31 Input/output library [input.output]

31.5 Iostreams base classes [iostreams.base]

31.5.3 Class template fpos [fpos]

31.5.3.1 General [fpos.general]

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namespace std { template<class stateT> class fpos { public: // [fpos.members], members stateT state() const; void state(stateT); private: stateT st; // exposition only }; }

31.5.3.2 Members [fpos.members]

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void state(stateT s);
1
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Effects: Assigns s to st.
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stateT state() const;
2
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Returns: Current value of st.

31.5.3.3 Requirements [fpos.operations]

1
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An fpos type specifies file position information.
It holds a state object whose type is equal to the template parameter stateT.
Type stateT shall meet the Cpp17DefaultConstructible (Table 30), Cpp17CopyConstructible (Table 32), Cpp17CopyAssignable (Table 34), and Cpp17Destructible (Table 35) requirements.
If is_trivially_copy_constructible_v<stateT> is true, then fpos<stateT> has a trivial copy constructor.
If is_trivially_copy_assignable_v<stateT> is true, then fpos<stateT> has a trivial copy assignment operator.
If is_trivially_destructible_v<stateT> is true, then fpos<stateT> has a trivial destructor.
In addition, the expressions shown in Table 136 are valid and have the indicated semantics.
In that table,
  • (1.1)
    P refers to a specialization of fpos,
  • (1.2)
    p and q refer to values of type P or const P,
  • (1.3)
    pl and ql refer to modifiable lvalues of type P,
  • (1.4)
    O refers to type streamoff, and
  • (1.5)
    o and o2 refer to values of type streamoff or const streamoff.
Table 136: Position type requirements [tab:fpos.operations]
Expression
Return type
Operational
Assertion/note
semantics
pre-/post-condition
P(o)
P
converts from offset
Effects: Value-initializes the state object.
P p(o);
P p = o;
Effects: Value-initializes the state object.

Postconditions: p == P(o) is true.
P()
P
P(0)
P p;
P p(0);
O(p)
streamoff
converts to offset
P(O(p)) == p
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.
p != q
bool
!(p == q)
p + o
P
+ offset
Remarks: With ql = p + o;, then: ql - o == p
pl += o
P&
+= offset
Remarks: With ql = pl; before the +=, then: pl - o == ql
p - o
P
- offset
Remarks: With ql = p - o;, then: ql + o == p
pl -= o
P&
-= offset
Remarks: With ql = pl; before the -=, then: pl + o == ql
o + p
convertible to P
p + o
P(o + p) == p + o
p - q
streamoff
distance
p == q + (p - q)
2
#
Stream operations that return a value of type traits​::​pos_type return P(O(-1)) as an invalid value to signal an error.
If this value is used as an argument to any istream, ostream, or streambuf member that accepts a value of type traits​::​pos_type then the behavior of that function is undefined.