Is it true? Yes. Technically speaking, no part of: int *p = (int*)malloc(sizeof(int)); actually creates an object of type int, so dereferencing p is UB since there is no actual int there. Do we really have to call default constructor that is trivial to start the life time of the object? Do you have to … Read more
Yes this behavior is undefined but for different reasons than most people are aware of. First, using an unitialized value is by itself not undefined behavior, but the value is simply indeterminate. Accessing this then is UB if the value happens to be a trap representation for the type. Unsigned types rarely have trap representations, … Read more
Yes, it permits anything to happen. The note is just giving examples. The definition is pretty clear: Undefined behavior: behavior for which this International Standard imposes no requirements. Frequent point of confusion: You should understand that “no requirement” also means means the implementation is NOT required to leave the behavior undefined or do something bizarre/nondeterministic! … Read more
This is not undefined behavior. Although foo is uninitialized, you are using it a way that is allowed by the standard. After space is allocated for an object but before it is fully initialized, you are allowed to use it limited ways. Both binding a reference to that variable and taking its address are allowed. … Read more
This is a compiler bug. Although getting impossible results for undefined behaviour is a valid consequence, there is actually no undefined behaviour in your code. What’s happening is that the compiler thinks the behaviour is undefined, and optimises accordingly. If c is defined as int8_t, and int8_t promotes to int, then c– is supposed to … Read more
As the C FAQ says: It’s not clear if it’s legal or portable, but it is rather popular. and: … an official interpretation has deemed that it is not strictly conforming with the C Standard, although it does seem to work under all known implementations. (Compilers which check array bounds carefully might issue warnings.) The … Read more
Both (a) and (b) result in undefined behavior. It’s always undefined behavior to call a member function through a null pointer. If the function is static, it’s technically undefined as well, but there’s some dispute. The first thing to understand is why it’s undefined behavior to dereference a null pointer. In C++03, there’s actually a … Read more
When the standard says it’s undefined behavior, it means it. Anything can happen. “Anything” includes “usually integers wrap around, but on occasion weird stuff happens”. Yes, on x86 CPUs, integers usually wrap the way you expect. This is one of those exceptions. The compiler assumes you won’t cause undefined behavior, and optimizes away the loop … Read more
The confusion is that C explicitly permits type-punning through a union, whereas C++ (c++11) has no such permission. c11 6.5.2.3 Structure and union members 95) If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate … Read more
I guess the question that needs to be answered why well-intentioned people would write the checks in the first place. The most common case is probably if you have a class that is part of a naturally occurring recursive call. If you had: struct Node { Node* left; Node* right; }; in C, you might … Read more