std::string capacity() method
- since C++20
- since C++11
- until C++11
// Const version only
constexpr size_type capacity() const noexcept;
// Const version only
size_type capacity() const noexcept;
// Const version only
size_type capacity() const;
Returns the number of characters that the string has currently allocated space for.
Parameters
(none)
Return value
Capacity of the currently allocated storage.
Complexity
Constant - O(1).
Exceptions
(none)
Notes
Memory locations obtained from the allocator but not available for storing any element are not counted in the allocated storage.
note
The null terminator is not an element of the std::basic_string
.
Example
Main.cpp
#include <iostream>
#include <iomanip>
#include <string>
void show_capacity(std::string const& s)
{
std::cout << std::quoted(s) << " has capacity " << s.capacity() << ".\n";
}
int main()
{
std::string s{"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::string g;
auto old_cap {g.capacity()};
for(int mark{}; 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();
}
}
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
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