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 the 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().

[edit] Parameters

[edit] 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.

[edit] 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.

[edit] 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. " <<