First, note that due to compiler idiosyncrasies you can't reliably assert that floating point arithmetic conforms to IEEE 754, e.g. both Visual C++ and g++ have flags that yield marginally faster operations at the cost of e.g. NaN values not comparing correctly wrt. IEEE 754.
But I'm reasonably sure that if std::numeric_limits::is_iec559
is true, then the bit level representation of ordinary values conforms to IEEE 754.
I.e.
static_assert( std::numeric_limits<double>::is_iec559, "IEEE 754 floating point" );
“IEC 559” is effectively just another name for “IEEE 754”; it’s the same standard.