Io.Writer

As explained in Go in Action, Second Edition

io.Writer is the Go interface for writing byte data to a destination.

io.Writer is Go’s abstraction for sending byte data to a destination. Functions that accept it can write to files, terminals, memory buffers, network connections, or other compatible destinations without changing their core logic. section 10.3 section 2.8.2

The interface separates what a function writes from where the output goes. A processing function can accept an io.Writer alongside an io.Reader, then support file input, operating-system input, file output, or screen output through the same design. section 10.3

Formatted output to a selected destination

var writer io.Writer writer = os.Stdout fmt.Fprintf(writer, "Output written to: %s\n", outputFile) Replacing writer with another compatible destination redirects the formatted message without changing the call. This pattern fits CLI tools and logging systems. section 10.3

Use io.Writer when:

• A function should write output without depending on a concrete file or terminal type. section 2.8.2
• A CLI or conversion utility needs to send results or status messages to selectable destinations. section 10.3 section 10.0
• Several destinations should receive the same stream through io.MultiWriter. section 10.3

Figure 10.1 io.Reader and io.Writer interface composition with multiple package implementations

Browse the interface relationships and package implementations that let one Writer-based function operate across multiple destinations. section 10.3

How Go in Action, Second Edition covers it

How deep it goes

Explains interface-based destination selection, formatted output, and composition, but stops short of detailing the Writer method signature.

Examples

Demonstrates file and screen output, fmt.Fprintf status messages, a JSON/XML conversion CLI, and simultaneous output with io.MultiWriter.

What you can do after

Apply io.Writer with fmt.Fprintf to route formatted output through interchangeable destinations.

How it connects

Connects Writer with Reader, Closer, WriteCloser, fmt.Fprintf, io.MultiWriter, files, standard input, and network connections.

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  Go by Exampleio.Writer is a Go interface whose Write method accepts a byte sequence and returns the number of bytes written and an error.

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Related concepts

• os.Fileimplemented by · os.File is a Go standard-library type representing an open file or other operating-system file handle and providing associated file operations.
• write operationhas part · A write operation transfers data to a destination or changes stored data or other state.
• contractkind of · An API contract is a specification of the operations, data, and behavioral expectations that govern interactions between software components.
• io.Copyused by · io.Copy is a Go standard-library function that copies bytes from an io.Reader to an io.Writer until input is exhausted or an error occurs.
• http.ResponseWriterincludes · http.ResponseWriter is the Go interface through which an HTTP handler constructs a response by writing its headers, status, and body.
• natural language interfacekind of · A natural language interface enables interaction with a system through human-language text or speech.

This explainer was generated by Manning's knowledge graph from the text of Go in Action, Second Edition. The section links point to the parts of the book it draws on.