C++ programmer's guide to undefined behavior: part 1 of 11

by Dmitry Sviridkin

Many modern programming languages, especially newer ones, forbid implicit type conversions. So, in Rust, Haskell, or Kotlin, we can't just use float and int in the same arithmetic expression without explicitly stating in the code to convert one to the other. Python isn't as strict but still keeps strings, characters, and numbers from mixing. C++ doesn't forbid implicit conversion, which leads to a lot of erroneous code. Moreover, such code can contain both defined (but unexpected) and undefined behavior.

Why do arrays have to be deleted via delete[] in C++

by Mikhail Gelvih

This pointer in no case should be passed to the usual operator delete. Most likely, it will just remove the first element of the array and leave the others intact. Note that I wrote "most likely" for a reason, because no one can predict every possible outcome and the way the program will behave. It all depends on what objects were in the array and whether their destructors did something important. As a result, we get the traditional undefined behavior. This is not what you would expect when trying to delete an array.

The Code Analyzer is wrong. Long live the Analyzer!

by Andrey Karpov

As you can see, once upon a time std::make_pair was taking arguments by value. If std::unique_ptr had existed at that time, then the code above would have been indeed incorrect. Whether this code would work or not would be a matter of luck. In practice, of course, this situation would never have occurred, since std::unique_ptr appeared in C++11 as a replacement for std::auto_ptr. Let's go back to our time. Starting with C++11, the constructor started to use move semantics.

PVS-Studio 6.26 Released

by Andrey Karpov

A developer believes that this is a bug in the compiler. Nevertheless, it is the author of the code who is wrong. The function does not work correctly due to the fact that undefined behavior occurs in it. The compiler follows that in the r variable a certain sum is calculated. Overflow of the r variable must not happen. Otherwise, it is undefined behavior, which doesn't have to be considered or taken into account by a compiler. So, the compiler thinks that since the value of r variable after ending the loop cannot be negative, then r & 0x7fffffff operation is not needed to reset the sigh bit and the compiler just returns the value of r variable from the function.

Closing Keynote Announced: John Regehr on Undefined Behavior and Compiler Optimizations

John will share some of the insights he’s gained from his research into compilers.

He will discuss what undefined behavior means to the compiler and how compiler writers use it in surprising ways generate better code.

Why it is important to check what the malloc function returned

by Andrey Karpov

There are 4 reasons at once, each of them is enough to prove that it is so necessary to write a check after you call the malloc function. If someone from your team doesn't write the checks, make him read this article.

1. Null pointer dereference is undefined behavior
2. Null pointer dereference is a vulnerability
3. Where are guarantees that dereferencing of exactly a null pointer will occur?
4. Where are the guarantees that memset fills the memory in a direct order?

Null Pointer Dereferencing Causes Undefined Behavior

by Andrey Karpov

This code is incorrect in both C and C++ when the podhd pointer equals 0. If the pointer equals 0, undefined behavior occurs.

C++ Papers for Issaquah - Library, Graphics, Networking, Numerics & Undefined Behavior

by Jens Weller

The 4th and last part about the C++ Papers for Issaquah. I already covered the first batch of proposals from the Library subgroup in the previous part, now its all about papers from Library, Graphics, Networking, Numerics and Undefined Behavior. A very diverse part.