Why Does `fmt.Println` Prefer `Error()` Over `String()` in Go?-Golang-php.cn

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Why Does `fmt.Println` Prefer `Error()` Over `String()` in Go?

Susan Sarandon

Nov 30, 2024 am 04:22 AM

Why Does `fmt.Println` Prefer `Error()` Over `String()` in Go?

Error() Overrides String() in fmt.Println

In the Go programming language, when a type implements both String() and Error() methods, the fmt.Println function gives precedence to Error(). This means that if both methods are implemented for a particular type, fmt.Println will use the output generated by Error().

To understand why this is the case, it is important to look at the documentation for the fmt package. According to the documentation, there are specific rules that govern how fmt formats different types of data. These rules are applied in the following order:

If the data implements the Formatter interface, the Formatter method is used to format the data.
If the %#v format is used and the data implements the GoStringer interface, the GoStringer method is used to format the data.
If the data implements the error interface, the Error method is used to format the data.
If the data implements the String() method, the String() method is used to format the data.

In the example provided, the Person type implements both String() and Error() methods. However, when the fmt.Println function is called to print the Person type, the Error() method is called instead of the String() method. This is because the Error() method takes precedence over the String() method as per the rules outlined in the fmt package documentation.

Therefore, when a type implements both String() and Error() methods, the fmt.Println function will always use the output of the Error() method. This is because the Error() method is ranked higher than the String() method in the order of formatting rules.

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