std::string resize() method

since C++20

// (1) Non const version only
constexpr void resize( size_type count );

// (2) Non const version only
constexpr void resize( size_type count, CharT ch );

until C++20

// (1) Non const version only
void resize( size_type count );

// (2) Non const version only
void resize( size_type count, CharT ch );

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.

important

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

Parameters

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

Return value

(none)

Complexity

important

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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

Main.cpp

#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';
        }
    }
}

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