[char.traits.require]

# 27 Strings library [[strings]](./#strings)

## 27.2 Character traits [[char.traits]](char.traits#require)

### 27.2.2 Character traits requirements [char.traits.require]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/strings.tex#L72)

In Table [87](#tab:char.traits.req "Table 87: Character traits requirements"),X denotes a traits class defining types and functions for the
character container typeC;c andd denote values of typeC;p andq denote values of typeconst C*;s denotes a value of typeC*;n,i andj denote values of typesize_t;e andf denote values of typeX​::​int_type;pos denotes a value of typeX​::​pos_type;
andr denotes an lvalue of typeC[.](#1.sentence-1)

No expression which is part of the character traits requirements
specified in [char.traits.require]
shall exit via an exception[.](#1.sentence-2)

Table [87](#tab:char.traits.req) — Character traits requirements [[tab:char.traits.req]](./tab:char.traits.req)  

| [🔗](#tab:char.traits.req-row-1)<br>**Expression** | **Return type** | **Assertion/note** | **Complexity** |
| --- | --- | --- | --- |
| [🔗](#tab:char.traits.req-row-2) |  | **pre-/post-condition** |  |
| [🔗](#tab:char.traits.req-row-3)<br>X​::​char_type | C |  |  |
| [🔗](#tab:char.traits.req-row-4)<br>X​::​int_type |  | (described in [[char.traits.typedefs]](char.traits.typedefs "27.2.3 Traits typedefs")) |  |
| [🔗](#tab:char.traits.req-row-5)<br>X​::​off_type |  | (described in [[iostreams.limits.pos]](iostreams.limits.pos "31.2.3 Positioning type limitations") and [[iostream.forward]](iostream.forward "31.3 Forward declarations")) |  |
| [🔗](#tab:char.traits.req-row-6)<br>X​::​pos_type |  | (described in [[iostreams.limits.pos]](iostreams.limits.pos "31.2.3 Positioning type limitations") and [[iostream.forward]](iostream.forward "31.3 Forward declarations")) |  |
| [🔗](#tab:char.traits.req-row-7)<br>X​::​state_type |  | (described in [[char.traits.typedefs]](char.traits.typedefs "27.2.3 Traits typedefs")) |  |
| [🔗](#tab:char.traits.req-row-8)<br>X​::​eq(c,d) | bool | *Returns*: whether c is to be treated as equal to d[.](#tab:char.traits.req-row-8-column-3-sentence-1) | constant |
| [🔗](#tab:char.traits.req-row-9)<br>X​::​lt(c,d) | bool | *Returns*: whether c is to be treated as less than d[.](#tab:char.traits.req-row-9-column-3-sentence-1) | constant |
| [🔗](#tab:char.traits.req-row-10)<br>X​::​compare(p,q,n) | int | *Returns*: 0 if for each i in [0, n), X​::​eq(p[i],q[i]) is true; else, a negative value if, for some j in [0, n), X​::​lt(p[j],q[j]) is true and for each i in [0, j) X​::​eq(p[i],q[i]) is true; else a positive value[.](#tab:char.traits.req-row-10-column-3-sentence-1) | linear |
| [🔗](#tab:char.traits.req-row-11)<br>X​::​length(p) | size_t | *Returns*: the smallest i such that X​::​eq(p[i],charT()) is true[.](#tab:char.traits.req-row-11-column-3-sentence-1) | linear |
| [🔗](#tab:char.traits.req-row-12)<br>X​::​find(p,n,c) | const X​::​char_type* | *Returns*: the smallest q in [p, p+n) such that X​::​eq(*q,c) is true, nullptr otherwise[.](#tab:char.traits.req-row-12-column-3-sentence-1) | linear |
| [🔗](#tab:char.traits.req-row-13)<br>X​::​move(s,p,n) | X​::​char_type* | for each i in [0, n), performs X​::​assign(s[i],p[i])[.](#tab:char.traits.req-row-13-column-3-sentence-1)<br>Copies correctly even where the ranges [p, p+n) and [s, s+n) overlap[.](#tab:char.traits.req-row-13-column-3-sentence-2)<br>   *Returns*: s[.](#tab:char.traits.req-row-13-column-3-sentence-3) | linear |
| [🔗](#tab:char.traits.req-row-14)<br>X​::​copy(s,p,n) | X​::​char_type* | *Preconditions*: The ranges [p, p+n) and [s, s+n) do not overlap[.](#tab:char.traits.req-row-14-column-3-sentence-1)<br>*Returns*: s[.](#tab:char.traits.req-row-14-column-3-sentence-2)<br>   for each i in [0, n), performs X​::​assign(s[i],p[i])[.](#tab:char.traits.req-row-14-column-3-sentence-3) | linear |
| [🔗](#tab:char.traits.req-row-15)<br>X​::​assign(r,d) | (not used) | assigns r=d[.](#tab:char.traits.req-row-15-column-3-sentence-1) | constant |
| [🔗](#tab:char.traits.req-row-16)<br>X​::​assign(s,n,c) | X​::​char_type* | for each i in [0, n), performs X​::​assign(s[i],c)[.](#tab:char.traits.req-row-16-column-3-sentence-1)<br>   *Returns*: s[.](#tab:char.traits.req-row-16-column-3-sentence-2) | linear |
| [🔗](#tab:char.traits.req-row-17)<br>X​::​not_eof(e) | int_type | *Returns*: e if X​::​eq_int_type(e,X​::​eof()) is false, otherwise a value f such that X​::​eq_int_type(f,X​::​eof()) is false[.](#tab:char.traits.req-row-17-column-3-sentence-1) | constant |
| [🔗](#tab:char.traits.req-row-18)<br>X​::​to_char_type(e) | X​::​char_type | *Returns*: if for some c, X​::​eq_int_type(e,X​::​to_int_type(c)) is true, c; else some unspecified value[.](#tab:char.traits.req-row-18-column-3-sentence-1) | constant |
| [🔗](#tab:char.traits.req-row-19)<br>X​::​to_int_type(c) | X​::​int_type | *Returns*: some value e, constrained by the definitions of to_char_type and eq_int_type[.](#tab:char.traits.req-row-19-column-3-sentence-1) | constant |
| [🔗](#tab:char.traits.req-row-20)<br>X​::​eq_int_type(e,f) | bool | *Returns*: for all c and d, X​::​eq(c,d) is equal to X​::​eq_int_type(X​::​to_int_type(c), X​::​to_int_type(d)); otherwise, yields true if e and f are both copies of X​::​eof(); otherwise, yields false if one of e and f is a copy of X​::​eof() and the other is not; otherwise the value is unspecified[.](#tab:char.traits.req-row-20-column-3-sentence-1) | constant |
| [🔗](#tab:char.traits.req-row-21)<br>X​::​eof() | X​::​int_type | *Returns*: a value e such that X​::​eq_int_type(e,X​::​to_int_type(c)) is false for all values c[.](#tab:char.traits.req-row-21-column-3-sentence-1) | constant |

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/strings.tex#L197)

The class template

[🔗](#lib:char_traits)

template<class charT> struct char_traits; is provided in the header [<string>](string.syn#header:%3cstring%3e "27.4.2 Header <string> synopsis [string.syn]") as a basis for explicit specializations[.](#2.sentence-1)