W3cubDocs

/C++

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

(1) 
int compare( const basic_string& str ) const;
 (until C++11) 
int compare( const basic_string& str ) const noexcept;
 (since C++11)
(until C++20) 
constexpr int compare( const basic_string& str ) const noexcept;
 (since C++20)
(2) 
int compare( size_type pos1, size_type count1,
             const basic_string& str ) const;
 (until C++20) 
constexpr int compare( size_type pos1, size_type count1,
                       const basic_string& str ) const;
 (since C++20)
(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)
(until C++20) 
constexpr int compare( size_type pos1, size_type count1,
                       const basic_string& str,
                       size_type pos2, size_type count2 = npos ) const;
 (since C++20)
(4) 
int compare( const CharT* s ) const;
 (until C++20) 
constexpr int compare( const CharT* s ) const;
 (since C++20)
(5) 
int compare( size_type pos1, size_type count1,
             const CharT* s ) const;
 (until C++20) 
constexpr int compare( size_type pos1, size_type count1,
                       const CharT* s ) const;
 (since C++20)
(6) 
int compare( size_type pos1, size_type count1,
             const CharT* s, size_type count2 ) const;
 (until C++20) 
constexpr int compare( size_type pos1, size_type count1,
                       const CharT* s, size_type count2 ) const;
 (since C++20)
(7) 
template< class StringViewLike >
int compare( const StringViewLike& t ) const noexcept(/* see below */);
 (since C++17)
(until C++20) 
template< class StringViewLike >
constexpr int
    compare( const StringViewLike& t ) const noexcept(/* see below */);
 (since C++20)
(8) 
template< class StringViewLike >
int compare( size_type pos1, size_type count1,
             const StringViewLike& t ) const;
 (since C++17)
(until C++20) 
template< class StringViewLike >
constexpr int compare( size_type pos1, size_type count1,
                       const StringViewLike& t ) const;
 (since C++20)
(9) 
template< class StringViewLike >
int compare( size_type pos1, size_type count1,
             const StringViewLike& t,
             size_type pos2, size_type count2 = npos) const;
 (since C++17)
(until C++20) 
template< class StringViewLike >
constexpr int compare( size_type pos1, size_type count1,
                       const StringViewLike& t,
                       size_type pos2, size_type count2 = npos) const;
 (since C++20)

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()).
  • If 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). The characters in [s, s + count2) may include null characters.
  • If count1 > size() - pos1, the substring is [pos1, size()).
7-9) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then
7) compares this string to sv;
8) 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);
9) 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)).
These overloads participate in overload resolution only if std::is_convertible_v<const StringViewLike&,
std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const StringViewLike&, 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 std::basic_string_view.

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

Possible implementation

template<class CharT, class Traits, class Alloc>
int std::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

#include <cassert>
#include <iomanip>
#include <iostream>
#include <string>
#include <string_view>
 
void print_compare_result(std::string_view str1, 
                          std::string_view str2,
                          int compare_result)
{
    if (compare_result < 0)
        std::cout << std::quoted(str1) << " comes before "
                  << std::quoted(str2) << ".\n";
    else if (compare_result > 0)
        std::cout << std::quoted(str2) << " comes before "
                  << std::quoted(str1) << ".\n";
    else
        std::cout << std::quoted(str1) << " and "
                  << std::quoted(str2) << " are the same.\n";
}
 
int main()
{
    std::string batman{"Batman"};
    std::string superman{"Superman"};
    int compare_result{0};
 
    // 1) Compare with other string
    compare_result = batman.compare(superman);
    std::cout << "1) ";
    print_compare_result("Batman", "Superman", compare_result);
 
    // 2) Compare substring with other string
    compare_result = batman.compare(3, 3, superman);
    std::cout << "2) ";
    print_compare_result("man", "Superman", compare_result);
 
    // 3) Compare substring with other substring
    compare_result = batman.compare(3, 3, superman, 5, 3);
    std::cout << "3) ";
    print_compare_result("man", "man", compare_result);
 
    // Compare substring with other substring
    // defaulting to end of other string
    assert(compare_result == batman.compare(3, 3, superman, 5));
 
    // 4) Compare with char pointer
    compare_result = batman.compare("Superman");
    std::cout << "4) ";
    print_compare_result("Batman", "Superman", compare_result);
 
    // 5) Compare substring with char pointer
    compare_result = batman.compare(3, 3, "Superman");
    std::cout << "5) ";
    print_compare_result("man", "Superman", compare_result);
 
    // 6) Compare substring with char pointer substring
    compare_result = batman.compare(0, 3, "Superman", 5);
    std::cout << "6) ";
    print_compare_result("Bat", "Superman", compare_result);
}

Output:

1) "Batman" comes before "Superman".
2) "Superman" comes before "man".
3) "man" and "man" are the same.
4) "Batman" comes before "Superman".
5) "Superman" comes before "man".
6) "Bat" comes before "Superman".

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 5 C++98 the parameter count2 of overload (6)
had a default argument npos 		default argument removed,
split to overloads (5) and (6)
LWG 847 C++98 there was no exception safety guarantee added strong exception safety guarantee
LWG 2946 C++17 overload (7) caused ambiguity in some cases avoided by making it a template
P1148R0 C++17 noexcept for overload (7) was accidently
dropped by the resolution of LWG2946 		restored

See also

(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
 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)
(C++17)
 compares two views
(public member function of std::basic_string_view<CharT,Traits>)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/string/basic_string/compare