std::string compare() method
- since C++20
- since C++17
- since C++14
- since C++11
- until C++11
// (1) Const version only
constexpr int compare( const basic_string& str ) const noexcept;
// (2) Const version only
constexpr int compare( size_type pos1, size_type count1,
const basic_string& str ) const;
// (3) Const version only
constexpr int compare( size_type pos1, size_type count1,
const basic_string& str,
size_type pos2, size_type count2 = npos ) const;
// (4) Const version only
constexpr int compare( const CharT* s ) const;
// (5) Const version only
constexpr int compare( size_type pos1, size_type count1,
const CharT* s ) const;
// (6) Const version only
constexpr int compare( size_type pos1, size_type count1,
const CharT* s, size_type count2 ) const;
// (7) Const version only
template < class StringViewLike >
constexpr int compare( const StringViewLike& t ) const noexcept(/* see below */);
// (8) Const version only
template < class StringViewLike >
constexpr int compare( const StringViewLike& t ) const noexcept(/* see below */);
// (9) Const version only
template < class StringViewLike >
constexpr int compare( size_type pos1, size_type count1,
const StringViewLike& t,
size_type pos2, size_type count2 = npos) const;
// (1) Const version only
int compare( const basic_string& str ) const noexcept;
// (2) Const version only
int compare( size_type pos1, size_type count1,
const basic_string& str ) const;
// (3) Const version only
int compare( size_type pos1, size_type count1,
const basic_string& str,
size_type pos2, size_type count2 = npos ) const;
// (4) Const version only
int compare( const CharT* s ) const;
// (5) Const version only
int compare( size_type pos1, size_type count1,
const CharT* s ) const;
// (6) Const version only
int compare( size_type pos1, size_type count1,
const CharT* s, size_type count2 ) const;
// (7) Const version only
template < class StringViewLike >
int compare( const StringViewLike& t ) const noexcept(/* see below */);
// (8) Const version only
template < class StringViewLike >
int compare( const StringViewLike& t ) const noexcept(/* see below */);
// (9) Const version only
template < class StringViewLike >
int compare( size_type pos1, size_type count1,
const StringViewLike& t,
size_type pos2, size_type count2 = npos) const;
// (1) Const version only
int compare( const basic_string& str ) const noexcept;
// (2) Const version only
int compare( size_type pos1, size_type count1,
const basic_string& str ) const;
// (3) Const version only
int compare( size_type pos1, size_type count1,
const basic_string& str,
size_type pos2, size_type count2 = npos ) const;
// (4) Const version only
int compare( const CharT* s ) const;
// (5) Const version only
int compare( size_type pos1, size_type count1,
const CharT* s ) const;
// (6) Const version only
int compare( size_type pos1, size_type count1,
const CharT* s, size_type count2 ) const;
// (1) Const version only
int compare( const basic_string& str ) const noexcept;
// (2) Const version only
int compare( size_type pos1, size_type count1,
const basic_string& str ) const;
// (3) Const version only
int compare( size_type pos1, size_type count1,
const basic_string& str,
size_type pos2, size_type count2 ) const;
// (4) Const version only
int compare( const CharT* s ) const;
// (5) Const version only
int compare( size_type pos1, size_type count1,
const CharT* s ) const;
// (6) Const version only
int compare( size_type pos1, size_type count1,
const CharT* s, size_type count2 ) const;
// (1) Const version only
int compare( const basic_string& str ) const;
// (2) Const version only
int compare( size_type pos1, size_type count1,
const basic_string& str ) const;
// (3) Const version only
int compare( size_type pos1, size_type count1,
const basic_string& str,
size_type pos2, size_type count2 ) const;
// (4) Const version only
int compare( const CharT* s ) const;
// (5) Const version only
int compare( size_type pos1, size_type count1,
const CharT* s ) const;
// (6) Const version only
int compare( size_type pos1, size_type count1,
const CharT* s, size_type count2 ) const;
Compares two character sequences.
-
(1) Compares this string to
str
. -
(2) Compares a [ pos1, pos1 + count1 ) substring of this string to
str
.
Ifcount1 > 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
.
Ifcount1 > 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 lengthTraits::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 lengthTraits::length(s)
.
Ifcount1 > 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 ).
Ifcount1 > size() - pos1
the substring is [ pos1, size() ).noteThe characters in the range [ s, s + count2) may include null characters.
-
(7) Implicitly converts
t
to a string viewsv
as if bystd::basic_string_view<CharT, Traits> sv = t;
, then compares this string tosv
.Overload ResolutionThis overload participates in overload resolution only if
std::is_convertible_v<const StringViewLike&, std::basic_string_view<CharT, Traits>>
istrue
andstd::is_convertible_v<const StringViewLike&, const CharT*>
isfalse
. -
(8) Implicitly converts
t
to a string viewsv
as if bystd::basic_string_view<CharT, Traits> sv = t;
, then compares a [ pos1, pos1 + count1 ) substring of this string tosv
, as if bystd::basic_string_view<CharT, Traits>(*this).substr(pos1, count1).compare(sv)
.Overload ResolutionThis overload participates in overload resolution only if
std::is_convertible_v<const StringViewLike&, std::basic_string_view<CharT, Traits>>
istrue
andstd::is_convertible_v<const StringViewLike&, const CharT*>
isfalse
. -
(9) Implicitly converts
t
to a string viewsv
as if bystd::basic_string_view<CharT, Traits> sv = t;
, then compares a [ pos1, pos1 + count1 ) substring of this string to a substring [ pos2, pos2 + count2 ) ofsv
as if bystd::basic_string_view<CharT, Traits>(*this).substr(pos1, count1).compare(sv.substr(pos2, count2))
.Overload ResolutionThis overload participates in overload resolution only if
std::is_convertible_v<const StringViewLike&, std::basic_string_view<CharT, Traits>>
istrue
andstd::is_convertible_v<const StringViewLike&, const CharT*>
isfalse
.
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:
- (1) Calculate the number of characters to compare, as if by
size_type rlen = std::min(count1, count2)
- (2) Then compare the sequences as if by
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:
This section requires improvement. You can help by editing this doc page.
Parameters
str
- other string to compare tos
- pointer to the character string to compare tocount1
- number of characters of this string to comparepos1
- position of the first character in this string to comparecount2
- number of characters of the given string to comparepos2
- position of the first character of the given string to comparet
- object (convertible tostd::basic_string_view
) to compare to
Returns 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.
Complexity
This section requires improvement. You can help by editing this doc page.
Exceptions
The overloads (2-3), (5-6), and (8-9) throw std::out_of_range
if the argument is out of range.
-
(7)
- since C++17
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 the operation would result in size() > max_size()
, throws std::length_error
.
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()
{
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) " <<
(
compare_result < 0 ? "Batman comes before Superman.\n" :
compare_result > 0 ? "Superman comes before Batman.\n" :
"Superman and Batman are the same.\n"
);
// 2) Compare substring with other string
compare_result = batman.compare(3, 3, superman);
std::cout << "2) " <<
(
compare_result < 0 ? "man comes before Superman.\n" :
compare_result > 0 ? "Superman comes before man.\n" :
"man and Superman are the same.\n"
);
// 3) Compare substring with other substring
compare_result = batman.compare(3, 3, superman, 5, 3);
std::cout << "3) " <<
(
compare_result < 0 ? "man comes before man.\n" :
compare_result > 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_result == batman.compare(3, 3, superman, 5));
// 4) Compare with char pointer
compare_result = batman.compare("Superman");
std::cout << "4) " <<
(
compare_result < 0 ? "Batman comes before Superman.\n" :
compare_result > 0 ? "Superman comes before Batman.\n" :
"Superman and Batman are the same.\n"
);
// 5) Compare substring with char pointer
compare_result = batman.compare(3, 3, "Superman");
std::cout << "5) " <<
(
compare_result < 0 ? "man comes before Superman.\n" :
compare_result > 0 ? "Superman comes before man.\n" :
"man and Superman are the same.\n"
);
// 6) Compare substring with char pointer substring
compare_result = batman.compare(0, 3, "Superman", 5);
std::cout << "6) " <<
(
compare_result < 0 ? "Bat comes before Super.\n" :
compare_result > 0 ? "Super comes before Bat.\n" :
"Super and Bat are the same.\n"
);
}
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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