30 Localization library [localization]

30.4 Standard locale categories [locale.categories]

30.4.4 The numeric punctuation facet [facet.numpunct]

30.4.4.1 Class template numpunct [locale.numpunct]

30.4.4.1.1 General [locale.numpunct.general]

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namespace std { template<class charT> class numpunct : public locale::facet { public: using char_type = charT; using string_type = basic_string<charT>; explicit numpunct(size_t refs = 0); char_type decimal_point() const; char_type thousands_sep() const; string grouping() const; string_type truename() const; string_type falsename() const; static locale::id id; protected: ~numpunct(); // virtual virtual char_type do_decimal_point() const; virtual char_type do_thousands_sep() const; virtual string do_grouping() const; virtual string_type do_truename() const; // for bool virtual string_type do_falsename() const; // for bool }; }
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numpunct<> specifies numeric punctuation.
The specializations required in Table 104 ([locale.category]), namely numpunct<​wchar_t> and numpunct<char>, provide classic "C" numeric formats, i.e., they contain information equivalent to that contained in the "C" locale or their wide character counterparts as if obtained by a call to widen.
2
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The syntax for number formats is as follows, where digit represents the radix set specified by the fmtflags argument value, and thousands-sep and decimal-point are the results of corresponding numpunct<charT> members.
Integer values have the format:
intval:
sign units
sign:
+
-
units:
digits
digits thousands-sep units
digits:
digit digits
and floating-point values have:
floatval:
sign units fractional exponent
sign decimal-point digits exponent
fractional:
decimal-point digits
exponent:
e sign digits
e:
e
E
where the number of digits between thousands-seps is as specified by do_grouping().
For parsing, if the digits portion contains no thousands-separators, no grouping constraint is applied.

30.4.4.1.2 Members [facet.numpunct.members]

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char_type decimal_point() const;
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Returns: do_decimal_point().
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char_type thousands_sep() const;
2
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Returns: do_thousands_sep().
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string grouping() const;
3
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Returns: do_grouping().
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string_type truename() const; string_type falsename() const;
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Returns: do_truename() or do_falsename(), respectively.

30.4.4.1.3 Virtual functions [facet.numpunct.virtuals]

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char_type do_decimal_point() const;
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Returns: A character for use as the decimal radix separator.
The required specializations return '.' or L'.'.
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char_type do_thousands_sep() const;
2
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Returns: A character for use as the digit group separator.
The required specializations return ',' or L','.
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string do_grouping() const;
3
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Returns: A string vec used as a vector of integer values, in which each element vec[i] represents the number of digits251 in the group at position i, starting with position 0 as the rightmost group.
If vec.size() <= i, the number is the same as group (i - 1); if (i < 0 || vec[i] <= 0 || vec[i] == CHAR_MAX), the size of the digit group is unlimited.
4
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The required specializations return the empty string, indicating no grouping.
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string_type do_truename() const; string_type do_falsename() const;
5
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Returns: A string representing the name of the boolean value true or false, respectively.
6
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In the base class implementation these names are "true" and "false", or L"true" and L"false".
251)251)
Thus, the string "\003" specifies groups of 3 digits each, and "3" probably indicates groups of 51 (!) digits each, because 51 is the ASCII value of "3".