessEvent event; When execute(1) is run, because 1 is even, the exception is thrown, the call fails and the CatchEvent event is released.