The result of int c=0; cout<<c++<<c;

Question

I think it should be 01 but someone says its \"undefined\", any reason for that?

Answer 1

The reason it is undefined is that the compiler is free to calculate function parameters in any order. Consider if you where calling a function (because you are, but it's easier to envision when it's in function syntax):

  cout.output(c++).output(c);

The compiler may hit the parameters in reverse order, forward order, or whatever. It may call the first output before calculating the parameter to the second output or it may do both and then call.