Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | string‎ | basic string
 
 
 
std::basic_string
Member functions
Element access
Iterators
Capacity
Modifiers
Search
Operations
basic_string::compare
Constants
Deduction guides (C++17)
Non-member functions
I/O
Comparison
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Numeric conversion
(C++11)(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Helper classes
 
(1)
int compare( const basic_string& str ) const;
(until C++11)
int compare( const basic_string& str ) const noexcept;
(since C++11)
(constexpr since C++20)
int compare( size_type pos1, size_type count1,
             const basic_string& str ) const;
(2) (constexpr 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)
(constexpr since C++20)
int compare( const CharT* s ) const;
(4) (constexpr since C++20)
int compare( size_type pos1, size_type count1,
             const CharT* s ) const;
(5) (constexpr since C++20)
int compare( size_type pos1, size_type count1,
             const CharT* s, size_type count2 ) const;
(6) (constexpr since C++20)
template< class StringViewLike >
int compare( const StringViewLike& t ) const noexcept(/* see below */);
(7) (since C++17)
(constexpr since C++20)
template< class StringViewLike >

int compare( size_type pos1, size_type count1,

             const StringViewLike& t ) const;
(8) (since C++17)
(constexpr since C++20)
template< class StringViewLike >

int compare( size_type pos1, size_type count1,
             const StringViewLike& t,

             size_type pos2, size_type count2 = npos) const;
(9) (since C++17)
(constexpr since C++20)

Compares two character sequences.

1) Compares this string to str.
2) Compares a [pos1pos1 + count1) substring of this string to str.
  • If count1 > size() - pos1, the substring is [pos1size()).
3) Compares a [pos1pos1 + count1) substring of this string to a substring [pos2pos2 + count2) of str.
  • If count1 > size() - pos1, the first substring is [pos1size()).
  • If count2 > str.size() - pos2, the second substring is [pos2str.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 [pos1pos1 + 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 [pos1size()).
6) Compares a [pos1pos1 + count1) substring of this string to the characters in the range [ss + count2). The characters in [ss + count2) may include null characters.
  • If count1 > size() - pos1, the substring is [pos1size()).
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 [pos1pos1 + 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 [pos1pos1 + count1) substring of this string to a substring [pos2pos2 + 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

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 std::basic_string_view.

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

[edit] Possible implementation

overload (1)
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;
}

[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 <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".

[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 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 accidentally
dropped by the resolution of LWG2946
restored

[edit] 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) [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]
compares two views
(public member function of std::basic_string_view<CharT,Traits>) [edit]