NAME¶

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

Synopsis¶

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

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

size_type pos2,
size_type
count2 ) const;
int compare(
size_type pos1,
size_type
count1,
(since
const C++14)
basic_string& (until
str, C++20)

size_type pos2,
size_type
count2 = npos )
const;
constexpr int
compare(
size_type pos1,
size_type
count1,

const (since
basic_string& C++20)
str,

size_type pos2,
size_type
count2 = npos )
const;
int compare( (until
const CharT* s C++20)
) const;
constexpr int
compare( const (since
CharT* s ) C++20)
const;
int compare(
size_type pos1,
size_type (until
count1, C++20)
const CharT* s
) const;
constexpr int
compare(
size_type pos1, (since
size_type C++20)
count1,
const CharT* s
) const;
int compare(
size_type pos1,
size_type (until
count1, C++20)
const CharT* s, (1)
size_type
count2 ) const;
constexpr int
compare(
size_type pos1,
size_type (since
count1, (2) C++20)
const CharT* s,
size_type
count2 ) const;
template <
class
StringViewLike
> (3) (since
int compare( C++17)
const (until
StringViewLike& C++20)
t ) const
noexcept(/* see
below */);
template <
class
StringViewLike
>
constexpr int (since
compare( const C++20)
StringViewLike&
t ) const
noexcept(/* see
below */);
template <
class
StringViewLike
> (4)
(since
int compare( C++17)
size_type pos1, (5) (until
size_type C++20)
count1,

const
StringViewLike&
t ) const;
template < (6)
class
StringViewLike
>

constexpr int
compare( (since
size_type pos1, (7) C++20)
size_type
count1,

const
StringViewLike&
t ) const;
template <
class
StringViewLike
> (8)

int compare(
size_type pos1, (since
size_type C++17)
count1, (until
const C++20)
StringViewLike&
t,

size_type pos2,
size_type
count2 = npos)
const;
template < (9)
class
StringViewLike
>

constexpr int
compare(
size_type pos1,
size_type (since
count1, C++20)
const
StringViewLike&
t,

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()). 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 participates 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.
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 participates 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.
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
participates 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 basic_string_view.

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¶

// Run this code

#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

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 avoided by making it a
some cases template
P1148R0 C++17 noexcept for overload (7) was accidently restored
dropped by 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)
compare compares two views
(C++17) (public member function of
std::basic_string_view<CharT,Traits>)