I've run into the following error:
class NormalClass
{
public:
constexpr NormalClass() : arr{}, debug_ptr((int*)arr)
{
//'reinterpret_cast' is not a constant expression
//cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
}
public:
char arr[5];
int* debug_ptr;
};
constinit NormalClass normal;
int main()
{
}
Originally this was in a template, with (T*) cast. Why is it that a cast like this isn't allowed in a constexpr?
See https://en.cppreference.com/w/cpp/language/constant_expression
Core constant expressions
A core constant expression is any expression whose evaluation would not evaluate any one of the following:
Why can't reinterpret_cast be used in a constant expression?
Because the C++ language spec says so. See [expr.const]/5.15:
An expression
Eis a core constant expression unless the evaluation ofE, ... would evaluate one of the following:5.15 a
reinterpret_cast([expr.reinterpret.cast]);
Also note that reinterpreting char[] as int and then accessing it is undefined behavior - a strict aliasing violation. As soon as the pointer debug_ptr is dereferenced in the actual program, there will be UB.