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

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< cpp‎ | string‎ | basic string
(exception guarantees per 21.4.1[string.require])
(Added LWG issue #2250 DR (part 7/16).)
 
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{{cpp/string/basic_string/title | resize}}
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{{cpp/string/basic_string/title|resize}}
 
{{cpp/string/basic_string/navbar}}
 
{{cpp/string/basic_string/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
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{{dcl | num=1 |
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{{|num=1|
 
void resize( size_type count );
 
void resize( size_type count );
 
}}
 
}}
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{{dcl | num=2 |
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{{|num=2|
 
void resize( size_type count, CharT ch );
 
void resize( size_type count, CharT ch );
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
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Resizes the string to contain {{tt|count}} characters.
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Resizes the string to contain {{|count}} characters.
  
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If the current size is less than {{tt|count}}, additional characters are appended.
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If the current size is less than {{|count}}, additional characters are appended
  
−
If the current size is greater than {{tt|count}}, the string is reduced to its first {{tt|count}} elements.
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{{tt|}} is
 +
to {{|}}.
  
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The first version initializes new characters to {{c|CharT()}}, the second version initializes new characters to {{tt|ch}}.
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{{c|}}, the to {{|}} .
  
 
===Parameters===
 
===Parameters===
−
 
 
{{par begin}}
 
{{par begin}}
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{{par | count | new size of the string}}
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{{par|count|new size of the string}}
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{{par | ch | character to initialize the new characters with }}
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{{par|ch|character to initialize the new characters with}}
 
{{par end}}
 
{{par end}}
−
 
−
===Return value===
 
−
(none)
 
  
 
===Exceptions===
 
===Exceptions===
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{{lc|std::length_error}} if {{c|count > max_size()}}.
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{{lc|std::length_error}} if {{c|count > max_size()}}.
 
Any exceptions thrown by corresponding {{tt|Allocator}}.
 
Any exceptions thrown by corresponding {{tt|Allocator}}.
  
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{{rev inl|since=c++11|
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{{strong exception guarantee}}
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If an exception is thrown for any reason, this function has no effect (strong exception guarantee).}}
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===Example===
 
===Example===
 
{{example
 
{{example
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| p=true
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|=
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| code =
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#include <iostream>
 
#include <iostream>
 
#include <stdexcept>
 
#include <stdexcept>
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int main()
 
int main()
 
{
 
{
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     std::string s;
 
     std::string s;
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+
−
     try {
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     try
 +
{
 
         // size is OK, no length_error
 
         // size is OK, no length_error
 
         // (may throw bad_alloc)
 
         // (may throw bad_alloc)
 
         s.resize(s.max_size() - 1, 'x');
 
         s.resize(s.max_size() - 1, 'x');
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    } catch (std::bad_alloc&) {
 
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        std::cout << "1. bad alloc\n";
 
 
     }
 
     }
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+
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     try {
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 +
 +
 +
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     try
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{
 
         // size is OK, no length_error
 
         // size is OK, no length_error
 
         // (may throw bad_alloc)
 
         // (may throw bad_alloc)
 
         s.resize(s.max_size(), 'x');
 
         s.resize(s.max_size(), 'x');
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    } catch (std::bad_alloc& exc) {
 
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        std::cout << "2. bad alloc\n";
 
 
     }
 
     }
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     try {
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 +
 +
 +
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     try
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{
 
         // size is BAD, throw length_error
 
         // size is BAD, throw length_error
 
         s.resize(s.max_size() + 1, 'x');
 
         s.resize(s.max_size() + 1, 'x');
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     } catch (std::length_error&) {
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     }
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         std::cout << "3. length error\n";
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catch (std::length_error& )
 +
{
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         std::cout << "3. error\n;
 
     }
 
     }
 
}
 
}
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| output=
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1. bad alloc
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|output=
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2. bad alloc
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3. length error
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1.
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2.
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3. error
 
}}
 
}}
  
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===Complexity===
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======
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Linear in the size of the string.
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 +
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the
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===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/string/basic_string/dsc size}}
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{{dsc inc|cpp/string/basic_string/dsc size
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}}
 
{{dsc end}}
 
{{dsc end}}
  
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[[de:cpp/string/basic string/resize]]
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deesfritjaptruzh
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[[es:cpp/string/basic string/resize]]
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[[fr:cpp/string/basic string/resize]]
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[[it:cpp/string/basic string/resize]]
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[[ja:cpp/string/basic string/resize]]
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[[pt:cpp/string/basic string/resize]]
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[[ru:cpp/string/basic string/resize]]
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[[zh:cpp/string/basic string/resize]]
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Latest revision as of 19:02, 11 December 2024

 
C++
 
Strings library
 
std::basic_string
 
void resize( size_type count );
 (1) (constexpr since C++20)
void resize( size_type count, CharT ch );
 (2) (constexpr since C++20)

Resizes the string to contain count characters.

If the current size is less than count, additional characters are appended:

1) Initializes appended characters to CharT() ('\0' if CharT is char).
2) Initializes appended characters to ch.

If the current size is greater than count, the string is reduced to its first count elements.

Contents

[edit] Parameters

count - new size of the string
ch - character to initialize the new characters with

[edit] Exceptions

std::length_error if count > max_size() is true. Any exceptions thrown by corresponding Allocator.

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

[edit] Example

Run this code
#include <iomanip>
#include <iostream>
#include <stdexcept>
 
int main()
{
    const unsigned desired_length{8};
    std::string long_string("Where is the end?");
    std::string short_string("H");
 
    std::cout << "Basic functionality:\n"
              << "Shorten:\n"
              << "1. Before: " << std::quoted(long_string) << '\n';
    long_string.resize(desired_length);
    std::cout << "2. After:  " << std::quoted(long_string) << '\n';
 
    std::cout << "Lengthen with a given value 'a':\n"
              << "3. Before: " << std::quoted(short_string) << '\n';
    short_string.resize(desired_length, 'a');
    std::cout << "4. After:  " << std::quoted(short_string) << '\n';
 
    std::cout << "Lengthen with char() == " << static_cast<int>(char()) << '\n'
              << "5. Before: " << std::quoted(short_string) << '\n';
    short_string.resize(desired_length + 3);
    std::cout << "6. After:  \"";
    for (char c : short_string)
        std::cout << (c == char() ? '@' : c);
    std::cout << "\"\n\n";
 
    std::cout << "Errors:\n";
    std::string s;
 
    try
    {
        // size is OK, no length_error
        // (may throw bad_alloc)
        s.resize(s.max_size() - 1, 'x');
    }
    catch (const std::bad_alloc& ex)
    {
        std::cout << "1. Exception: " << ex.what() << '\n';
    }
 
    try
    {
        // size is OK, no length_error
        // (may throw bad_alloc)
        s.resize(s.max_size(), 'x');
    }
    catch (const std::bad_alloc& ex)
    {
        std::cout << "2. Exception: " << ex.what() << '\n';
    }
 
    try
    {
        // size is BAD, throw length_error
        s.resize(s.max_size() + 1, 'x');
    }
    catch (const std::length_error& ex)
    {
        std::cout << "3. Length error: " << ex.what() << '\n';
    }
}

Possible output: 

Basic functionality:
Shorten:
1. Before: "Where is the end?"
2. After:  "Where is"
Lengthen with a given value 'a':
3. Before: "H"
4. After:  "Haaaaaaa"
Lengthen with char() == 0
5. Before: "Haaaaaaa"
6. After:  "Haaaaaaa@@@"
 
Errors:
1. Exception: std::bad_alloc
2. Exception: std::bad_alloc
3. Length error: basic_string::_M_replace_aux

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 847 C++98 there was no exception safety guarantee added strong exception safety guarantee
LWG 2250 C++98 the behavior was undefined if
count > max_size() is true 		always throws an exception in this case

[edit] See also

 returns the number of characters
(public member function) [edit]
reserves storage
(public member function) [edit]
(DR*)
 reduces memory usage by freeing unused memory
(public member function) [edit]
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