i have a function that returns a double value (eg. 1.76896558753375)

then i append the value to a QVector<double> with some_vector.append(value)

after append, the value is 1.76897 in the qvector.

why is this happening and how can it be prevented?

stepping through qvector's append function:
Qt Code: 
  1. template <typename T>
  2. void QVector<T>::append(const T &t)
  3. {
  4.     const T copy(t);
  5.     const bool isTooSmall = uint(d->size + 1) > d->alloc;
  6.     if (!isDetached() || isTooSmall) {
  7.         QArrayData::AllocationOptions opt(isTooSmall ? QArrayData::Grow : QArrayData::Default);
  8.         reallocData(d->size, isTooSmall ? d->size + 1 : d->alloc, opt);
  9.     }
  10.     if (QTypeInfo<T>::isComplex)
  11.         new (d->end()) T(copy);
  12.     else
  13.         *d->end() = copy;
  14.     ++d->size;
  15. }
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both t and copy are the correct un-rounded value until ++d->size, after that line, the rounded value is appended instead of the full value.