In the same spirit than the creation of an instance in a std::map when calling operator[] , I would like to emplace (direclty in the expression) in an std::optional only if has_value() is false. In other words, I would like to one-line this code :

std::vector>>> data;
//...
if(condition)
{
  if(!data[42].has_value()) data[42].emplace();
  data[42].value()[3].push_back("foo");
}

Something like :

if(condition)
  data[42].value_or_emplace()[3].push_back("foo");

I know I can create my own tool like this :

template 
T& value_or_emplace(std::optional& o)
{
  return (o.has_value() ? o.value() : o.emplace());
}

if(condition)
  value_or_emplace(data[42])[3].push_back("foo");

But I expect something I missed in the standard library.