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tutorial: update discussion of variadic functions
R=rsc CC=golang-dev https://golang.org/cl/1677042
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@ -938,14 +938,19 @@ implements <code>Printf</code>, <code>Fprintf</code>, and so on.
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Within the <code>fmt</code> package, <code>Printf</code> is declared with this signature:
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<p>
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<pre>
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Printf(format string, v ...) (n int, errno os.Error)
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Printf(format string, v ...interface{}) (n int, errno os.Error)
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</pre>
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<p>
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That <code>...</code> represents the variadic argument list that in C would
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be handled using the <code>stdarg.h</code> macros but in Go is passed using
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an empty interface variable (<code>interface {}</code>) and then unpacked
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using the reflection library. It's off topic here but the use of
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reflection helps explain some of the nice properties of Go's <code>Printf</code>,
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The token <code>...</code> introduces a variable-length argument list that in C would
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be handled using the <code>stdarg.h</code> macros.
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In Go, variadic functions are passed a slice of the arguments of the
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specified type. In <code>Printf</code>'s case, the declaration says <code>...interface{}</code>
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so the actual type is a slice of empty interface values, <code>[]interface{}</code>.
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<code>Printf</code> can examine the arguments by iterating over the slice
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and, for each element, using a type switch or the reflection library
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to interpret the value.
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It's off topic here but such run-time type analysis
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helps explain some of the nice properties of Go's <code>Printf</code>,
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due to the ability of <code>Printf</code> to discover the type of its arguments
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dynamically.
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<p>
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@ -622,13 +622,18 @@ We've seen simple uses of the package "fmt", which
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implements "Printf", "Fprintf", and so on.
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Within the "fmt" package, "Printf" is declared with this signature:
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Printf(format string, v ...) (n int, errno os.Error)
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Printf(format string, v ...interface{}) (n int, errno os.Error)
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That "..." represents the variadic argument list that in C would
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be handled using the "stdarg.h" macros but in Go is passed using
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an empty interface variable ("interface {}") and then unpacked
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using the reflection library. It's off topic here but the use of
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reflection helps explain some of the nice properties of Go's "Printf",
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The token "..." introduces a variable-length argument list that in C would
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be handled using the "stdarg.h" macros.
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In Go, variadic functions are passed a slice of the arguments of the
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specified type. In "Printf"'s case, the declaration says "...interface{}"
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so the actual type is a slice of empty interface values, "[]interface{}".
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"Printf" can examine the arguments by iterating over the slice
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and, for each element, using a type switch or the reflection library
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to interpret the value.
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It's off topic here but such run-time type analysis
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helps explain some of the nice properties of Go's "Printf",
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due to the ability of "Printf" to discover the type of its arguments
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dynamically.
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