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std::basic_string::compare

From cppreference.com
< cpp‎ | string‎ | basic string
 
 
 
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

Contents

[edit] 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

[edit] 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

[edit] 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.

[edit] 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

[edit] 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;
}

[edit] 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.

[edit] 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

[edit] See also

lexicographically compares two strings
(function template) [edit]
returns a substring
(public member function) [edit]
defines lexicographical comparison and hashing of strings
(class template) [edit]
compares two strings in accordance to the current locale
(function) [edit]
returns true if one range is lexicographically less than another
(function template) [edit]