W3cubDocs

/C++

std::basic_string::compare

int compare( const basic_string& str ) const;
 (1) 
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;
 (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. First, calculates the number of characters to compare, as if by size_type rlen = std::min(size(), str.size()). Then compares by calling Traits::compare(data(), str.data(), rlen). For standard strings this function performs character-by-character lexicographical comparison. If the result is zero (the strings are equal so far), then their sizes are compared as follows:
Condition Result Return value
Traits::compare(data, arg, rlen) < 0 data is less than arg <0
Traits::compare(data, arg, rlen) == 0 size(data) < size(arg) data is less than arg <0
size(data) == size(arg) data is equal to arg ​0​
size(data) > size(arg) data is greater than arg >0
Traits::compare(data, arg, rlen) > 0 data is greater than arg >0
2) Compares a [pos1, pos1+count1) substring of this string to str as if by basic_string(*this, pos1, count1).compare(str) (until C++17)compare(pos1, count1, std::basic_string_view<CharT, Traits>(str)) (since C++17)
3) Compares a [pos1, pos1+count1) substring of this string to a substring [pos2, pos2+count2) of str as if by basic_string(*this, pos1, count1).compare(basic_string(str, pos2, count2)) (until C++17)compare(pos1, count1, std::basic_string_view<CharT, Traits>(str), pos2, count2) (since C++17)
4) Compares this string to the null-terminated character sequence beginning at the character pointed to by s, as if by compare(basic_string(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, as if by basic_string(*this, pos, count1).compare(basic_string(s))
6) Compares a [pos1, pos1+count1) substring of this string to the first count2 characters of the character array whose first character is pointed to by s, as if by basic_string(*this, pos, count1).compare(basic_string(s, count2)). (Note: the characters from s to 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, similar to (1) except for using sv.size() and sv.data() instead of str.size() and str.data(). 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>(data(), size()).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>(data(), size()).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.

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

1)

(none).

 (until C++11)
noexcept specification:
noexcept
 (since C++11)
2-6) May throw the exceptions thrown by the corresponding basic_string constructors.
7) Throws nothing unless the initialization of sv throws an exception. (since C++17)

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

#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)

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