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

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< cpp‎ | string‎ | basic string
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Revision as of 17:31, 2 June 2022

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

Parameters

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

Return value

(none)

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 guarantee).(since C++11)

Example

Run this code
#include <iostream>
#include <iomanip>
#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: " << quoted( long_string ) << '\n';
    long_string.resize( desired_length );
    std::cout << "2. After:  " << quoted( long_string ) << '\n';
 
 
    std::cout << "Lengthen with a given value 'a':\n"
              << "3. Before: " << quoted( short_string ) << '\n';
    short_string.resize( desired_length, 'a' );
    std::cout << "4. After:  " << quoted( short_string ) << '\n';
 
 
    std::cout << "Lengthen with char() == " << static_cast<int>(char()) << '\n'
              << "5. Before: " << 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

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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