fmtErrorf

fmtErrorf is a function in the Go programming language that formats an error message and returns it as an error value. It is part of the standard library package fmt, which provides I/O formatting primitives. The function signature is:

func Errorf(format string, a ...interface{}) error

The format string works with the same verbs and flags as fmt.Printf. Each verb may be preceded by optional flags, width, and precision specifiers. The arguments following the format string are used to replace the verbs in order.

Typical usage is to create simple descriptive errors that contain variable information. For example:

err := fmt.Errorf("read %d bytes: %w", n, io.ErrUnexpectedEOF)

Here the %w verb is used to wrap an existing error, enabling error unwrapping with errors.Is and errors.As. The resulting error implements the Error interface and can be inspected with fmt.Errorf within other functions.

Compared to constructing errors with errors.New, fmtErrorf provides automatic string formatting, useful for complex messages or when including multiple variables. It also supports error wrapping, which simplifies chaining errors in layered systems. Because it returns a value that satisfies the error interface, fmtErrorf is commonly used in APIs, logging, and internal error handling to produce consistent and informative error messages.