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fmt: document order of application of %T, %p and the special interfaces

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Their priority was not documented.

Fixes #7571.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/80360043
This commit is contained in:
Rob Pike 2014-03-26 17:18:24 +11:00
parent 34767046d7
commit 274fc7600d
1 changed files with 14 additions and 9 deletions
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23
src/pkg/fmt/doc.go
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@ -115,20 +115,25 @@
fmt.Printf("%v\n", i)
will print 23.
If an operand implements interface Formatter, that interface
can be used for fine control of formatting. Similarly, if an
operand implements the GoStringer interface, that will be
invoked if the '%#v' verb is used to format the operand.
Except when printed using the the verbs %T and %p, special
formatting considerations apply for operands that implement
certain interfaces. In order of application:
1. If an operand implements the Formatter interface, it will
be invoked. Formatter provides fine control of formatting.
2. If the %v verb is used with the # flag (%#v) and the operand
implements the GoStringer interface, that will be invoked.
If the format (which is implicitly %v for Println etc.) is valid
for a string (%s %q %v %x %X), the following two rules also apply:
for a string (%s %q %v %x %X), the following two rules apply:
1. If an operand implements the error interface, the Error method
will be used to convert the object to a string, which will then
3. If an operand implements the error interface, the Error method
will be invoked to convert the object to a string, which will then
be formatted as required by the verb (if any).
2. If an operand implements method String() string, that method
will be used to convert the object to a string, which will then
4. If an operand implements method String() string, that method
will be invoked to convert the object to a string, which will then
be formatted as required by the verb (if any).
For compound operands such as slices and structs, the format