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std::basic_string<CharT,Traits,Allocator>::compare - cppreference.com

int compare( const basic_string& str ) const;

int compare( size_type pos1, size_type count1,
             const basic_string& str ) const;

int compare( const CharT* s ) const;

int compare( size_type pos1, size_type count1,
             const CharT* s ) const;

int compare( size_type pos1, size_type count1,
             const CharT* s, size_type count2 ) const;

template< class StringViewLike >
int compare( const StringViewLike& t ) const noexcept(/* see below */);

Compares two character sequences.

1) Compares this string to str.

Compares a

substring of this string to

.

• If count1 > size() - pos1, the substring is [pos1, size()).

Compares a

substring of this string to a substring

of

.

• 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).

Compares a

substring of this string to the null-terminated character sequence beginning at the character pointed to by

with length

.

• If count1 > size() - pos1, the substring is [pos1, size()).

Compares a

substring of this string to the characters in the range

. The characters in

may include null characters.

• If count1 > size() - pos1, the substring is [pos1, size()).

7) compares this string to sv;

compares a

substring of this string to

, as if by

;

compares a

substring of this string to a substring

of

, as if by

.

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:

• 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.

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

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

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;
}

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.

#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", "Super", 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 "Super".

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