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template<class Operation> constexprvoid resize_and_overwrite( size_type count, Operation op); | (since C++23) | |
Resizes the string to contain at mostcount characters, using the user-provided operationop to modify the possibly indeterminate contents and set the length. This avoids the cost of initializing a suitably-sizedstd::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:
resize_and_overwrite. Letp denote the pointer to the first character in the storage.[p+ k, p+ count] may have indeterminate values.[p, p+ r) (which sets the length of*this tor). Invalidates all pointers and references to the range[p, p+ count].Ifr is not of aninteger-like type, the program is ill-formed.
If any of the following conditions is satisfied, the behavior is undefined:
[0, count].[p, p+ r) has an indeterminate value.Implementations are recommended to avoid unnecessary copies and allocations by, e.g., makingp 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 ifcount is less than or equal tocapacity().
Contents |
| count | - | the maximal possible new size of the string |
| op | - | the function object used for setting the new contents of the string |
std::length_error ifcount> max_size().Any exceptions thrown by correspondingAllocator.
If an exception is thrown fromstd::move(op)(p, count), the behavior is undefined. Otherwise, if an exception is thrown, this function has no effect.
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 toresize_and_overwrite.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_string_resize_and_overwrite | 202110L | (C++23) | std::basic_string::resize_and_overwrite |
Link to test the example:compiler explorer.
#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); constexprstd::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{constauto 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{returnstd::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:"};constauto 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;conststd::string_view fruit: fruits){char* last= first+ fruit.size()+1;if(last> end)break;*first++=' '; std::ranges::copy(fruit, first); first= last;} constauto 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: 27In Operation(); final_size: 26Finally, food.size: 26, food.capacity: 30, food: "Food: apple banana coconut"
| changes the number of characters stored (public member function)[edit] |
