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Difference between revisions of "cpp/string/basic string/at"

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< cpp‎ | string‎ | basic string
m (Text replace - "{{params}}" to "===Parameters===")
(Added LWG issue #2207 DR.)
 
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{{cpp/string/basic_string/title | at}}
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{{cpp/string/basic_string/title|at}}
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{{cpp/string/basic_string/sidebar}}
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{{cpp/string/basic_string/navbar}}
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{{ddcl list begin}}
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{{dcl begin}}
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{{ddcl list item |  
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{{dcla|anchor=no|num=1|constexpr=c++20|
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reference      at( size_type pos );
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CharT& at( size_type pos );
 
}}
 
}}
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{{ddcl list item |  
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{{dcla|anchor=no|num=2|constexpr=c++20|
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const_reference at( size_type pos ) const;
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const CharT& at( size_type pos ) const;
 
}}
 
}}
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{{ddcl list end}}
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{{dcl end}}
  
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Returns a reference to the character at specified location {{tt|pos}}. Bounds checking is performed, exception of type {{tt|todo|out_of_range}} will be thrown on invalid access.
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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===
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{{param list begin}}
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{{par begin}}
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{{param list item | pos | position of the character to return}}
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{{par|pos|position of the character to return}}
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{{param list end}}
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{{par end}}
  
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<!-- ======== -->
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===Return value===
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{{returns}}
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Reference to the requested character.
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reference to the requested character
+
  
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<!-- ======== -->
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===Exceptions===
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{{complex}}
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Throws {{lc|std::out_of_range}} if {{c|1=pos >= size()}}.
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{{complex constant}}
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<!-- ======== -->
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{{cpp/strong exception safety guarantee|plural=yes}}
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{{see also}}
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{{dcl list begin}}
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===Complexity===
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{{dcl list template | cpp/string/basic_string/dcl list operator_at }}
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Constant.
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{{dcl list end}}
+
 
 +
===Example===
 +
{{example
 +
|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';
 +
    }
 +
}
 +
|p=true
 +
|output=
 +
abx
 +
string size = 3
 +
string capacity = 7
 +
basic_string::at
 +
}}
 +
 
 +
===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 inc|cpp/string/basic_string/dsc operator_at}}
 +
{{dsc inc|cpp/string/basic_string_view/dsc {{SUBPAGENAMEE}}}}
 +
{{dsc end}}
 +
 
 +
{{langlinks|de|es|fr|it|ja|pl|pt|ru|zh}}

Latest revision as of 21:45, 14 November 2024

 
C++
 
Strings library
 
std::basic_string
Member functions
(C++23)
Element access
basic_string::at
(DR*)
(DR*)
(C++17)
Iterators
(C++11)
(C++11)
(C++11)
(C++11)
Capacity
(DR*)
Modifiers
(C++23)
(DR*)
(C++23)
(C++23)
(C++23)  
Search
Operations
(C++20)
(C++20)
(C++23)
Constants
Non-member functions
(C++20)(C++20)
I/O
Comparison
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Numeric conversions
(C++11)(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Literals
(C++14)
Helper classes
(C++11)
Deduction guides (C++17)

 
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) [edit]
accesses the specified character with bounds checking
(public member function of std::basic_string_view<CharT,Traits>) [edit]
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