Answer

If a user-defined type implements the method

String() string

then when this value is used in fmt functions (for example, fmt.Println, fmt.Printf("%v"), log package), this method is called to obtain a string representation.

You should implement String() for user-defined entity types to get meaningful human-readable output instead of the default from fmt (%v). It is recommended to make the output concise, contextually appropriate, and secure (without leaking sensitive data) if possible.

Example implementation:

type Order struct {
    ID     string
    Amount int
}

func (o Order) String() string {
    return fmt.Sprintf("Order[%s]: %d", o.ID, o.Amount)
}

func main() {
    o := Order{"A123", 99}
    fmt.Println(o)
}
// Output: Order[A123]: 99

If the method is implemented with errors or inappropriately, it leads to logs with meaningless or dangerous output, making it difficult to debug.

Trick question

What if your type implements the String() method but you use it as a pointer and not as a value? Will String() be called?

Answer: The String() method will be called if it is implemented for the pointer type. If it is implemented only on the value, but in the code the type is a pointer, the method will be invoked thanks to Go's auto-dereferencing for methods without a pointer receiver. Usually, this is not a problem, but if the method is implemented only on the pointer receiver and you call it for a value, it will result in a compilation error.

Example:

type X struct{}
func (x *X) String() string { return "ptr" }

fmt.Println(X{}) // error: X does not implement String
fmt.Println(&X{}) // ok, the method will be called

Examples of real errors due to ignorance of the nuances of the topic

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Story

In a large project, the error type implemented String() only on pointer receiver (func (err *MyErr) String() string). As a result, when returning an error value (not a pointer) in the logs, it displayed a useless {} instead of a useful message. The bug was not noticed for a long time.

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Story

One of the types stored sensitive data, and due to improper implementation of String() (fields were output explicitly), user passwords began to appear in production logs. A security audit was required.

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Story

Using automatic generation of String() led to a situation where upon updating the structure, the new output format did not match the previous one, making logs unreadable and breaking log parsing by third-party systems.