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Difference between revisions of "cpp/string/basic string view/starts with"

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m (~)
m (@1@ s/x/sv, per talk & [https://eel.is/c++draft/string.view.template#string.view.ops-20]. 'x' was an original param name in <string_view>.)
 
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{{cpp/string/basic_string_view/title | starts_with}}
+
{{cpp/string/basic_string_view/title|starts_with}}
 
{{cpp/string/basic_string_view/navbar}}
 
{{cpp/string/basic_string_view/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl | num=1 |since=c++20|1=
+
{{dcl|num=1|since=c++20|1=
 
constexpr bool starts_with( basic_string_view sv ) const noexcept;
 
constexpr bool starts_with( basic_string_view sv ) const noexcept;
 
}}
 
}}
−
{{dcl | num=2 |since=c++20|1=
+
{{dcl|num=2|since=c++20|1=
−
constexpr bool starts_with( CharT c ) const noexcept;
+
constexpr bool starts_with( CharT ch ) const noexcept;
 
}}
 
}}
−
{{dcl | num=3 |since=c++20|1=
+
{{dcl|num=3|since=c++20|1=
 
constexpr bool starts_with( const CharT* s ) const;
 
constexpr bool starts_with( const CharT* s ) const;
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
Checks if the string view begins with the given prefix, where  
+
Checks if the string view begins with the given prefix, where
−
@1@ the prefix is a string view. Effectively returns {{c|1=substr(0, sv.size()) == sv}}
+
@1@ the prefix is a string view. Effectively returns {{c|1=basic_string_view(data(), std::min(size(), sv.size())) == sv}}.
−
@2@ the prefix is a single character. Effectively returns {{c|1=!empty() && Traits::eq(front(), c)}}
+
@2@ the prefix is a single character. Effectively returns {{c|1=!empty() && Traits::eq(front(), ch)}}.
−
@3@ the prefix is a null-terminated character string. Effectively returns {{c|1=starts_with(basic_string_view(s))}}
+
@3@ the prefix is a null-terminated character string. Effectively returns {{c|1=starts_with(basic_string_view(s))}}.
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | sv | a string view which may be a result of implicit conversion from {{tt|std::basic_string}}}}
+
{{par|sv|a string view which may be a result of implicit conversion from {{tt|std::basic_string}}}}
−
{{par | c | a single character}}
+
{{par|ch|a single character}}
−
{{par | s | a null-terminated character string}}
+
{{par|s|a null-terminated character string}}
−
{{par end}}  
+
{{par end}}
  
 
===Return value===
 
===Return value===
 
{{c|true}} if the string view begins with the provided prefix, {{c|false}} otherwise.
 
{{c|true}} if the string view begins with the provided prefix, {{c|false}} otherwise.
 +
 +
===Notes===
 +
{{feature test macro|__cpp_lib_starts_ends_with|String prefix and suffix checking: {{lc|starts_with()}} and {{lc|ends_with()}}|std=C++20|value=201711L}}
  
 
===Example===
 
===Example===
−
{{example |
+
{{example
−
| code=
+
|code=
−
#include <iostream>
+
#include <cassert>
 
#include <string_view>
 
#include <string_view>
  
−
auto main() -> int
+
int main()
 
{
 
{
 
     using namespace std::literals;
 
     using namespace std::literals;
  
−
     std::cout
+
     assert
−
        << std::boolalpha
+
    (""
−
 
+
         // (1) starts_with( basic_string_view )
−
         // bool starts_with(basic_string_view x) const noexcept;
+
         && "https://cppreference.com"sv.starts_with("http"sv) == true
−
         << "https://cppreference.com"sv.starts_with("http"sv) << ' ' // true
+
         && "https://cppreference.com"sv.starts_with("ftp"sv) == false
−
         << "https://cppreference.com"sv.starts_with("ftp"sv) << ' '  // false
+
−
 
+
         // (2) starts_with( CharT )
−
         // bool starts_with(CharT x) const noexcept;
+
         && "C++20"sv.starts_with('C') == true
−
         << "C++20"sv.starts_with('C') << ' ' // true
+
         && "C++20"sv.starts_with('J') == false
−
         << "C++20"sv.starts_with('J') << ' ' // false
+
−
 
+
         // (3) starts_with( const CharT* )
−
         // bool starts_with(const CharT* x) const;
+
         && std::string_view("string_view").starts_with("string") == true
−
         << std::string_view("string_view").starts_with("string") << ' ' // true
+
         && std::string_view("string_view").starts_with("String") == false
−
         << std::string_view("string_view").starts_with("String") << ' ' // false
+
    );
−
        << '\n';
+
 
}
 
}
−
| output=
 
−
true false true false true false
 
 
}}
 
}}
  
 
===See also===
 
===See also===
−
 
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/string/basic_string_view/dsc ends_with}}
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{{dsc inc|cpp/string/basic_string_view/dsc ends_with}}
−
{{dsc inc | cpp/string/basic_string/dsc starts_with}}
+
{{dsc inc|cpp/string/basic_string/dsc starts_with}}
−
{{dsc inc | cpp/string/basic_string/dsc ends_with}}
+
{{dsc inc|cpp/string/basic_string/dsc ends_with}}
−
{{dsc inc | cpp/string/basic_string_view/dsc compare}}
+
{{dsc inc|cpp/string/basic_string/dsc contains}}
 +
{{dsc inc|cpp/string/basic_string_view/dsc contains}}
 +
{{dsc inc|cpp/string/basic_string_view/dsc compare}}
 
{{dsc end}}
 
{{dsc end}}
  
 
{{langlinks|de|es|fr|it|ja|pl|pt|ru|zh}}
 
{{langlinks|de|es|fr|it|ja|pl|pt|ru|zh}}

Latest revision as of 12:45, 10 May 2024

 
C++
 
Strings library
 
std::basic_string_view
 
constexpr bool starts_with( basic_string_view sv ) const noexcept;
 (1) (since C++20)
constexpr bool starts_with( CharT ch ) const noexcept;
 (2) (since C++20)
constexpr bool starts_with( const CharT* s ) const;
 (3) (since C++20)

Checks if the string view begins with the given prefix, where

1) the prefix is a string view. Effectively returns basic_string_view(data(), std::min(size(), sv.size())) == sv.
2) the prefix is a single character. Effectively returns !empty() && Traits::eq(front(), ch).
3) the prefix is a null-terminated character string. Effectively returns starts_with(basic_string_view(s)).

Contents

[edit] Parameters

sv - a string view which may be a result of implicit conversion from std::basic_string
ch - a single character
s - a null-terminated character string

[edit] Return value

true if the string view begins with the provided prefix, false otherwise.

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_starts_ends_with 201711L (C++20) String prefix and suffix checking: starts_with() and ends_with()

[edit] Example

Run this code
#include <cassert>
#include <string_view>
 
int main()
{
    using namespace std::literals;
 
    assert
    (""
        // (1) starts_with( basic_string_view )
        && "https://cppreference.com"sv.starts_with("http"sv) == true
        && "https://cppreference.com"sv.starts_with("ftp"sv) == false
 
        // (2) starts_with( CharT )
        && "C++20"sv.starts_with('C') == true
        && "C++20"sv.starts_with('J') == false
 
        // (3) starts_with( const CharT* )
        && std::string_view("string_view").starts_with("string") == true
        && std::string_view("string_view").starts_with("String") == false
    );
}

[edit] See also

(C++20)
 checks if the string view ends with the given suffix
(public member function) [edit]
(C++20)
 checks if the string starts with the given prefix
(public member function of std::basic_string<CharT,Traits,Allocator>) [edit]
(C++20)
 checks if the string ends with the given suffix
(public member function of std::basic_string<CharT,Traits,Allocator>) [edit]
(C++23)
 checks if the string contains the given substring or character
(public member function of std::basic_string<CharT,Traits,Allocator>) [edit]
(C++23)
 checks if the string view contains the given substring or character
(public member function) [edit]
compares two views
(public member function) [edit]
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