std::string resize() method

since C++20

// (1) Non const version onlyconstexpr void resize( size_type count );// (2) Non const version onlyconstexpr void resize( size_type count, CharT ch );

until C++20

// (1) Non const version onlyvoid resize( size_type count );// (2) Non const version onlyvoid 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() == 05. Before: "Haaaaaaa"6. After:  "Haaaaaaa@@@" Errors:1. Exception: std::bad_alloc2. Exception: std::bad_alloc3. Length error: basic_string::_M_replace_aux