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

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< cpp‎ | string‎ | basic string
(Added LWG issue #847 DR (part 15).)
m (→‎Synopsis: +constexpr since)
 
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{{cpp/string/basic_string/navbar}}
 
{{cpp/string/basic_string/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
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{{dcl rev multi|num=1
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{{dcl|num=1|notes={{mark constexpr since c++20}}|
−
|until1=c++20|dcl1=
+
 
void resize( size_type count );
 
void resize( size_type count );
−
|dcl2=
 
−
constexpr void resize( size_type count );
 
 
}}
 
}}
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{{dcl rev multi|num=2
+
{{dcl|num=2|notes={{mark constexpr since c++20}}|
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|until1=c++20|dcl1=
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void resize( size_type count, CharT ch );
 
void resize( size_type count, CharT ch );
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|dcl2=
 
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constexpr void resize( size_type count, CharT ch );
 
 
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<!--
 
<!--
 
===Complexity===
 
===Complexity===
−
Linear in the size of the string. -->
+
Linear in the size of the string.-->
  
 
===Exceptions===
 
===Exceptions===
Line 58: Line 52:
 
     std::cout << "Basic functionality:\n"
 
     std::cout << "Basic functionality:\n"
 
               << "Shorten:\n"
 
               << "Shorten:\n"
−
               << "1. Before: " << quoted(long_string) << '\n';
+
               << "1. Before: " << std::quoted(long_string) << '\n';
 
     long_string.resize(desired_length);
 
     long_string.resize(desired_length);
−
     std::cout << "2. After:  " << quoted(long_string) << '\n';
+
     std::cout << "2. After:  " << std::quoted(long_string) << '\n';
 
      
 
      
 
     std::cout << "Lengthen with a given value 'a':\n"
 
     std::cout << "Lengthen with a given value 'a':\n"
−
               << "3. Before: " << quoted(short_string) << '\n';
+
               << "3. Before: " << std::quoted(short_string) << '\n';
 
     short_string.resize(desired_length, 'a');
 
     short_string.resize(desired_length, 'a');
−
     std::cout << "4. After:  " << quoted(short_string) << '\n';
+
     std::cout << "4. After:  " << std::quoted(short_string) << '\n';
 
      
 
      
 
     std::cout << "Lengthen with char() == " << static_cast<int>(char()) << '\n'
 
     std::cout << "Lengthen with char() == " << static_cast<int>(char()) << '\n'
−
               << "5. Before: " << quoted(short_string) << '\n';
+
               << "5. Before: " << std::quoted(short_string) << '\n';
 
     short_string.resize(desired_length + 3);
 
     short_string.resize(desired_length + 3);
 
     std::cout << "6. After:  \"";
 
     std::cout << "6. After:  \"";

Latest revision as of 11:15, 21 April 2024

 
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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] Return value

(none)

[edit] Exceptions

std::length_error if count > max_size(). 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

[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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