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)