Difference between revisions of "cpp/string/basic string/resize"
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< cpp | string | basic string
Andreas Krug (Talk | contribs) m ({{c}}, headers sorted, fmt) |
(Added LWG issue #2250 DR (part 7/16).) |
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{{cpp/string/basic_string/navbar}} | {{cpp/string/basic_string/navbar}} | ||
{{dcl begin}} | {{dcl begin}} | ||
| − | {{ | + | {{|num=1|=c++20| |
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void resize( size_type count ); | void resize( size_type count ); | ||
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}} | }} | ||
| − | {{ | + | {{|num=2|=c++20| |
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void resize( size_type count, CharT ch ); | void resize( size_type count, CharT ch ); | ||
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}} | }} | ||
{{dcl end}} | {{dcl end}} | ||
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If the current size is less than {{c|count}}, additional characters are appended: | If the current size is less than {{c|count}}, additional characters are appended: | ||
| − | @1@ Initializes appended characters to {{c|CharT()}} ({{c|'\0'}} if {{tt|CharT}} is {{c|char}}). | + | @1@ Initializes appended characters to {{c|CharT()}} ({{c|'\0'}} if {{tt|CharT}} is {{c|char}}). |
@2@ Initializes appended characters to {{c|ch}}. | @2@ Initializes appended characters to {{c|ch}}. | ||
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{{par|ch|character to initialize the new characters with}} | {{par|ch|character to initialize the new characters with}} | ||
{{par end}} | {{par end}} | ||
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===Exceptions=== | ===Exceptions=== | ||
| − | {{lc|std::length_error}} if {{c|count > max_size()}}. | + | {{lc|std::length_error}} if {{c|count > max_size()}}. |
Any exceptions thrown by corresponding {{tt|Allocator}}. | Any exceptions thrown by corresponding {{tt|Allocator}}. | ||
| − | {{ | + | {{strong exception guarantee}} |
| − | + | ||
===Example=== | ===Example=== | ||
{{example | {{example | ||
| − | |code = | + | |code= |
#include <iomanip> | #include <iomanip> | ||
#include <iostream> | #include <iostream> | ||
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std::string long_string("Where is the end?"); | std::string long_string("Where is the end?"); | ||
std::string short_string("H"); | std::string short_string("H"); | ||
| − | + | ||
std::cout << "Basic functionality:\n" | std::cout << "Basic functionality:\n" | ||
<< "Shorten:\n" | << "Shorten:\n" | ||
| − | << "1. Before: " << quoted(long_string) << '\n'; | + | << "1. Before: " << quoted(long_string) << '\n'; |
long_string.resize(desired_length); | long_string.resize(desired_length); | ||
| − | std::cout << "2. After: " << quoted(long_string) << '\n'; | + | std::cout << "2. After: " << quoted(long_string) << '\n'; |
| − | + | ||
std::cout << "Lengthen with a given value 'a':\n" | std::cout << "Lengthen with a given value 'a':\n" | ||
| − | << "3. Before: " << quoted(short_string) << '\n'; | + | << "3. Before: " << quoted(short_string) << '\n'; |
short_string.resize(desired_length, 'a'); | short_string.resize(desired_length, 'a'); | ||
| − | std::cout << "4. After: " << quoted(short_string) << '\n'; | + | std::cout << "4. After: " << quoted(short_string) << '\n'; |
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std::cout << "Lengthen with char() == " << static_cast<int>(char()) << '\n' | std::cout << "Lengthen with char() == " << static_cast<int>(char()) << '\n' | ||
| − | << "5. Before: " << quoted(short_string) << '\n'; | + | << "5. Before: " << quoted(short_string) << '\n'; |
short_string.resize(desired_length + 3); | short_string.resize(desired_length + 3); | ||
std::cout << "6. After: \""; | std::cout << "6. After: \""; | ||
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std::cout << (c == char() ? '@' : c); | std::cout << (c == char() ? '@' : c); | ||
std::cout << "\"\n\n"; | std::cout << "\"\n\n"; | ||
| − | + | ||
std::cout << "Errors:\n"; | std::cout << "Errors:\n"; | ||
std::string s; | std::string s; | ||
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try | try | ||
{ | { | ||
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std::cout << "1. Exception: " << ex.what() << '\n'; | std::cout << "1. Exception: " << ex.what() << '\n'; | ||
} | } | ||
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try | try | ||
{ | { | ||
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std::cout << "2. Exception: " << ex.what() << '\n'; | std::cout << "2. Exception: " << ex.what() << '\n'; | ||
} | } | ||
| − | + | ||
try | try | ||
{ | { | ||
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3. Length error: basic_string::_M_replace_aux | 3. Length error: basic_string::_M_replace_aux | ||
}} | }} | ||
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===See also=== | ===See also=== | ||
Latest revision as of 19:02, 11 December 2024
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.
Contents |
[edit] Parameters
| count | - | new size of the string |
| ch | - | character to initialize the new characters with |
[edit] Exceptions
std::length_error if count > max_size() is true.
Any exceptions thrown by corresponding Allocator.
If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).
[edit] 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
[edit] 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 |
| LWG 2250 | C++98 | the behavior was undefined if count > max_size() is true |
always throws an exception in this case |
[edit] See also
| returns the number of characters (public member function) | |
| reserves storage (public member function) | |
| (DR*) |
reduces memory usage by freeing unused memory (public member function) |
