std::basic_string::resize(3)

NAME
std::basic_string::resize - std::basic_string::resize

Synopsis
   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.

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 safety guarantee).

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
   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 added strong exception safety
                      guarantee guarantee

See also
   size returns the number of characters
   length (public member function)
   reserve reserves storage
                 (public member function)
   shrink_to_fit reduces memory usage by freeing unused memory
   (DR*) (public member function)