From cppreference.com
referenceat(size_typepos); (1) (constexpr since C++20)const_referenceat(size_typepos)const; (2) (constexpr since C++20)Returns a reference to the element at specified location pos, with bounds checking.
If pos is not within the range of the container, an exception of type
is thrown.
Parameters
pos - position of the element to return Return value
Reference to the requested element
Exceptions
if pos>=size().
Complexity
Constant.
Example
Run this code
#include<chrono>#include<cstddef>#include<iostream>#include<vector>#include<stdexcept>intmain(){std::vector<int>data{1,2,4,5,5,6};// Set element 1data.at(1)=88;// Read element 2std::cout<<"Element at index 2 has value "<<data.at(2)<<'\n';std::cout<<"data size = "<<data.size()<<'\n';try{// Try to set an element at random position >= size()automoon_phase=[]{returnstd::chrono::system_clock::now().time_since_epoch().count()%8;};data.at(data.size()+moon_phase())=13;}catch(conststd::out_of_range&ex){std::cout<<ex.what()<<'\n';}// Print final valuesstd::cout<<"data:";for(intelem:data)std::cout<<' '<<elem;std::cout<<'\n';}Possible output:
Element at index 2 has value 4 data size = 6 vector::_M_range_check: __n (which is 8) >= this->size() (which is 6) data: 1 88 4 5 5 6 See also