std::basic_string<CharT,Traits,Allocator>::compare

 

C++

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Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

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Regular expressions library (C++11)

Atomic operations library (C++11)

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Filesystem library (C++17)

Technical Specifications

 

Strings library

Null-terminated strings

Byte strings

Multibyte strings

Wide strings

Classes

basic_string

basic_string_view (C++17)

char_traits

 

std::basic_string

Member functions basic_string::basic_string basic_string::operator= basic_string::assign basic_string::get_allocator Element access basic_string::at basic_string::operator[] basic_string::front (C++11) basic_string::back (C++11) basic_string::data basic_string::c_str basic_string::operator basic_string_view (C++17) Iterators basic_string::beginbasic_string::cbegin (C++11) basic_string::endbasic_string::cend (C++11) basic_string::rbeginbasic_string::crbegin (C++11) basic_string::rendbasic_string::crend (C++11) Capacity basic_string::empty basic_string::sizebasic_string::length basic_string::max_size basic_string::reserve basic_string::capacity basic_string::shrink_to_fit (C++11)

Operations basic_string::clear basic_string::insert basic_string::erase basic_string::push_back basic_string::pop_back (C++11) basic_string::append basic_string::operator+= basic_string::compare basic_string::starts_with (C++20) basic_string::ends_with (C++20) basic_string::replace basic_string::substr basic_string::copy basic_string::resize basic_string::swap Search basic_string::find basic_string::rfind basic_string::find_first_of basic_string::find_first_not_of basic_string::find_last_of basic_string::find_last_not_of Constants basic_string::npos

Non-member functions operator+ swap(std::basic_string) operator""s (C++14) erase(std::basic_string)erase_if(std::basic_string) (C++20)(C++20) I/O operator<<operator>> getline Comparison operator==operator!=operator<operator>operator<=operator>= Numeric conversion (C++11) stoistolstoll stoulstoull stofstodstold to_string to_wstring Helper classes hash<std::string>hash<std::wstring>hash<std::u32string>hash<std::u16string>hash<std::u8string> (C++11)(C++11)(C++11)(C++11)(C++20) hash<std::pmr::string>hash<std::pmr::wstring>hash<std::pmr::u32string>hash<std::pmr::u16string>hash<std::pmr::u8string> (C++20)(C++20)(C++20)(C++20)(C++20) Deduction guides(C++17)

 

	(1)
int compare( const basic_string& str ) const;			(until C++11)
int compare( const basic_string& str ) const noexcept;			(since C++11)
int compare( size_type pos1, size_type count1,              const basic_string& str ) const;		(2)
		(3)
int compare( size_type pos1, size_type count1,              const basic_string& str,              size_type pos2, size_type count2 ) const;				(until C++14)
int compare( size_type pos1, size_type count1,              const basic_string& str,              size_type pos2, size_type count2 = npos ) const;				(since C++14)
int compare( const CharT* s ) const;			(4)
int compare( size_type pos1, size_type count1,              const CharT* s ) const;			(5)
int compare( size_type pos1, size_type count1,              const CharT* s, size_type count2 ) const;			(6)
template < class T > int compare( const T& t ) const noexcept(/* see below */);			(7)	(since C++17)
template < class T > int compare( size_type pos1, size_type count1,              const T& t ) const;			(8)	(since C++17)
template < class T > int compare( size_type pos1, size_type count1,              const T& t,              size_type pos2, size_type count2 = npos) const;			(9)	(since C++17)

Compares two character sequences.

1) Compares this string to str.

2) Compares a [pos1, pos1+count1) substring of this string to str. If count1 > size() - pos1 the substring is [pos1, size()).

3) Compares a [pos1, pos1+count1) substring of this string to a substring [pos2, pos2+count2) of str. If count1 > size() - pos1 the first substring is [pos1, size()). Likewise, count2 > str.size() - pos2 the second substring is [pos2, str.size()).

4) Compares this string to the null-terminated character sequence beginning at the character pointed to by s with length Traits::length(s).

5) Compares a [pos1, pos1+count1) substring of this string to the null-terminated character sequence beginning at the character pointed to by s with length Traits::length(s) If count1 > size() - pos1 the substring is [pos1, size()).

6) Compares a [pos1, pos1+count1) substring of this string to the characters in the range [s, s + count2). If count1 > size() - pos1 the substring is [pos1, size()). (Note: the characters in the range [s, s + count2) may include null characters.)

7) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then compares this string to sv. This overload only participates in overload resolution if std::is_convertible_v<const T&, std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const T&, const CharT*> is false.

8) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then compares a [pos1, pos1+count1) substring of this string to sv, as if by std::basic_string_view<CharT, Traits>(*this).substr(pos1, count1).compare(sv). This overload only participates in overload resolution if std::is_convertible_v<const T&, std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const T&, const CharT*> is false.

9) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then compares a [pos1, pos1+count1) substring of this string to a substring [pos2, pos2+count2) of sv as if by std::basic_string_view<CharT, Traits>(*this).substr(pos1, count1).compare(sv.substr(pos2, count2)). This overload only participates in overload resolution if std::is_convertible_v<const T&, std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const T&, const CharT*> is false.

A character sequence consisting of count1 characters starting at data1 is compared to a character sequence consisting of count2 characters starting at data2 as follows. First, calculate the number of characters to compare, as if by size_type rlen = std::min(count1, count2). Then compare the sequences by calling Traits::compare(data1, data2, rlen). For standard strings this function performs character-by-character lexicographical comparison. If the result is zero (the character sequences are equal so far), then their sizes are compared as follows:

Condition		Result	Return value
Traits::compare(data1, data2, rlen) < 0		data1 is less than data2	<0
Traits::compare(data1, data2, rlen) == 0	size1 < size2	data1 is less than data2	<0
	size1 == size2	data1 is equal to data2	​0​
	size1 > size2	data1 is greater than data2	>0
Traits::compare(data1, data2, rlen) > 0		data1 is greater than data2	>0

Parameters

str	-	other string to compare to
s	-	pointer to the character string to compare to
count1	-	number of characters of this string to compare
pos1	-	position of the first character in this string to compare
count2	-	number of characters of the given string to compare
pos2	-	position of the first character of the given string to compare
t	-	object (convertible to std::basic_string_view) to compare to

Return value

negative value if *this appears before the character sequence specified by the arguments, in lexicographical order

zero if both character sequences compare equivalent

positive value if *this appears after the character sequence specified by the arguments, in lexicographical order

Exceptions

The overloads taking parameters named pos1 or pos2 throws std::out_of_range if the argument is out of range.

7)

noexcept specification:  

noexcept(std::is_nothrow_convertible_v<const T&, std::basic_string_view<CharT, Traits>>)

8-9) Throws anything thrown by the conversion to basic_string_view.

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 2946	C++17	string_view overload causes ambiguity in some cases	avoided by making it a template

Possible implementation

template<class CharT, class Traits, class Alloc> int basic_string<CharT, Traits, Alloc>::compare(const std::basic_string& s) const noexcept { size_type lhs_sz = size(); size_type rhs_sz = s.size(); int result = traits_type::compare(data(), s.data(), std::min(lhs_sz, rhs_sz)); if (result != 0) return result; if (lhs_sz < rhs_sz) return -1; if (lhs_sz > rhs_sz) return 1; return 0; }

Notes

For the situations when three-way comparison is not required, std::basic_string provides the usual relational operators (<, <=, ==, >, etc).

By default (with the default std::char_traits), this function is not locale-sensitive. See std::collate::compare for locale-aware three-way string comparison.

Example

Run this code

#include <cassert>
#include <string>
#include <iostream>
 
int main() 
{
    // 1) Compare with other string
    {
        int compare_value{
            std::string{"Batman"}.compare(std::string{"Superman"})
        };
        std::cout << (
            compare_value < 0 ? "Batman comes before Superman\n" :
            compare_value > 0 ? "Superman comes before Batman\n" :
            "Superman and Batman are the same.\n"
        );
    }
 
    // 2) Compare substring with other string
    {
        int compare_value{
            std::string{"Batman"}.compare(3, 3, std::string{"Superman"})
        };
        std::cout << (
            compare_value < 0 ? "man comes before Superman\n" :
            compare_value > 0 ? "Superman comes before man\n" :
            "man and Superman are the same.\n"
        );
    }
 
    // 3) Compare substring with other substring
    {
        std::string a{"Batman"};
        std::string b{"Superman"};
 
        int compare_value{a.compare(3, 3, b, 5, 3)};
 
        std::cout << (
            compare_value < 0 ? "man comes before man\n" :
            compare_value > 0 ? "man comes before man\n" :
            "man and man are the same.\n"
        );
        // Compare substring with other substring
        // defaulting to end of other string
        assert(compare_value == a.compare(3, 3, b, 5));
    }
 
    // 4) Compare with char pointer
    {
        int compare_value{std::string{"Batman"}.compare("Superman")};
 
        std::cout << (
            compare_value < 0 ? "Batman comes before Superman\n" :
            compare_value > 0 ? "Superman comes before Batman\n" :
            "Superman and Batman are the same.\n"
        );
    }
 
    // 5) Compare substring with char pointer
    {
        int compare_value{std::string{"Batman"}.compare(3, 3, "Superman")};
 
        std::cout << (
            compare_value < 0 ? "man comes before Superman\n" :
            compare_value > 0 ? "Superman comes before man\n" :
            "man and Superman are the same.\n"
        );
    }
 
    // 6) Compare substring with char pointer substring
    {
        int compare_value{std::string{"Batman"}.compare(0, 3, "Superman", 5)};
 
        std::cout << (
            compare_value < 0 ? "Bat comes before Super\n" :
            compare_value > 0 ? "Super comes before Bat\n" :
            "Super and Bat are the same.\n"
        );
    }
}

Output:

Batman comes before Superman
Superman comes before man
man and man are the same.
Batman comes before Superman
Superman comes before man
Bat comes before Super

See also

operator==operator!=operator<operator>operator<=operator>=	lexicographically compares two strings (function template)
substr	returns a substring (public member function)
collate	defines lexicographical comparison and hashing of strings (class template)
strcoll	compares two strings in accordance to the current locale (function)
lexicographical_compare	returns true if one range is lexicographically less than another (function template)