Why does using a temporary object in the range-based for initializer result in a crash?

by

The range initialization line of a for(:) loop does not extend lifetime of anything but the final temporary (if any). Any other temporaries are discarded prior to the for(:) loop executing.

Now, do not despair; there is an easy fix to this problem. But first a walk through of what is going wrong.

The code for(auto x:exp){ /* code */ } expands to, basically:

{
  auto&& __range=exp;
  auto __it=std::begin(__range);
  auto __end=std::end(__range);
  for(; __it!=__end;++__it){
    auto x=*__it;
    /* code */
  }
}

(With a modest lies on the __it and __end lines, and all variables starting with __ have no visible name. Also I am showing C++17 version, because I believe in a better world, and the differences do not matter here.)

Your exp creates a temporary object, then returns a reference to within it. The temporary dies after that line, so you have a dangling reference in the rest of the code.

Fixing it is relatively easy. To fix it:

std::string const& func() const& // notice &
{
    return m.find("key")->second;
}
std::string func() && // notice &&
{
    return std::move(m.find("key")->second);
}

do rvalue overloads and return moved-into values by value when consuming temporaries instead of returning references into them.

Then the

auto&& __range=exp;

line does reference lifetime extension on the by-value returned string, and no more dangling references.

As a general rule, never return a range by reference to a parameter that could be an rvalue.

Appendix: Wait, && and const& after methods? rvalue references to *this?

C++11 added rvalue references. But the this or self parameter to functions is special. To select which overload of a method based on the rvalue/lvalue-ness of the object being invoked, you can use & or && after the end of the method.

This works much like the type of a parameter to a function. && after the method states that the method should be called only on non-const rvalues; const& means it should be called for constant lvalues. Things that don’t exactly match follow the usual precidence rules.

When you have a method that returns a reference into an object, make sure you catch temporaries with a && overload and either don’t return a reference in those cases (return a value), or =delete the method.

More Related Contents:

  1. “unpacking” a tuple to call a matching function pointer
  2. Why does reading a record struct fields from std::istream fail, and how can I fix it?
  3. Does C++11 allow vector?
  4. shared_ptr to an array : should it be used?
  5. When does a constexpr function get evaluated at compile time?
  6. Compiling C++11 with g++
  7. How much is too much with C++11 auto keyword?
  8. Why do I get the same sequence for every run with std::random_device with mingw gcc4.8.1?
  9. How do I pass a unique_ptr argument to a constructor or a function?
  10. What does std::match_results::size return?
  11. Where is the lock for a std::atomic?
  12. What are the rules for the “…” token in the context of variadic templates?
  13. Difference between C++03 throw() specifier C++11 noexcept
  14. Why do some people use swap for move assignments?
  15. Default, value and zero initialization mess
  16. How can I output the value of an enum class in C++11
  17. What exactly is a ‘side-effect’ in C++?
  18. How Does std::enable_if work?
  19. Vector of std::function with different signatures
  20. decltype as a return type in class member function
  21. Why are std::atomic objects not copyable?
  22. Is it smart to replace boost::thread and boost::mutex with c++11 equivalents?
  23. The implementation of std::forward
  24. Move or Named Return Value Optimization (NRVO)?
  25. Why does destructor disable generation of implicit move methods?
  26. C++11: Replace all non-owning raw pointers with std::shared_ptr()?
  27. Can we apply content not explicitly cited from the normative references to the C++ standard?
  28. “What happened to my SFINAE” redux: conditional template class members?
  29. Concatenate compile-time strings in a template at compile time?
  30. When will C++0x be finished? [closed]

Leave a Comment