Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | string‎ | basic string
m (→‎Synopsis: added numbering for overloads, IMfO, this should always be done, even if explanation part does not use the numbers.)
(Added LWG issue #2207 DR.)
 
(5 intermediate revisions by 3 users not shown)
Line 2: Line 2:
 
{{cpp/string/basic_string/navbar}}
 
{{cpp/string/basic_string/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl rev multi|num=1|until1=c++20
+
{{dcla|anchor=no|num=1|constexpr=c++20|
−
|dcl1=
+
CharT& at( size_type pos );
−
reference at( size_type pos );
+
−
|dcl2=
+
−
constexpr reference at( size_type pos );
+
 
}}
 
}}
−
{{dcl rev multi|num=2|until1=c++20
+
{{dcla|anchor=no|num=2|constexpr=c++20|
−
|dcl1=
+
const CharT& at( size_type pos ) const;
−
const_reference at( size_type pos ) const;
+
−
|dcl2=
+
−
constexpr const_reference at( size_type pos ) const;
+
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
Returns a reference to the character at specified location {{tt|pos}}. Bounds checking is performed, exception of type {{lc|std::out_of_range}} will be thrown on invalid access.
+
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===
Line 28: 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 42: Line 38:
 
{
 
{
 
     std::string s("message"); // for capacity
 
     std::string s("message"); // for capacity
−
 
+
   
 
     s = "abc";
 
     s = "abc";
−
     s.at(2) = 'x'; // ok
+
     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
 
     {
 
     {
Line 68: Line 64:
 
}}
 
}}
  
−
===See also===
+
===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}}

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]
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/string/basic_string/at&oldid=177668"