Difference between revisions of "cpp/string/basic string/front"
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< cpp | string | basic string
m (→See also: + crosslink) |
m (→Synopsis: +constexpr since) |
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| − | {{cpp/string/basic_string/title | front}} | + | {{cpp/string/basic_string/title|front}} |
{{cpp/string/basic_string/navbar}} | {{cpp/string/basic_string/navbar}} | ||
{{dcl begin}} | {{dcl begin}} | ||
| − | {{dcl | + | {{dcl|num=1|notes={{mark constexpr since c++20}}| |
| − | | | + | |
CharT& front(); | CharT& front(); | ||
| − | |||
| − | |||
}} | }} | ||
| − | {{dcl | + | {{dcl|num=2|notes={{mark constexpr since c++20}}| |
| − | | | + | |
const CharT& front() const; | const CharT& front() const; | ||
| − | |||
| − | |||
}} | }} | ||
{{dcl end}} | {{dcl end}} | ||
| − | Returns reference to the first character in the string. The behavior is undefined if {{ | + | Returns reference to the first character in the string. The behavior is undefined if {{lc|empty()}} is {{c/core|true}}. |
===Parameters=== | ===Parameters=== | ||
| Line 22: | Line 16: | ||
===Return value=== | ===Return value=== | ||
| − | + | Reference to the first character, equivalent to {{c|operator[](0)}}. | |
===Complexity=== | ===Complexity=== | ||
| − | Constant | + | Constant. |
| + | |||
| + | ===Notes=== | ||
| + | In libstdc++, {{tt|front()}} is [https://gcc.gnu.org/onlinedocs/libstdc++/manual/strings.html#strings.string.shrink not available] in C++98 mode. | ||
===Example=== | ===Example=== | ||
{{example | {{example | ||
| − | | code= | + | |code= |
#include <iostream> | #include <iostream> | ||
#include <string> | #include <string> | ||
| − | + | ||
int main() | int main() | ||
{ | { | ||
| − | |||
std::string s("Exemplary"); | std::string s("Exemplary"); | ||
| − | char& | + | char& f1 = s.front(); |
| − | + | f1 = 'e'; | |
std::cout << s << '\n'; // "exemplary" | std::cout << s << '\n'; // "exemplary" | ||
| − | + | ||
| − | + | ||
| − | + | ||
std::string const c("Exemplary"); | std::string const c("Exemplary"); | ||
| − | char const& | + | char const& f2 = c.front(); |
| − | std::cout << & | + | std::cout << &f2 << '\n'; // "Exemplary" |
| − | + | ||
} | } | ||
| − | | output= | + | |output= |
exemplary | exemplary | ||
Exemplary | Exemplary | ||
}} | }} | ||
| − | === | + | ===Defect reports=== |
| + | {{dr list begin}} | ||
| + | {{dr list item|wg=lwg|dr=534|std=C++98|before={{lc|std::basic_string}} did not have the member function {{tt|front()}}|after=added}} | ||
| + | {{dr list end}} | ||
| + | ===See also=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/string/basic_string/dsc back}} | + | {{dsc inc|cpp/string/basic_string/dsc back}} |
| − | {{dsc inc | cpp/string/basic_string_view/dsc {{SUBPAGENAMEE}}}} | + | {{dsc inc|cpp/string/basic_string_view/dsc {{SUBPAGENAMEE}}}} |
{{dsc end}} | {{dsc end}} | ||
{{langlinks|de|es|fr|it|ja|pt|ru|zh}} | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} | ||
Latest revision as of 05:43, 22 April 2024
| CharT& front(); |
(1) | (constexpr since C++20) |
| const CharT& front() const; |
(2) | (constexpr since C++20) |
Returns reference to the first character in the string. The behavior is undefined if empty() is true.
Contents |
[edit] Parameters
(none)
[edit] Return value
Reference to the first character, equivalent to operator[](0).
[edit] Complexity
Constant.
[edit] Notes
In libstdc++, front() is not available in C++98 mode.
[edit] Example
Run this code
#include <iostream> #include <string> int main() { std::string s("Exemplary"); char& f1 = s.front(); f1 = 'e'; std::cout << s << '\n'; // "exemplary" std::string const c("Exemplary"); char const& f2 = c.front(); std::cout << &f2 << '\n'; // "Exemplary" }
Output:
exemplary Exemplary
[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 534 | C++98 | std::basic_string did not have the member function front()
|
added |
[edit] See also
| (DR*) |
accesses the last character (public member function) |
| accesses the first character (public member function of std::basic_string_view<CharT,Traits>)
|
