uff, so,

iostream>>(float&) is specified to be functionally identical to std::use_facet(num_get), which itself is defined to be identical to strtof from , which refers to the C11 standard, which …

Anyways, In C11 it is defined that the conversion is to be done "correctly rounded". And I'd tend to say that's what GCC does here. (And rounding a subnormal to 0 seems fine to me!)

But. Does it really matter? you will want to know when a number was too small to be represented sensibly. If I'm right with that assessment: use from_chars instead, make sure to check the error it sets for range errors. Here's a side-by-side comparison with your parse() function to highlight the different behaviours (Compiler Explorer):

#include <charconv> #include <iostream> #include <sstream> #include <string_view> void parse(const char* zstr) { std::istringstream s(zstr); float f; s >> f; std::cout << "parse: " << (bool)s; if (s) { std::cout << " (" << f << ")"; } std::cout << "\n"; } template <typename T> void parse_charconv(const std::string_view& sv) { T value; auto [first_fail, errorcode] = std::from_chars(sv.data(), sv.data() + sv.size(), value); constexpr std::errc noerror{}; std::cout << "charconv (" << typeid(T).name() << "): "; if (errorcode == noerror) { std::cout << "1 (" << value << ")"; } else if (errorcode == std::errc::invalid_argument) { std::cout << "i"; } else if (errorcode == std::errc::result_out_of_range) { std::cout << "R"; } else { std::cout << "?"; } std::cout << "\n"; } int main() { for (const char* str : {"1e-30", "1e-45", "1e-46", "banana pie"}) { std::cout << str << "\n"; parse(str); parse_charconv<float>(str); parse_charconv<double>(str); std::cout << "\n\n"; } }