#include <cassert>#include <cstring>#include <iostream>#include <vector> int main(){// Works with containersstd::vector<int> v{3,1,4};assert(std::size(v)==3); // And works with built-in arrays tooint a[]{-5,10,15};// Returns the number of elements (not bytes) as opposed to sizeofassert(std::size(a)==3);std::cout<<"size of a[]: "<< sizeof a<<'\n';// 12, if sizeof(int) == 4 // Provides a safe way (compared to sizeof) of getting string buffer sizeconstchar str[]="12345";// These are fine and give the correct resultassert(std::size(str)==6);assert(sizeof(str)==6); // But use of sizeof here is a common source of bugsconstchar* str_decayed="12345";// std::cout << std::size(str_decayed) << '\n'; // Usefully fails to compilestd::cout<< sizeof(str_decayed)<<'\n';// Prints the size of the pointer! // Since C++20 the signed size (std::ssize) is availableauto i= std::ssize(v);for(--i; i!=-1;--i)std::cout<< v[i]<<(i?' ':'\n');assert(i==-1); // Note that the string literal includes the ending null character, which// will be part of the constructed characters array. This makes std::size// behave differently from std::strlen and std::string::size:constexprchar symbols[]="0123456789";     static_assert(std::size(symbols)==11);    static_assert(std::string(symbols).size()==10);assert(std::strlen(symbols)==10);}

size of a[]: 1284 1 3