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()).


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()).


std::out_of_range if pos > size().


Linear in count.


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().


#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";


pos exceeds string size

See also

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

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