std::string starts_with() method

od C++20

// (1) Const version only
constexpr bool starts_with( std::basic_string_view<CharT,Traits> sv ) const noexcept;

// (2) Const version only
constexpr bool starts_with( CharT c ) const noexcept;

// (3) Const version only
constexpr bool starts_with( const CharT* s ) const;

Checks if the string begins with the given prefix.

The prefix may be one of the following:

• (1) A string view sv (which may be a result of implicit conversion from another std::basic_string).

• (2) A single character c.

• (3) A null-terminated character string s.

All three overloads effectively do

std::basic_string_view<CharT, Traits>(data(), size()).starts_with(x)

where x is the parameter.

Parameters

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

Return value

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

Complexity

• (1) Linear in the size of sv - O(sv.size()).
• (2) Constant - O(1).
• (3) Linear in the size of s - O(std::strlen(s)).

Notes

Feature testing macro: __cpp_lib_starts_ends_with.

Example

#include <iostream>
#include <string_view>
#include <string>
 
template <typename PrefixType>
void test_prefix_print(const std::string& str, PrefixType prefix)
{
    std::cout << '\'' << str << "' starts with '" << prefix << "': " <<
        str.starts_with(prefix) << '\n';
}
 
int main()
{
    std::boolalpha(std::cout);    
    auto helloWorld = std::string("hello world");
 
    test_prefix_print(helloWorld, std::string_view("hello"));
 
    test_prefix_print(helloWorld, std::string_view("goodbye"));
 
    test_prefix_print(helloWorld, 'h');
 
    test_prefix_print(helloWorld, 'x');
}

Output

'hello world' starts with 'hello': true
'hello world' starts with 'goodbye': false
'hello world' starts with 'h': true
'hello world' starts with 'x': false