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correct and clarify the rules about integer conversions.

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DELTA=15  (6 added, 1 deleted, 8 changed)
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Rob Pike 2009-09-11 11:51:00 -07:00
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commit 61dd8363ba
1 changed files with 14 additions and 9 deletions
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doc/go_spec.html
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@ -3890,22 +3890,27 @@ to a variable of type T.
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2) The conversion succeeds if the value would be assignment-compatible 2) The conversion succeeds if the value would be assignment-compatible
to a variable of type T if the value type or T or any of their component to a variable of type T if the value's type, or T, or any of their component
types are unnamed (§<a href="#Type_identity_and_compatibility">Type identity and compatibility</a>). types are unnamed (§<a href="#Type_identity_and_compatibility">Type identity and compatibility</a>).
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3) Between integer types. If the value is a signed quantity, it is 3a) From an ideal number to an integer type.
The ideal number must be representable in the result type; it must not overflow.
For example, <code>uint8(0xFF)</code> is legal but <code>int8(0xFF)</code> is not.
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3b) From a non-ideal integer value to an integer type. If the value is a signed quantity, it is
sign extended to implicit infinite precision; otherwise it is zero sign extended to implicit infinite precision; otherwise it is zero
extended. It is then truncated to fit in the result type size. extended. It is then truncated to fit in the result type's size.
For example, <code>uint32(int8(0xFF))</code> is <code>0xFFFFFFFF</code>. For example, if <code>x := uint16(0x10F0)</code>, then <code>uint32(int8(x)) == 0xFFFFFFF0</code>.
The conversion always yields a valid value; there is no signal for overflow. The conversion always yields a valid value; there is no indication of overflow.
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4) Between integer and floating point types, or between floating point 4) Between integer and floating point types, or between floating point
types. To avoid overdefining the properties of the conversion, for types. To avoid overdefining the properties of the conversion, for
now it is defined as a ``best effort'' conversion. The conversion now it is defined as a ``best effort'' conversion. The conversion
always succeeds but the value may be a NaN or other problematic always succeeds but the value may be a NaN or other problematic
result. <font color=red>TODO: clarify?</font> result. <font color=red>TODO: clarify</font>
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5) Strings permit three special conversions: 5) Strings permit three special conversions:
@ -3915,7 +3920,7 @@ result. <font color=red>TODO: clarify?</font>
representation of the integer. representation of the integer.
<pre> <pre>
string(0x65e5) // "\u65e5" string(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
</pre> </pre>
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@ -3924,9 +3929,9 @@ string(0x65e5) // "\u65e5"
concatenation of the individual integers converted to strings. concatenation of the individual integers converted to strings.
If the slice value is <code>nil</code>, the result is the empty string. If the slice value is <code>nil</code>, the result is the empty string.
<pre> <pre>
string([]int{0x65e5, 0x672c, 0x8a9e}) // "\u65e5\u672c\u8a9e" string([]int{0x65e5, 0x672c, 0x8a9e}) // "\u65e5\u672c\u8a9e" == "日本語"</pre>
</pre>
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5c) Converting a slice of bytes yields a string whose successive 5c) Converting a slice of bytes yields a string whose successive
bytes are those of the slice. If the slice value is <code>nil</code>, bytes are those of the slice. If the slice value is <code>nil</code>,