Why does Golang assertion fail? How to avoid it?

Golang is a rapidly developing programming language. Its syntax is concise and its concurrency performance is superior. It is loved and used by more and more developers. However, even with such an efficient language, some unexpected problems will inevitably arise. In Golang, one of the common problems is assertion failure (Assertion Failed).

Assertion failure means that during the running of the program, certain conditions are not met, causing the program to crash. For example, if you use an assertion to determine the length of a certain slice, the program will fail to assert when the length of the slice is less than a known limit. At this time, the program will throw a panic error message, causing the program to be unable to continue execution.

Although this kind of problem seems not easy to happen, in fact it can really crash your program. Therefore, we need to understand why assertions fail and how to avoid it.

Reasons for assertion failure

1. Uninitialized variables

In Golang, if a variable is not explicitly initialized, its value Defaults to zero value for this type. For some values ​​(such as integer types), the zero value is 0, and for other values, it is nil or an empty slice.

Therefore, if you operate on an uninitialized variable, such as a slice or array, the assertion will fail. Because the default value of these variables is a nil value, there is no way to get their length or index. In this case, to avoid assertion failures, you need to ensure that you initialize or assign a variable-length data structure or array element before reading it.

2. Array or slice range out of bounds

In Golang, access to arrays and slices is based on subscripts. This means that if you access an array or slice using an invalid subscript, the assertion will fail. This situation is very common and is especially error-prone when using multiple index variables in a loop.

To avoid assertion failures, you need to check whether the variable is out of bounds before accessing the array or slice. You can use comparison operators to check if it is greater than or equal to the length or less than 0.

For example, the following code snippet shows how to safely traverse a slice:

slice := []int{1, 2, 3, 4, 5}
for i := 0; i < len(slice); i++ {
    if i >= len(slice) || i < 0 {
        // 索引越界
        break
    }
    // 访问数据
    fmt.Println(slice[i])
}

3. Type assertion failed

In Golang, we often need to Convert an interface type to an actual type. Typically, we use type assertions to perform this type conversion. However, if the type judgment fails, it will cause the assertion to fail.

There may be many reasons for type judgment failure. For example, if you try to convert a pointer to an int to a string, the assertion fails. Alternatively, if you try to convert an interface type to a type that cannot be converted, the assertion will fail.

To avoid this, you need to make sure you do correct type determination and always use reflection to check if the interface type matches the target type.

How to avoid assertion failures

1. Ensure valid values ​​of variables through initialization and assignment

In Golang, uninitialized variables usually have default values The value is nil. Therefore, to avoid assertion failures, you need to initialize or assign a valid value to the variable before using it.

For example, the following is the declaration and initialization of a slice:

// 声明切片
var slice []int

// 初始化切片
slice = make([]int, 0)

Alternatively, you can declare and initialize a variable using a short variable declaration:

slice := make([]int, 0)

2. Bounds checking on indexes

In order to avoid the range of an array or slice going out of bounds, you need to bounds check the index. You can use comparison operators to check whether the index is greater than or equal to zero and less than the length of the array or slice.

For example, the following is a safe slice traversal:

// 安全获取数组元素的值
func safeGet(slice []int, index int) int {
    if index >= len(slice) || index < 0 {
        return -1
    }
    return slice[index]
}

func main() {
    slice := []int{1, 2, 3, 4, 5}

    // 遍历切片元素
    for i := 0; i < len(slice); i++ {
        fmt.Println(safeGet(slice, i))
    }
}

3. Before using type assertions, check if the type matches

Finally, when you use the type When asserting, make sure to check that the interface type matches the target type before making the assertion. Otherwise, you may encounter assertion failures.

To avoid this situation, you can use the reflection mechanism to detect the type. For example, the following code snippet shows how to use reflection to check whether a value is of type string:

// 判断一个值是否是字符串类型
func isString(val interface{}) bool {
    tt := reflect.TypeOf(val)
    if tt.Kind() == reflect.String {
        return true
    }
    return false
}

func main() {
    fmt.Println(isString("Hello, world!"))  // 输出 true 
    fmt.Println(isString(123))  // 输出 false 
}

Conclusion

Golang assertion failure is a common problem, but it is also easily overlooked question. To avoid this, you need to initialize and assign variables, perform bounds checks on arrays or slices, and make sure variable types match before making type assertions. Only in this way can you ensure that your program does not crash due to an assertion failure.

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