How to construct a or exception without throwing?

Question

Exceptions defined in (e.g. std::logic_error, std::runtime_error and their subclasses such as std::system_erro

Answer 1

if (haveError)
    throw std::runtime_error("BOO!"); // May throw std::bad_alloc instead?!

by the time you reach this throw you should have already taken care of all clean up so in most cases, having a std::bad_alloc thrown instead of a std::runtime_error won't make much of a difference really.

The only exceptional case, I can thing of is when exceptions are used to control the flow of a program - I tend to do this very often with code such as:

try { 
  auto face = detectFace(); // may throw a custom no_face_exception or a many_faces_exception
  // do work with face
} 
catch (no_face_exception& e) {
  std::cout << "no face detected\n";
}
catch (many_faces_exception& e) {
  std::cout << "too many faces detected\n";
}

In this particular case failure to allocate memory would cause detectFace to throw a std::bad_alloc instead which would cause a disastrous crash. First allocating the exceptions before throwing, as you suggested, would not change anything at all - the program would still crash with a std::bad_alloc since the allocation would still fail. The only way around this problem would be to simply catch the std::bad_alloc:

catch (std::bad_alloc& e) {
  std::cout << "bad alloc reported\n";
}