W3cubDocs

/C++

std::basic_string<CharT,Traits,Allocator>::resize

	(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:

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

#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: " << 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

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