At least one reason why it’s UB is that string.h can introduce macros. For internal implementation reasons, those macros might have been written on the assumption that strcmp is the “real” strcmp function. If you define strcmp to be something else and then use those macros, strcmp will expand to my_strcmp in the macros, with … Read more
The section that allowed the implicit int rule in C89 would be section 3.5.2 Type specifiers which says (emphasis mine): int , signed , signed int , or no type specifiers Keith Thompson in the comments points out that in C90 the section is 6.5.2 and says, The only difference is some introductory material required … Read more
As pointed out in Stargateur’s answer, the reason for this is the lifetime of temporaries. While Rust does not have a full specification yet, the language reference still is quite good and at least gives a hint to understand the behaviour. Here is the relevant part from the section about temporary lifetimes: [T]he lifetime of … Read more
This is a gcc bug, filed 83258. In C++14, we used to have a linkage requirement for non-type template parameters of pointer type. But in C++17 (as a result of N4268), the parameter just needs to be a converted constant expression of the correct type, with a few other restrictions (none of which are relevant … Read more
Some preliminary paragraphs first: [basic] 3 An entity is a value, object, reference, function, enumerator, type, class member, template, template specialization, namespace, parameter pack, or this. [dcl.type.simple] 4 The type denoted by decltype(e) is defined as follows: if e is an unparenthesized id-expression or an unparenthesized class member access ([expr.ref]), decltype(e) is the type of … Read more
According to the C++03 ISO spec (§23.1.1, Table 67) (and as @AndyProwl has mentioned, in §23.2.3, table 11 of the C++11 ISO spec), as part of sequence requirements, the operation a.insert(p, i, j) in a sequence container has this precondition: i, j are not iterators into a. In other words, sequence containers are allowed to … Read more
Not only is there no requirement that the objects be arranged any particular way, but many tuple implementations actually put the second object before the first one.
The attempt to write to *dst is UB if the caller passes you a pointer to a const object, or a pointer to a string literal. But if the caller passes you a pointer to data that in fact is mutable, then behavior is defined. Creating a const char* that points to a modifiable char … Read more
It is still possible for an allocation to generate a pointer that satisfies the condition despite the pointer not pointing into the region. This will happen, for example, on an 80286 in protected mode, which is used by Windows 3.x in Standard mode and OS/2 1.x. In this system, pointers are 32-bit values, split into … Read more
Because the behavior of copy elision changes from C++17; for this case copy elision is mandatory. Mandatory elision of copy/move operations Under the following circumstances, the compilers are required to omit the copy and move construction of class objects, even if the copy/move constructor and the destructor have observable side-effects. The objects are constructed directly … Read more