std::basic_string<CharT,Traits,Allocator>:: resize_and_overwrite

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 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.
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 ] .

If r is not of an integer-like type , the program is ill-formed.

If any of the following conditions is satisfied, the behavior is undefined:

• std :: move ( op ) ( p, count ) throws an exception.
• std :: move ( op ) ( p, count ) modifies p or count .
• r is not in the range [ ​ 0 ​ , count ] .
• 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 capacity ( ) .

Parameters

Exceptions

std::length_error if count > 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 .

Example

#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';
}

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