# 【Assert】

`assert()` 必須指被用來檢測內部錯誤，意思就是用來測試或 debug，並且用來檢測不變量（不該被改變的常數等）。

以下的例子是 `num` 這個變數必須只為 `0`，在我們的預想之中它得是個常數不該被更改，所以為了確認我們沒有忽略它被改變的情況，這裡加上 `assert()` 在 `num` 被改變時使程式強制中止。

```solidity
pragma solidity ^0.8.11;

contract error {
    function testAssert(uint _i) public view {
        assert(_i == 0);
    }
}
```

Asserted 會被觸發的情況及性質如下：

* `assert(X)` 當X 之值為`false`
* 踩到了超出範圍的 index
* 除以0或取除以0之餘數（5/0 or 23%0）
* 位元移動運算（Byteshifting）時位移量為負
* 轉換負值或過大的數字成`enum`資料結構

Assert 和 Require的比較

* 當`require`面對Revert operation(0xfd) 時會送回剩餘的gas
* 當`assert`面對Invalid operation(0xfe) 時會消耗全部的gas
* Assert 用於檢測完全不該在合約裡出現的情況
* Require 用來檢測使用者的輸入等錯誤情況


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