From cppreference.com
size_typecapacity()const;(noexcept since C++11)
(constexpr since C++20)Returns the number of characters that the string has currently allocated space for.
Parameters
(none)
Return value
Capacity of the currently allocated storage, i.e. the storage available for storing elements.
Complexity
Constant.
Notes
Memory locations obtained from the allocator but not available for storing any element are not counted in the allocated storage. Note that the null terminator is not an element of the
.
Example
Run this code
#include<iomanip>#include<iostream>#include<string>voidshow_capacity(std::stringconst&s){std::cout<<std::quoted(s)<<" has capacity "<<s.capacity()<<".\n";}intmain(){std::strings{"Exemplar"};show_capacity(s);s+=" is an example string.";show_capacity(s);s.clear();show_capacity(s);std::cout<<"\nDemonstrate the capacity's growth policy.""\nSize: Capacity: Ratio:\n"<<std::left;std::stringg;autoold_cap{g.capacity()};for(intmark{};mark!=5;++mark){while(old_cap==g.capacity())g.push_back('.');std::cout<<std::setw(7)<<g.size()<<std::setw(11)<<g.capacity()<<std::setw(10)<<g.capacity()/static_cast<float>(old_cap)<<'\n';old_cap=g.capacity();}}Possible output:
"Exemplar" has capacity 15. "Exemplar is an example string." has capacity 30. "" has capacity 30. Demonstrate the capacity's growth policy. Size: Capacity: Ratio: 16 30 2 31 60 2 61 120 2 121 240 2 241 480 2 See also