From cppreference.com
size_typemax_size()constnoexcept;(since C++11)Returns the maximum number of elements the container is able to hold due to system or library implementation limitations, i.e. std::distance(begin(),end()) for the largest container.
Return value
Maximum number of elements.
Complexity
Constant.
Notes
This value typically reflects the theoretical limit on the size of the container, at most std::numeric_limits<difference_type>::max(). At runtime, the size of the container may be limited to a value smaller than max_size() by the amount of RAM available.
Example
Run this code
#include<iostream>#include<locale>#include<unordered_map>intmain(){std::unordered_multimap<char,char>p;std::unordered_multimap<long,char>q;std::cout.imbue(std::locale("en_US.UTF-8"));std::cout<<std::uppercase<<"p.max_size() = "<<std::dec<<p.max_size()<<" = 0x"<<std::hex<<p.max_size()<<'\n'<<"q.max_size() = "<<std::dec<<q.max_size()<<" = 0x"<<std::hex<<q.max_size()<<'\n';}Possible output:
p.max_size() = 1,152,921,504,606,846,975 = 0xFFF,FFF,FFF,FFF,FFF q.max_size() = 768,614,336,404,564,650 = 0xAAA,AAA,AAA,AAA,AAA See also
returns the number of elements
(public member function)