std::basic_string::resize_and_overwrite(3)

NAME
std::basic_string::resize_and_overwrite - std::basic_string::resize_and_overwrite

Synopsis
   template< class Operation > (since C++23)
   constexpr void resize_and_overwrite( size_type count, Operation op );
   Resizes the string to contain at most count characters, using the user-provided
   operation op to modify the possibly indeterminate contents and set the length. This
   avoids the cost of initializing a suitably-sized std::string when it is intended to
   be used as a char array to be populated by, e.g., a C API call.
   This function performs following steps:
    1. Obtains contiguous storage that contains count + 1 characters, and makes its
       first k characters equal to the first k characters of *this, where k is the
       smaller of count and the result of this->size() before the call to
       resize_and_overwrite. Let p denote the pointer to the first character in the
       storage.
          * The equality is determined as if by checking this->compare(0, k, p, k) ==
            0.
          * The characters in [p + k, p + count] may have indeterminate values.
    2. Evaluates std::move(op)(p, count). Let r be the return value of std::move(op)(p,
       count).
    3. Replaces the contents of *this with [p, p + r) (which sets the length of *this
       to r). Invalidates all pointers and references to the range [p, p + count].
   The program is ill-formed if r does not have an integer-like type. The behavior is
   undefined if std::move(op)(p, count) throws an exception or modifies p or count, r
   is not in the range [0, count], or any character in range [p, p + r) has an
   indeterminate value.
   Implementations are recommended to avoid unnecessary copies and allocations by,
   e.g., making p equal to the pointer to beginning of storage of characters allocated
   for *this after the call, which can be identical to the existing storage of *this if
   count is less than or equal to this->capacity().

Parameters
   count - the maximal possible new size of the string
   op - the function object used for setting the new contents of the string

Return value
   (none)

Exceptions
   std::length_error if count > this->max_size(). Any exceptions thrown by
   corresponding Allocator.
   If an exception is thrown from std::move(op)(p, count), the behavior is undefined.
   Otherwise, if an exception is thrown, this function has no effect.

Notes
   resize_and_overwrite invalidates all iterators, pointers, and references into *this,
   regardless whether reallocation occurs. Implementations may assume that the contents
   of the string are not aliased after the call to resize_and_overwrite.
         Feature-test macro Value Std Feature __cpp_lib_string_resize_and_overwrite 202110L (C++23) std::basic_string::resize_and_overwrite

Example
   Link to test the example: compiler explorer. // Run this code
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
 #include <cstring>
 #include <iomanip>
 #include <iostream>
 #include <string>
 #include <string_view>
 static_assert(__cpp_lib_string_resize_and_overwrite);
 constexpr std::string_view fruits[]{"apple", "banana", "coconut", "date", "elderberry"};
 int main()
 {
     // A simple case, append only fruits[0]. The string size will be increased.
     std::string s{"Food: "};
     s.resize_and_overwrite(16, [sz = s.size()](char* buf, std::size_t buf_size) noexcept
     {
         const auto to_copy = std::min(buf_size - sz, fruits[0].size());
         std::memcpy(buf + sz, fruits[0].data(), to_copy);
         return sz + to_copy;
     });
     std::cout << "1. " << std::quoted(s) << '\n';
     // The size shrinking case. Note, that the user's lambda is always invoked.
     s.resize_and_overwrite(10, [](char* buf, int n) noexcept
     {
         return std::find(buf, buf + n, ':') - buf;
     });
     std::cout << "2. " << std::quoted(s) << '\n';
     std::cout << "3. Copy data until the buffer is full. Print data and sizes.\n";
     std::string food{"Food:"};
     const auto resize_to{27};
     std::cout << "Initially, food.size: " << food.size()
               << ", food.capacity: " << food.capacity()
               << ", resize_to: " << resize_to
               << ", food: " << std::quoted(food) << '\n';
     food.resize_and_overwrite(
         resize_to,
         [food_size = food.size()](char* p, std::size_t n) noexcept -> std::size_t
         {
             // p[0]..p[n] is the assignable range
             // p[0]..p[min(n, food_size) - 1] is the readable range
             // (contents initially equal to the original string)
             // Debug print:
             std::cout << "In Operation(); n: " << n << '\n';
             // Copy fruits to the buffer p while there is enough space.
             char* first = p + food_size;
             for (char* const end = p + n; const std::string_view fruit : fruits)
             {
                 char* last = first + fruit.size() + 1;
                 if (last > end)
                     break;
                 *first++ = ' ';
                 std::ranges::copy(fruit, first);
                 first = last;
             }
             const auto final_size{static_cast<std::size_t>(first - p)};
             // Debug print:
             std::cout << "In Operation(); final_size: " << final_size << '\n';
             assert(final_size <= n);
             return final_size; // Return value is the actual new length
                        // of the string, must be in range 0..n
         });
     std::cout << "Finally, food.size: " << food.size()
               << ", food.capacity: " << food.capacity()
               << ", food: " << std::quoted(food) << '\n';
 }

Possible output:
 1. "Food: apple"
 2. "Food"
 3. Copy data until the buffer is full. Print data and sizes.
 Initially, food.size: 5, food.capacity: 15, resize_to: 27, food: "Food:"
 In Operation(); n: 27
 In Operation(); final_size: 26
 Finally, food.size: 26, food.capacity: 30, food: "Food: apple banana coconut"

See also
   resize changes the number of characters stored
          (public member function)