std::vector capacity() method

od C++20

// prism-push-types:size_type
// Const version only
constexpr size_type capacity() const noexcept;

od C++11

// prism-push-types:size_type
// Const version only
size_type capacity() const noexcept;

do C++11

// prism-push-types:size_type
// Const version only
size_type capacity() const;

Returns the number of elements that the container has currently allocated space for.

Parameters

(none)

Return value

Capacity of the currently allocated storage.

Complexity

Constant - O(1).

Example

Main.cpp

#include <iomanip>
#include <iostream>
#include <vector>
 
int main()
{
    int sz = 100;
    std::vector<int> v;
 
    auto cap = v.capacity();
    std::cout << "Initial size: " << v.size() << ", capacity: " << cap << '\n';
 
    std::cout << "\nDemonstrate the capacity's growth policy."
                 "\nSize:  Capacity:  Ratio:\n" << std::left;
    while (sz-- > 0) {
        v.push_back(sz);
        if (cap != v.capacity()) {
            std::cout << std::setw( 7) << v.size()
                      << std::setw(11) << v.capacity()
                      << std::setw(10) << v.capacity() / static_cast<float>(cap) << '\n';
            cap = v.capacity();
        }
    }
 
    std::cout << "\nFinal size: " << v.size() << ", capacity: " << v.capacity() << '\n';
}

Output

Initial size: 0, capacity: 0
 
Demonstrate the capacity's growth policy.
Size:  Capacity:  Ratio:
1      1          inf
2      2          2
3      4          2
5      8          2
9      16         2
17     32         2
33     64         2
65     128        2
 
Final size: 100, capacity: 128