NAME¶

std::basic_string::compare - std::basic_string::compare

Synopsis¶

(noexcept since
int compare( const basic_string& str ) const; (1) C++11)
(constexpr since
C++20)
int compare( size_type pos1, size_type count1, (2) (constexpr since
const basic_string& str ) const; C++20)
int compare( size_type pos1, size_type count1,

const basic_string& str, (until C++14)

size_type pos2, size_type count2
) const;
int compare( size_type pos1, size_type count1,
(since C++14)
const basic_string& str, (constexpr since
C++20)
size_type pos2, size_type count2
= npos ) const;
int compare( const CharT* s ) const; (4) (constexpr since
C++20)
int compare( size_type pos1, size_type count1, (5) (constexpr since
const CharT* s ) const; C++20)
int compare( size_type pos1, size_type count1, (3) (constexpr since
const CharT* s, size_type count2 (6) C++20)
) const;
template< class StringViewLike > (since C++17)
int compare( const StringViewLike& t ) const (7) (constexpr since
noexcept(/* see below */); C++20)
template< class StringViewLike >
(since C++17)
int compare( size_type pos1, size_type count1, (8) (constexpr since
C++20)
const StringViewLike& t ) const;
template< class StringViewLike >

int compare( size_type pos1, size_type count1, (since C++17)
const StringViewLike& t, (9) (constexpr since
C++20)
size_type pos2, size_type count2
= npos) const;

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 <0
data2
size1 < size2 data1 is less than <0
data2
Traits::compare(data1, data2, rlen) == size1 == data1 is equal to 0
0 size2 data2
size1 > size2 data1 is greater >0
than data2
Traits::compare(data1, data2, rlen) > 0 data1 is greater >0
than data2

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¶

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;
}

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¶

// Run this code

#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".

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

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

See also¶

operator==
operator!=
operator<
operator>
operator<=
operator>= lexicographically compares two strings
operator<=> (function template)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(C++20)
substr returns a substring
(public member function)
collate defines lexicographical comparison and hashing of strings
(class template)
strcoll compares two strings in accordance to the current locale
(function)
returns true if one range is lexicographically less than
lexicographical_compare another
(function template)
compares two views
compare (public member function of
std::basic_string_view<CharT,Traits>)

Category:¶

* conditionally noexcept