std::cout << sizeof(float) << "n";  // this gives 4 on my machine "debian 64 bit" with "gcc 6.3.0"
std::cout << std::numeric_limits<float>::digits10  << "n"; // gives 6


std::cout << sizeof(double) << "n";  // gives 8
std::cout << std::numeric_limits<double>::digits10 <<  "n"; // gives 15

std::cout << sizeof(long double) <<  "n";  // gives 16
std::cout << std::numeric_limits<long double>::digits10  << "n"; // gives 18