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Difference between revisions of "cpp/string/basic string/capacity"

From cppreference.com
< cpp‎ | string‎ | basic string
m (r2.7.3) (Robot: Adding fr, ja, pl, pt, ru)
m (→‎Synopsis: {{ddcla}})
 
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{{cpp/string/basic_string/title | capacity}}
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{{cpp/string/basic_string/title|capacity}}
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{{cpp/string/basic_string/sidebar}}
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{{cpp/string/basic_string/navbar}}
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{{ddcl |  
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{{ddcla|noexcept=c++11|constexpr=c++20|
 
size_type capacity() const;
 
size_type capacity() const;
 
}}
 
}}
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===Return value===
 
===Return value===
−
 
+
Capacity of the currently allocated storage, i.e. the storage available for storing elements.
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capacity of the currently allocated storage
+
  
 
===Complexity===
 
===Complexity===
−
Constant
+
Constant.
  
−
===See also===
+
===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 {{lc|std::basic_string}}.
  
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{{dcl list begin}}
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===Example===
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{{dcl list template | cpp/string/basic_string/dcl list size}}
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{{example
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{{dcl list template | cpp/string/basic_string/dcl list reserve}}
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|code=
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{{dcl list end}}
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#include <iomanip>
 +
#include <iostream>
 +
#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();
 +
    }
 +
}
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|p=true
 +
|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===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/string/basic_string/dsc size}}
 +
{{dsc inc|cpp/string/basic_string/dsc reserve}}
 +
{{dsc end}}
  
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[[fr:cpp/string/basic string/capacity]]
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{{langlinks|de|es|fr|it|ja|pl|pt|ru|zh}}
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[[ja:cpp/string/basic string/capacity]]
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−
[[pl:cpp/string/basic string/capacity]]
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−
[[pt:cpp/string/basic string/capacity]]
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−
[[ru:cpp/string/basic string/capacity]]
+

Latest revision as of 13:47, 18 October 2024

 
C++
 
Strings library
 
std::basic_string
Member functions
(C++23)
Element access
(DR*)
(DR*)
(C++17)
Iterators
(C++11)
(C++11)
(C++11)
(C++11)
Capacity
basic_string::capacity
(DR*)
Modifiers
(C++23)
(DR*)
(C++23)
(C++23)
(C++23)  
Search
Operations
(C++20)
(C++20)
(C++23)
Constants
Non-member functions
(C++20)(C++20)
I/O
Comparison
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Numeric conversions
(C++11)(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Literals
(C++14)
Helper classes
(C++11)
Deduction guides (C++17)

 
size_type capacity() const;
 (noexcept since C++11)
(constexpr since C++20)

Returns the number of characters that the string has currently allocated space for.

Contents

[edit] Parameters

(none)

[edit] Return value

Capacity of the currently allocated storage, i.e. the storage available for storing elements.

[edit] Complexity

Constant.

[edit] 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 std::basic_string.

[edit] Example

Run this code
#include <iomanip>
#include <iostream>
#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();
    }
}

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

[edit] See also

 returns the number of characters
(public member function) [edit]
reserves storage
(public member function) [edit]
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