[fpos] # 31 Input/output library [[input.output]](./#input.output) ## 31.5 Iostreams base classes [[iostreams.base]](iostreams.base#fpos) ### 31.5.3 Class template fpos [fpos] #### [31.5.3.1](#general) General [[fpos.general]](fpos.general) [🔗](#lib:fpos) namespace std {template class fpos {public:// [[fpos.members]](#members "31.5.3.2 Members"), members stateT state() const; void state(stateT); private: stateT *st*; // *exposition only*};} #### [31.5.3.2](#members) Members [[fpos.members]](fpos.members) [🔗](#lib:state,fpos) `void state(stateT s); ` [1](#members-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L1659) *Effects*: Assigns s to *st*[.](#members-1.sentence-1) [🔗](#lib:state,fpos_) `stateT state() const; ` [2](#members-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L1670) *Returns*: Current value of *st*[.](#members-2.sentence-1) #### [31.5.3.3](#operations) Requirements [[fpos.operations]](fpos.operations) [1](#operations-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L1677) An fpos type specifies file position information[.](#operations-1.sentence-1) It holds a state object whose type is equal to the template parameter stateT[.](#operations-1.sentence-2) Type stateT shall meet 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) If is_trivially_copy_constructible_v is true, then fpos has a trivial copy constructor[.](#operations-1.sentence-4) If is_trivially_copy_assignable_v is true, then fpos has a trivial copy assignment operator[.](#operations-1.sentence-5) If is_trivially_destructible_v is true, then fpos has a trivial destructor[.](#operations-1.sentence-6) All specializations of fpos meet 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]"), 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) 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) In that table, - [(1.1)](#operations-1.1) P refers to a specialization of fpos, - [(1.2)](#operations-1.2) p and q refer to values of type P or const P, - [(1.3)](#operations-1.3) pl and ql refer to modifiable lvalues of type P, - [(1.4)](#operations-1.4) O refers to type streamoff, and - [(1.5)](#operations-1.5) o and o2 refer to values of type streamoff or const streamoff[.](#operations-1.sentence-9) Table [141](#tab:fpos.operations) — Position type requirements [[tab:fpos.operations]](./tab:fpos.operations) | [🔗](#tab:fpos.operations-row-1)
**Expression** | **Return type** | **Operational** | **Assertion/note** | | --- | --- | --- | --- | | [🔗](#tab:fpos.operations-row-2) | | **semantics** | **pre-/post-condition** | | [🔗](#tab:fpos.operations-row-3)
P(o) | P | converts from offset | *Effects*: Value-initializes the state object[.](#tab:fpos.operations-row-3-column-4-sentence-1) | | [🔗](#tab:fpos.operations-row-4)
P p(o); P p = o; | | | *Effects*: Value-initializes the state object[.](#tab:fpos.operations-row-4-column-4-sentence-1)
*Postconditions*: p == P(o) is true[.](#tab:fpos.operations-row-4-column-4-sentence-2) | | [🔗](#tab:fpos.operations-row-5)
P() | P | P(0) | | | [🔗](#tab:fpos.operations-row-6)
P p; | | P p(0); | | | [🔗](#tab:fpos.operations-row-7)
O(p) | streamoff | converts to offset | P(O(p)) == p | | [🔗](#tab:fpos.operations-row-8)
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) | | [🔗](#tab:fpos.operations-row-9)
p != q | bool | !(p == q) | | | [🔗](#tab:fpos.operations-row-10)
p + o | P | + offset | *Remarks*: With ql = p + o;, then: ql - o == p | | [🔗](#tab:fpos.operations-row-11)
pl += o | P& | += offset | *Remarks*: With ql = pl; before the +=, then: pl - o == ql | | [🔗](#tab:fpos.operations-row-12)
p - o | P | - offset | *Remarks*: With ql = p - o;, then: ql + o == p | | [🔗](#tab:fpos.operations-row-13)
pl -= o | P& | -= offset | *Remarks*: With ql = pl; before the -=, then: pl + o == ql | | [🔗](#tab:fpos.operations-row-14)
o + p | convertible to P | p + o | P(o + p) == p + o | | [🔗](#tab:fpos.operations-row-15)
p - q | streamoff | distance | p == q + (p - q) | [2](#operations-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L1790) 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) If this value is used as an argument to anyistream,ostream, orstreambuf member that accepts a value of typetraits​::​pos_type then the behavior of that function is undefined[.](#operations-2.sentence-2)