language-lawyer

问题 I am reading the C++11 draft standard and the section on [expr.typeid] mentions the following (emphasis mine): [...] When typeid is applied to an expression other than a glvalue of a polymorphic class type, the result refers to a std::type_info object representing the static type of the expression. Lvalue-to-rvalue (4.1), array-topointer (4.2), and function-to-pointer (4.3) conversions are not applied to the expression. If the type of the expression is a class type, the class shall be

问题 This question already has answers here : Alias of a template. Who's right? (2 answers) Equality of template aliases (1 answer) Closed 2 months ago . The context This question asked if it is possible to write a trait that can retrieve the template from a given instantiation. In a nutshell the question was: "Is it possible to write a f such that f<foo<int>>::type is foo ?" This answer presents something that I would have expected to be exactly what that question asked for, but the answer

问题 This is a code example from the C++20 spec ([basic.life]/8): struct C { int i; void f(); const C& operator=( const C& ); }; const C& C::operator=( const C& other) { if ( this != &other ) { this->~C(); // lifetime of *this ends new (this) C(other); // new object of type C created f(); // well-defined } return *this; } int main() { C c1; C c2; c1 = c2; // well-defined c1.f(); // well-defined; c1 refers to a new object of type C } Would the following be legal or undefined behavior : struct C {

问题 can anybody explaine the memory layout of std::vector<std::array<int, 5>> vec(2) does it provide contiguous memory block of a 2D array with 2 rows of 5 elements? To my understanding, the vector of vectors std::vector<std::vector<int>> vec(2, std::vector<int>(5)) provide the memory layout of two contiguous arrays of length 5 element s in different locations in memory. Will it be the same for the vector of arrays? 回答1: Arrays do not have any indirection, but just store their data "directly".

问题 The following code always prints the argument passed in to parameter a , regardless of the presence of a variable with the same name. Presumably because parameter identifiers are bound separately to variables in scope. Where are they positioned? Are they in the lexical environment? function foo(a, b = () => a) { var a = 1 console.log(b()) } foo() // undefined foo(2) // 2 Is it that var declarations end up in the special VariableEnvironment, while parameters are positioned in the

问题 Is there any guarantee that static class members are initialized before main is called? 回答1: I think no : [C++03: 3.6.2/3]: It is implementation-defined whether or not the dynamic initialization (8.5, 9.4, 12.1, 12.6.1) of an object of namespace scope is done before the first statement of main . If the initialization is deferred to some point in time after the first statement of main , it shall occur before the first use of any function or object defined in the same translation unit as the

问题 Is there any guarantee that static class members are initialized before main is called? 回答1: I think no : [C++03: 3.6.2/3]: It is implementation-defined whether or not the dynamic initialization (8.5, 9.4, 12.1, 12.6.1) of an object of namespace scope is done before the first statement of main . If the initialization is deferred to some point in time after the first statement of main , it shall occur before the first use of any function or object defined in the same translation unit as the

问题 Is there any guarantee that static class members are initialized before main is called? 回答1: I think no : [C++03: 3.6.2/3]: It is implementation-defined whether or not the dynamic initialization (8.5, 9.4, 12.1, 12.6.1) of an object of namespace scope is done before the first statement of main . If the initialization is deferred to some point in time after the first statement of main , it shall occur before the first use of any function or object defined in the same translation unit as the

问题 This question already has answers here : Deleted default constructor. Objects can still be created… sometimes (3 answers) When is a private constructor not a private constructor? (3 answers) Closed 2 years ago . Here's the code example in question: struct A { A() = delete; }; int main() { // A a(); // compiles, since it's a function declaration (most vexing parse) // A a; // does not compile, just as expected A a{}; // compiles, why? The default constructor is deleted. } Try it here with any

问题 Is it well defined in c++ to dereference a one-past-the-end pointer to an array type? Consider the following code : #include <cassert> #include <iterator> int main() { // An array of ints int my_array[] = { 1, 2, 3 }; // Pointer to the array using array_ptr_t = int(*)[3]; array_ptr_t my_array_ptr = &my_array; // Pointer one-past-the-end of the array array_ptr_t my_past_end = my_array_ptr + 1; // Is this valid? auto is_this_valid = *my_past_end; // Seems to yield one-past-the-end of my_array