NAME¶

std::basic_string::substr - std::basic_string::substr

Synopsis¶

basic_string substr( size_type pos = 0, size_type count = npos ) (until C++20)
const;
constexpr basic_string substr( size_type pos = 0, size_type count = (since C++20)
npos ) const;

Returns a substring [pos, pos+count). If the requested substring extends past the
end of the string, i.e. the count is greater than size() - pos (e.g. if count ==
npos), the returned substring is [pos, size()).

Parameters¶

pos - position of the first character to include
count - length of the substring

Return value¶

String containing the substring [pos, pos+count) or [pos, size()).

Exceptions¶

std::out_of_range if pos > size()

Complexity¶

Linear in count

Notes¶

The returned string is constructed as if by basic_string(data()+pos, count), which
implies that the returned string's allocator will be default-constructed — the new
allocator might not be a copy of this->get_allocator().

Example¶

// Run this code

#include <string>
#include <iostream>

int main()
{
std::string a = "0123456789abcdefghij";

// count is npos, returns [pos, size())
std::string sub1 = a.substr(10);
std::cout << sub1 << '\n';

// both pos and pos+count are within bounds, returns [pos, pos+count)
std::string sub2 = a.substr(5, 3);
std::cout << sub2 << '\n';

// pos is within bounds, pos+count is not, returns [pos, size())
std::string sub4 = a.substr(a.size()-3, 50);
// this is effectively equivalent to
// std::string sub4 = a.substr(17, 3);
// since a.size() == 20, pos == a.size()-3 == 17, and a.size()-pos == 3

std::cout << sub4 << '\n';

try {
// pos is out of bounds, throws
std::string sub5 = a.substr(a.size()+3, 50);
std::cout << sub5 << '\n';
} catch(const std::out_of_range& e) {
std::cout << "pos exceeds string size\n";
}
}

Output:¶

abcdefghij
567
hij
pos exceeds string size

See also¶

copy copies characters
(public member function)
size returns the number of characters
length (public member function)
find find characters in the string
(public member function)
npos special value. The exact meaning depends on the context
[static] (public static member constant)