std::basic_string::compare

From cppreference.com
< cpp‎ | string‎ | basic string
 
C++
 
Strings library
 
std::basic_string
 
(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

 lexicographically compares two strings
(function template)
returns a substring
(public member function)
defines lexicographical comparison and hashing of strings
(class template)
compares two strings in accordance to the current locale
(function)
returns true if one range is lexicographically less than another
(function template)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/string/basic_string/compare&oldid=107742"