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std::basic_...nd_overwrite(3) C++ Standard Libarystd::basic_...nd_overwrite(3) NAME std::basic_string::resize_and_overwrite - std::basic_string::re- size_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->com- pare(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 al- locations by, e.g., making p equal to the pointer to beginning of storage of char- acters 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 refer- ences into *this, regardless whether reallocation occurs. Implementations may assume that the contents of the string are not aliased after the call to resize_and_over- write. Feature-test macro Value Std Feature __cpp_lib_string_resize_and_overwrite 202110L (C++23) std::ba- sic_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) http://cppreference.com 2024.06.10 std::basic_...nd_overwrite(3)
NAME | Synopsis | Parameters | Return value | Exceptions | Notes | Example | Possible output: | See also
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