Question

My program has a class with a vector of Level objects named levels. In a member function there is this line:

levels.push_back(level::Level());

I made several changes to my program today, and that line of code started segfaulting:

0x0804d248 in void std::vector<yarl::level::Level, std::allocator<yarl::level::Level> >::emplace_back<yarl::level::Level>(yarl::level::Level&&) (this=0x0, 
__args#0=...) at /usr/include/c++/4.4/bits/vector.tcc:93
93      if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)

I thought that the Level object may have somehow become corrupt, so I declared it outside the function call so I could inspect it in gdb, like this:

level::Level foo();
levels.push_back(foo);

Turns out this doesn’t compile. It g++ gives two errors I haven’t seen before:

error: invalid conversion from 'level::Level (*)()' to 'int'
error: initializing argument 1 of 'level::Level::Level(int, int, int)'

Now, Level‘s constructor takes three integer parameters, each with default values. I thought it might have been complaining that I hadn’t passed those three parameters, even though they have defaults, so I changed the first line to pass the value of the defaults:

level::Level foo(1, 100, 100);

This now compiles, and still segfaults, but does so at a different spot (although an identical test):

0x0804c699 in std::vector<yarl::level::Level, std::allocator<yarl::level::Level> >::push_back (this=0x0, __x=...) at /usr/include/c++/4.4/bits/stl_vector.h:735
735     if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)

I realize this is too little code to expect you guys to be able to solve my problem, but perhaps someone could tell me a little more about what these errors mean? Those g++ errors especially; I don’t know why it wouldn’t accept an empty Level constructor given that all its parameters are default, and I have no idea what the (*)() part of the error means (it makes the error a very frustrating one to google).

Answer 1

level::Level foo();

That declares a function named foo that takes no arguments and returns a level::Level. This is known as the most vexing parse. This creates a level::Level using the default constructor:

level::Level foo;

As for your original problem, you appear to have a null this pointer:

... ::push_back (this=0x0, __x=...) ...

This could happen if levels is a reference to null. Did you create the reference by dereferencing a null pointer?