120 lines
7.0 KiB
Markdown
120 lines
7.0 KiB
Markdown
[fpos]
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# 31 Input/output library [[input.output]](./#input.output)
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## 31.5 Iostreams base classes [[iostreams.base]](iostreams.base#fpos)
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### 31.5.3 Class template fpos [fpos]
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#### [31.5.3.1](#general) General [[fpos.general]](fpos.general)
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[ð](#lib:fpos)
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namespace std {template<class stateT> class fpos {public:// [[fpos.members]](#members "31.5.3.2 Members"), members stateT state() const; void state(stateT); private: stateT *st*; // *exposition only*};}
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#### [31.5.3.2](#members) Members [[fpos.members]](fpos.members)
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[ð](#lib:state,fpos)
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`void state(stateT s);
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`
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[1](#members-1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L1659)
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*Effects*: Assigns s to *st*[.](#members-1.sentence-1)
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[ð](#lib:state,fpos_)
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`stateT state() const;
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`
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[2](#members-2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L1670)
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*Returns*: Current value of *st*[.](#members-2.sentence-1)
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#### [31.5.3.3](#operations) Requirements [[fpos.operations]](fpos.operations)
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[1](#operations-1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L1677)
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An fpos type specifies file position information[.](#operations-1.sentence-1)
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It holds a state object
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whose type is equal to the template parameter stateT[.](#operations-1.sentence-2)
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Type stateT shall meet
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the [*Cpp17DefaultConstructible*](utility.arg.requirements#:Cpp17DefaultConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") (Table [30](utility.arg.requirements#tab:cpp17.defaultconstructible "Table 30: Cpp17DefaultConstructible requirements")),[*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") (Table [32](utility.arg.requirements#tab:cpp17.copyconstructible "Table 32: Cpp17CopyConstructible requirements (in addition to Cpp17MoveConstructible)")),[*Cpp17CopyAssignable*](utility.arg.requirements#:Cpp17CopyAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]") (Table [34](utility.arg.requirements#tab:cpp17.copyassignable "Table 34: Cpp17CopyAssignable requirements (in addition to Cpp17MoveAssignable)")), and[*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") (Table [35](utility.arg.requirements#tab:cpp17.destructible "Table 35: Cpp17Destructible requirements")) requirements[.](#operations-1.sentence-3)
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If is_trivially_copy_constructible_v<stateT> is true,
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then fpos<stateT> has a trivial copy constructor[.](#operations-1.sentence-4)
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If is_trivially_copy_assignable_v<stateT> is true,
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then fpos<stateT> has a trivial copy assignment operator[.](#operations-1.sentence-5)
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If is_trivially_destructible_v<stateT> is true,
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then fpos<stateT> has a trivial destructor[.](#operations-1.sentence-6)
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All specializations of fpos meet
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the [*Cpp17DefaultConstructible*](utility.arg.requirements#:Cpp17DefaultConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]"),[*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]"),[*Cpp17CopyAssignable*](utility.arg.requirements#:Cpp17CopyAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]"),[*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]"),
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and [*Cpp17EqualityComparable*](utility.arg.requirements#:Cpp17EqualityComparable "16.4.4.2 Template argument requirements [utility.arg.requirements]") (Table [28](utility.arg.requirements#tab:cpp17.equalitycomparable "Table 28: Cpp17EqualityComparable requirements")) requirements[.](#operations-1.sentence-7)
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In addition, the expressions shown in Table [141](#tab:fpos.operations "Table 141: Position type requirements") are valid and have the indicated semantics[.](#operations-1.sentence-8)
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In that table,
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- [(1.1)](#operations-1.1)
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P refers to a specialization of fpos,
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- [(1.2)](#operations-1.2)
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p and q refer to values
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of type P or const P,
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- [(1.3)](#operations-1.3)
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pl and ql refer to modifiable lvalues of type P,
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- [(1.4)](#operations-1.4)
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O refers to type streamoff, and
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- [(1.5)](#operations-1.5)
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o and o2 refer to values
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of type streamoff or const streamoff[.](#operations-1.sentence-9)
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Table [141](#tab:fpos.operations) — Position type requirements [[tab:fpos.operations]](./tab:fpos.operations)
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| [ð](#tab:fpos.operations-row-1)<br>**Expression** | **Return type** | **Operational** | **Assertion/note** |
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| --- | --- | --- | --- |
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| [ð](#tab:fpos.operations-row-2) | | **semantics** | **pre-/post-condition** |
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| [ð](#tab:fpos.operations-row-3)<br>P(o) | P | converts from offset | *Effects*: Value-initializes the state object[.](#tab:fpos.operations-row-3-column-4-sentence-1) |
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| [ð](#tab:fpos.operations-row-4)<br>P p(o); P p = o; | | | *Effects*: Value-initializes the state object[.](#tab:fpos.operations-row-4-column-4-sentence-1)<br> *Postconditions*: p == P(o) is true[.](#tab:fpos.operations-row-4-column-4-sentence-2) |
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| [ð](#tab:fpos.operations-row-5)<br>P() | P | P(0) | |
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| [ð](#tab:fpos.operations-row-6)<br>P p; | | P p(0); | |
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| [ð](#tab:fpos.operations-row-7)<br>O(p) | streamoff | converts to offset | P(O(p)) == p |
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| [ð](#tab:fpos.operations-row-8)<br>p == q | bool | | *Remarks*: For any two values o and o2, if p is obtained from o converted to P or from a copy of such P value and if q is obtained from o2 converted to P or from a copy of such P value, then p == q is true only if o == o2 is true[.](#tab:fpos.operations-row-8-column-4-sentence-1) |
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| [ð](#tab:fpos.operations-row-9)<br>p != q | bool | !(p == q) | |
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| [ð](#tab:fpos.operations-row-10)<br>p + o | P | + offset | *Remarks*: With ql = p + o;, then: ql - o == p |
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| [ð](#tab:fpos.operations-row-11)<br>pl += o | P& | += offset | *Remarks*: With ql = pl; before the +=, then: pl - o == ql |
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| [ð](#tab:fpos.operations-row-12)<br>p - o | P | - offset | *Remarks*: With ql = p - o;, then: ql + o == p |
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| [ð](#tab:fpos.operations-row-13)<br>pl -= o | P& | -= offset | *Remarks*: With ql = pl; before the -=, then: pl + o == ql |
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| [ð](#tab:fpos.operations-row-14)<br>o + p | convertible to P | p + o | P(o + p) == p + o |
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| [ð](#tab:fpos.operations-row-15)<br>p - q | streamoff | distance | p == q + (p - q) |
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[2](#operations-2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L1790)
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Stream operations that return a value of typetraits::pos_type returnP(O(-1)) as an invalid value to signal an error[.](#operations-2.sentence-1)
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If this value is used as an argument to anyistream,ostream,
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orstreambuf member
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that accepts a value of typetraits::pos_type then the behavior of that function is undefined[.](#operations-2.sentence-2)
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