Difference between revisions of "cpp/string/basic string/at"
From cppreference.com
< cpp | string | basic string
m (Catch exceptions by const reference) |
(Added LWG issue #2207 DR.) |
||
| (12 intermediate revisions by 4 users not shown) | |||
| Line 1: | Line 1: | ||
| − | {{cpp/string/basic_string/title | at}} | + | {{cpp/string/basic_string/title|at}} |
{{cpp/string/basic_string/navbar}} | {{cpp/string/basic_string/navbar}} | ||
{{dcl begin}} | {{dcl begin}} | ||
| − | {{ | + | {{dcla|anchor=no|num=1|constexpr=c++20| |
| − | + | CharT& at( size_type pos ); | |
}} | }} | ||
| − | {{ | + | {{dcla|anchor=no|num=2|constexpr=c++20| |
| − | + | const CharT& at( size_type pos ) const; | |
}} | }} | ||
{{dcl end}} | {{dcl end}} | ||
| − | Returns a reference to the character at specified location {{ | + | Returns a reference to the character at specified location {{c|pos}}. Bounds checking is performed, exception of type {{lc|std::out_of_range}} will be thrown on invalid access. |
===Parameters=== | ===Parameters=== | ||
{{par begin}} | {{par begin}} | ||
| − | {{par | pos | position of the character to return}} | + | {{par|pos|position of the character to return}} |
{{par end}} | {{par end}} | ||
| Line 22: | Line 22: | ||
===Exceptions=== | ===Exceptions=== | ||
Throws {{lc|std::out_of_range}} if {{c|1=pos >= size()}}. | Throws {{lc|std::out_of_range}} if {{c|1=pos >= size()}}. | ||
| + | |||
| + | {{cpp/strong exception safety guarantee|plural=yes}} | ||
===Complexity=== | ===Complexity=== | ||
| Line 27: | Line 29: | ||
===Example=== | ===Example=== | ||
| − | {{ | + | {{example |
| − | example | + | |code= |
| − | | code= | + | |
| − | + | ||
#include <iostream> | #include <iostream> | ||
| + | #include <stdexcept> | ||
#include <string> | #include <string> | ||
| Line 37: | Line 38: | ||
{ | { | ||
std::string s("message"); // for capacity | std::string s("message"); // for capacity | ||
| − | + | ||
s = "abc"; | s = "abc"; | ||
| − | s.at(2) = 'x'; // | + | s.at(2) = 'x'; // OK |
std::cout << s << '\n'; | std::cout << s << '\n'; | ||
| − | + | ||
std::cout << "string size = " << s.size() << '\n'; | std::cout << "string size = " << s.size() << '\n'; | ||
std::cout << "string capacity = " << s.capacity() << '\n'; | std::cout << "string capacity = " << s.capacity() << '\n'; | ||
| − | + | ||
| − | try { | + | try |
| − | // throw | + | { |
| + | // This will throw since the requested offset is greater than the current size. | ||
s.at(3) = 'x'; | s.at(3) = 'x'; | ||
} | } | ||
| − | catch (std::out_of_range const& exc) { | + | catch (std::out_of_range const& exc) |
| + | { | ||
std::cout << exc.what() << '\n'; | std::cout << exc.what() << '\n'; | ||
} | } | ||
} | } | ||
| − | | output= | + | |p=true |
| + | |output= | ||
abx | abx | ||
string size = 3 | string size = 3 | ||
| Line 60: | Line 64: | ||
}} | }} | ||
| − | === | + | ===Defect reports=== |
| + | {{dr list begin}} | ||
| + | {{dr list item|wg=lwg|dr=847|std=C++98|before=there was no exception safety guarantee|after=added strong exception safety guarantee}} | ||
| + | {{dr list item|wg=lwg|dr=2207|std=C++98|before=the behavior was undefined if {{c|1=pos >= size()}}|after=always throws an exception in this case}} | ||
| + | {{dr list end}} | ||
| + | ===See also=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/string/basic_string/dsc operator_at }} | + | {{dsc inc|cpp/string/basic_string/dsc operator_at}} |
| + | {{dsc inc|cpp/string/basic_string_view/dsc {{SUBPAGENAMEE}}}} | ||
{{dsc end}} | {{dsc end}} | ||
| − | + | {{langlinks|de|es|fr|it|ja|pl|pt|ru|zh}} | |
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | ||
Latest revision as of 21:45, 14 November 2024
CharT& at( size_type pos ); |
(1) | (constexpr since C++20) |
const CharT& at( size_type pos ) const; |
(2) | (constexpr since C++20) |
Returns a reference to the character at specified location pos. Bounds checking is performed, exception of type std::out_of_range will be thrown on invalid access.
Contents |
[edit] Parameters
| pos | - | position of the character to return |
[edit] Return value
Reference to the requested character.
[edit] Exceptions
Throws std::out_of_range if pos >= size().
If an exception is thrown for any reason, these functions have no effect (strong exception safety guarantee).
[edit] Complexity
Constant.
[edit] Example
Run this code
#include <iostream> #include <stdexcept> #include <string> int main() { std::string s("message"); // for capacity s = "abc"; s.at(2) = 'x'; // OK std::cout << s << '\n'; std::cout << "string size = " << s.size() << '\n'; std::cout << "string capacity = " << s.capacity() << '\n'; try { // This will throw since the requested offset is greater than the current size. s.at(3) = 'x'; } catch (std::out_of_range const& exc) { std::cout << exc.what() << '\n'; } }
Possible output:
abx string size = 3 string capacity = 7 basic_string::at
[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 2207 | C++98 | the behavior was undefined if pos >= size() | always throws an exception in this case |
[edit] See also
| accesses the specified character (public member function) | |
| accesses the specified character with bounds checking (public member function of std::basic_string_view<CharT,Traits>)
|
