Issue 3662: basic_string::append/assign(NTBS, pos, n) suboptimal

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3662. `basic_string::append/assign(NTBS, pos, n)` suboptimal

Section: 23.4.3.7.2 [string.append], 23.4.3.7.3 [string.assign] Status: New Submitter: Jonathan Wakely Opened: 2022-01-21 Last modified: 2022-01-30

Priority: 3

View all other issues in [string.append].

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Discussion:

This will allocate temporary string, then copy one byte from it:

std::string s;
s.append("a very very long string that doesn't fit in SSO buffer", 13, 1);

The problem is that there is no overload accepting an NTBS there, so it converts to a temporary string using:

append(const basic_string&, size_t, size_t = npos);

C++17 added a new overload for a string_view:

append(const T&, size_t, size_t = npos);

But that overload is constrained to not allow const char* arguments, so we still create a temporary string.

If we're going to accept those arguments, we should do so efficiently, without forcing an unnecessary allocation.

We could do better if we split the append(const T&, ...) overload into:

append(const T&, size_t);
append(const T&, size_t, size_t);

and then remove the !is_convertible_v<const T&, const charT*> constraint from the second overload (Option A in the proposed resolution).

Alternatively, we can leave the append(const T&, size_t, size_t) overload alone and just add one taking exactly the arguments used above. That seems simpler to me, and I prefer that (Option B in the Proposed Resolution).

The same problem applies to assign("...", pos, n).

[2022-01-30; Reflector poll]

Set priority to 3 after reflector poll.

Proposed resolution:

This wording is relative to N4901.

[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

Option A:

Modify 23.4.3.1 [basic.string.general], class template basic_string synopsis, as indicated:

[…]
// 23.4.3.7 [string.modifiers], modifiers
[…]
template<class T>
  constexpr basic_string& append(const T& t);
template<class T>
  constexpr basic_string& append(const T& t, size_type pos);
template<class T>
  constexpr basic_string& append(const T& t, size_type pos, size_type n = npos);
constexpr basic_string& append(const charT* s, size_type n);
[…]
template<class T>
  constexpr basic_string& assign(const T& t);
template<class T>
  constexpr basic_string& assign(const T& t, size_type pos);
template<class T>
  constexpr basic_string& assign(const T& t, size_type pos, size_type n = npos);
constexpr basic_string& assign(const charT* s, size_type n);
[…]

Modify 23.4.3.7.2 [string.append] as indicated:
```
template<class T>
  constexpr basic_string& append(const T& t);
```
-3- Constraints:
1. (3.1) — is_convertible_v<const T&, basic_string_view<charT, traits>> is true and
2. (3.2) — is_convertible_v<const T&, const charT*> is false.
-4- Effects: Equivalent to:
```
basic_string_view<charT, traits> sv = t;
return append(sv.data(), sv.size());
```
```
template<class T>
  constexpr basic_string& append(const T& t, size_type pos);
```
-?- Constraints:
1. (?.1) — is_convertible_v<const T&, basic_string_view<charT, traits>> is true and
2. (?.2) — is_convertible_v<const T&, const charT*> is false.
-?- Effects: Equivalent to:
```
basic_string_view<charT, traits> sv = t;
return append(sv.substr(pos));
```
```
template<class T>
  constexpr basic_string& append(const T& t, size_type pos, size_type n = npos);
```
-5- Constraints:
1. ~~(5.1) —~~ is_convertible_v<const T&, basic_string_view<charT, traits>> is true ~~and~~
2. ~~(5.2) — is_convertible_v<const T&, const charT*> is false~~.
-6- Effects: Equivalent to:
```
basic_string_view<charT, traits> sv = t;
return append(sv.substr(pos, n));
```
Modify 23.4.3.7.3 [string.assign] as indicated:
```
template<class T>
  constexpr basic_string& assign(const T& t);
```
-4- Constraints:
1. (4.1) — is_convertible_v<const T&, basic_string_view<charT, traits>> is true and
2. (4.2) — is_convertible_v<const T&, const charT*> is false.
-5- Effects: Equivalent to:
```
basic_string_view<charT, traits> sv = t;
return assign(sv.data(), sv.size());
```
```
template<class T>
  constexpr basic_string& assign(const T& t, size_type pos);
```
-?- Constraints:
1. (?.1) — is_convertible_v<const T&, basic_string_view<charT, traits>> is true and
2. (?.2) — is_convertible_v<const T&, const charT*> is false.
-?- Effects: Equivalent to:
```
basic_string_view<charT, traits> sv = t;
return assign(sv.substr(pos));
```
```
template<class T>
  constexpr basic_string& assign(const T& t, size_type pos, size_type n = npos);
```
-6- Constraints:
1. ~~(6.1) —~~ is_convertible_v<const T&, basic_string_view<charT, traits>> is true ~~and~~
2. ~~(6.2) — is_convertible_v<const T&, const charT*> is false~~.
-7- Effects: Equivalent to:
```
basic_string_view<charT, traits> sv = t;
return assign(sv.substr(pos, n));
```

Option B:

Modify 23.4.3.1 [basic.string.general], class template basic_string synopsis, as indicated:

[…]
// 23.4.3.7 [string.modifiers], modifiers
[…]
constexpr basic_string& append(const charT* s, size_type n);
constexpr basic_string& append(const charT* s);
constexpr basic_string& append(const charT* s, size_type pos, size_type n);
constexpr basic_string& append(size_type n, charT c);
[…]
constexpr basic_string& assign(const charT* s, size_type n);
constexpr basic_string& assign(const charT* s);
constexpr basic_string& assign(const charT* s, size_type pos, size_type n);
constexpr basic_string& assign(size_type n, charT c);
[…]

Modify 23.4.3.7.2 [string.append] as indicated:
```
constexpr basic_string& append(const charT* s, size_type n);
```
-7- Preconditions: [s, s + n) is a valid range.
-8- Effects: Appends a copy of the range [s, s + n) to the string.
-9- Returns: *this.
```
constexpr basic_string& append(const charT* s);
```
-10- Effects: Equivalent to: return append(s, traits::length(s));
```
constexpr basic_string& append(const charT* s, size_type pos, size_type n);
```
-?- Effects: Equivalent to: return append(basic_string_view<charT, traits>(s).substr(pos, n));
Modify 23.4.3.7.3 [string.assign] as indicated:
```
constexpr basic_string& assign(const charT* s, size_type n);
```
-8- Preconditions: [s, s + n) is a valid range.
-9- Effects: Replaces the string controlled by *this with a copy of the range [s, s + n).
-10- Returns: *this.
```
constexpr basic_string& assign(const charT* s);
```
-11- Effects: Equivalent to: return assign(s, traits::length(s));
```
constexpr basic_string& assign(const charT* s, size_type pos, size_type n);
```
-?- Effects: Equivalent to: return assign(basic_string_view<charT, traits>(s).substr(pos, n));

3662. basic_string::append/assign(NTBS, pos, n) suboptimal

3662. `basic_string::append/assign(NTBS, pos, n)` suboptimal