Would like to exploit the following behaviour in Ruby ary = Array.new(5) { |i| [i, j=2*i, k=j+1] } p ary #> [[0, 0, 1], [1, 2, 3], [2, 4, 5], [3, 6, 7], [4, 8, 9]] It works for my purposes, b...

#include <cstdio> #include <string> class A { std::string data; public: A() = default; explicit A (const char* data) : data(data) {} operator const char* () const; explicit o...

Consider the following example that compiles with clang but is rejected by edg, gcc and msvc. Demo #include <initializer_list> struct C { C(){} C(std::initializer_list<int> i = {3})...

I learned that when a class has a user-provided constructor then the compiler will not implicitly generate a default constructor. I wrote the following code, C d{}; in particular, and I expect it t...

In addition to an implicitly-defined copy assignment operator, if a class also defines an operator= without an lvalue-reference object parameter, which of the operators must be selected? Please con...

C standard say (6.3.1.1 Boolean, characters, and integers): 1 Every integer type has an integer conversion rank defined as follows: No two signed integer types shall have the same rank, even if t...

I recently learnt that constructors do not have names. I am also aware that a function has a type called a function type. For example, void func(int) { } In the above snippet, func has the functio...

I wrote the following program that compiles with EDG but is rejected by gcc, clang and msvc. Demo struct C { C(const C&&) = default; // EDG: ok, gcc: No, Clang: No }; int main() { } Afa...

Is the following C program guaranteed to exit with 0 or is the compiler allowed to identify the objects s and t with one another as is permitted in C++ as the so-called named return value optimizat...

Using -std=c++23, gcc accepts the following code while clang rejects it: struct outer { struct { int x; inline int get_x() const; } inner; }; inline int decltype(outer::inner)::get_x() const {...

I'm seeing some unexpected behavior when using the std::variant::operator<. In the situation where the type has an implicit bool conversion operator and its less operator is not a member functio...

In C++26, reading uninitialized variables is no longer undefined, it's "erroneous" now (What is erroneous behavior? How is it different from undefined behavior?). However, the wording for...

Consider the following two structs whose sizes are 8 and 1 bytes respectively: class eight { int i; char c; eight(const blub&) {} }; class one { char s; one(const blob&) {} }; Whe...

Generally _Atomic does not imply semantics of volatile, i.e. operations on the atomic object are not observable side effects that the compiler needs to preserve. As a consequence the compiler can o...

Generally std::atomic<T> does not imply semantics of volatile, i.e. operations on the atomic object are not observable side effects that the compiler needs to preserve. As a consequence the c...

I'm having a disagreement with some co-workers over the following code: int foo ( int a, int b ) { return b > 0 ? a / b : a; } Does this code exhibit undefined behavior? EDIT: The disagreem...

I wrote the following program in c++23. Here I've overloaded member functions. But for all cases different compilers give different result as shown below in comment. As you can see I have three cas...

Having class X, the following object initialization: new (ptr) X(X()); requires an accessible destructor even since C++17. Why is that, when the object is initialized by the default constructor di...

Does it works correctly (does nothing) when I use vector<T> v; v.erase(v.end()); I want to use something like v.erase(std::find(...)); Should I if is it v.end() or not? There is no info ...

In this recent question, some code was shown to have undefined behavior: a[++i] = foo(a[i-1], a[i]); because even though the actual call of foo() is a sequence point, the assignment is unsequenc...

There was some code floating around on Reddit that defined a member-function with an explicit this object parameter defined as type int. This made me wonder how this member-function could possibly ...

I have this example: #include <iostream> #define print(X) std::cout << X << std::endl struct B1 { B1(int _i = 5): i(_i) { print("B1 constructor"); }; int i; }; struc...

I am learning C++ using the books listed here. I wrote the following example(for purely academic reasons) that compiles with GCC but not with Clang and MSVC. Demo. struct C { static bool f() noexc...

Now we all sometimes have to work with binary data. In C++ we work with sequences of bytes, and since the beginning char was the our building block. Defined to have sizeof of 1, it is the byte. And...

I wrote the following program in C++23 that compiles with gcc and clang but is rejected by msvc. I want to know is this well-formed or ill-formed etc as per the standard. Live demo struct C { void...