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

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

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

#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

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