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

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

Return value

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

Exceptions

std::out_of_range if pos > size ( ) .

If an exception is thrown for any reason, these functions have no effect ( strong exception safety guarantee ).

Complexity

Linear in count .

Notes

The allocator of the returned string is default-constructed: the new allocator might not be a copy of get_allocator() .

Example

#include <iostream>
#include <string>
 
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& ex)
    {
        std::cout << ex.what() << '\n';
    }
}

abcdefghij
567
hij
basic_string::substr: __pos (which is 23) > this->size() (which is 20)

Defect reports

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

See also