From cppreference.com
basic_stringsubstr(size_typepos=0,size_typecount=npos)const; (1)(until C++23)
(constexpr since C++20)constexprbasic_stringsubstr(size_typepos=0,size_typecount=npos)const&;(since C++23)constexprbasic_stringsubstr(size_typepos=0,size_typecount=npos)&&; (2) (since C++23)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,
).
1) Equivalent to returnbasic_string(*this,pos,count);.
2) Equivalent to returnbasic_string(std::move(*this),pos,count);.
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,
).
Exceptions
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
.
Example
Run this code
#include<iostream>#include<string>intmain(){std::stringa="0123456789abcdefghij";// count is npos, returns [pos, size())std::stringsub1=a.substr(10);std::cout<<sub1<<'\n';// both pos and pos + count are within bounds, returns [pos, pos + count)std::stringsub2=a.substr(5,3);std::cout<<sub2<<'\n';// pos is within bounds, pos + count is not, returns [pos, size())std::stringsub4=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 == 3std::cout<<sub4<<'\n';try{// pos is out of bounds, throwsstd::stringsub5=a.substr(a.size()+3,50);std::cout<<sub5<<'\n';}catch(conststd::out_of_range&ex){std::cout<<ex.what()<<'\n';}}Possible output:
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.
DR Applied to Behavior as published Correct behavior
C++98 there was no exception safety guarantee added strong exception safety guarantee See also
constexprsize_typenpos[static]the special value size_type(-1), its exact meaning depends on the context
(C++26)
returns a sub-view
(public member function)
copies characters
(public member function)
returns the number of characters
(public member function)
finds the first occurrence of the given substring
(public member function)
returns a substring
(public member function of std::basic_string_view<CharT,Traits>)