undefined-behavior

问题 (related to my previous question) In QT, the QMap documentation says: The key type of a QMap must provide operator<() specifying a total order . However, in qmap.h , they seem to use something similar to std::less to compare pointers: /* QMap uses qMapLessThanKey() to compare keys. The default implementation uses operator<(). For pointer types, qMapLessThanKey() casts the pointers to integers before it compares them, because operator<() is undefined on pointers that come from different memory

问题 (related to my previous question) In QT, the QMap documentation says: The key type of a QMap must provide operator<() specifying a total order . However, in qmap.h , they seem to use something similar to std::less to compare pointers: /* QMap uses qMapLessThanKey() to compare keys. The default implementation uses operator<(). For pointer types, qMapLessThanKey() casts the pointers to integers before it compares them, because operator<() is undefined on pointers that come from different memory

问题 The C++ 20 draft [concept.default.init] does not precisely define default_initializable template<class T> concept default_initializable = constructible_from<T> && requires { T{}; } && is-default-initializable <T>; // exposition-only and describe what is-default-initializable should do with the following words: For a type T , is-default-initializable <T> is true if and only if the variable definition T t; is well-formed for some invented variable t; otherwise it is false. Access checking is

问题 Is this well defined behaviour or is it undefined / somehow else defined which foo (data type or identifier) sizeof will be operating on ? typedef int foo; int main(int argc, char *argv[]) { char foo; printf ("%u\r\n", sizeof(foo)); return 0; } If it is well defined, is there a way I could obtain the size of the datatype foo without declaring a variable of that type only to use sizeof on it? 回答1: C does not have explicit scope resolution, so identifiers (variable names, typedef names, struct

问题 I am aware that we cannot read data into an uninitialized char pointer using fgets. There are quite a few questions relating to this very point here on stackoverflow. All the answers point to the fact that you can't load data into an uninitialized pointer variable. The program shown in the first code snippet is able to populate the first uninitialized char pointer (*str2) using fgets but, crashes while trying to read data into the second uninitialized char pointer (*str3). I can get it to

问题 I am aware that we cannot read data into an uninitialized char pointer using fgets. There are quite a few questions relating to this very point here on stackoverflow. All the answers point to the fact that you can't load data into an uninitialized pointer variable. The program shown in the first code snippet is able to populate the first uninitialized char pointer (*str2) using fgets but, crashes while trying to read data into the second uninitialized char pointer (*str3). I can get it to

问题 I recently ran across the following situation: #include <iostream> int *p = 0; int f() { p = new int(10); return 0; } void g(int x, int *y = p) { std::cout << y << std::endl; } int main() { g(f()); } This is quite subtle, since you usually don't expect the default arguments to change during their evaluation for the function call. I had to take a look at the assembly to spot this error. Now my question is: Is this really undefined behavior, since there aren't any guarantees concerning the

问题 So... I was testing something and noticed that when I run this code: $arr = str_split("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890", 1); print_r(implode(array_rand(array_flip($arr), 16))); The Output is Refresh 1: BDFIJKPTVkl12789 Refresh 2: HIJKMQWYdfmorsw3 Refresh 3: FGHMNRVYfhknouw5 Refresh 4: AFIJKRSVeiuwx579 Refresh 5: DJORYZcgijlpqry1 Refresh 6: EISWbhjmoqr45689 Refresh 7: CDEFOTXdhkloqr27 Refresh 8: AEFIKLNORSknx349 Refresh 9: DEFHJMTVZcgpstz0 Refresh 10:

问题 This question already has answers here : Why does this implementation of offsetof() work? (8 answers) Closed 1 year ago . offsetof is defined like this in stddef.h : #define offsetof(type, member) ((size_t)&((type *)0)->member) Does this invoke undefined behavior due to the dereference of a NULL pointer? If not, why? 回答1: In normal C code, the behavior of ((size_t)&((type *)0)->member) is not specified by the C standard: First, per C 2018 6.5.2.3 4, about -> , ((type *)0)->member designates

问题 This question already has answers here : Why does this implementation of offsetof() work? (8 answers) Closed 1 year ago . offsetof is defined like this in stddef.h : #define offsetof(type, member) ((size_t)&((type *)0)->member) Does this invoke undefined behavior due to the dereference of a NULL pointer? If not, why? 回答1: In normal C code, the behavior of ((size_t)&((type *)0)->member) is not specified by the C standard: First, per C 2018 6.5.2.3 4, about -> , ((type *)0)->member designates