Possible Duplicate:
declaring a const instance of a class
Why does C++ require a user-provided default constructor to default-construct a const object?
My program like this:
class c
{
};
int main()
{
const c a;
return 0;
}
when I compile it using g++, it prompt:
main.cpp:10:7: note: ‘const class c’ has no user-provided default constructor
Why, this is just an empty class and do not do anything, why I have to provide a user-provided constructor.
Because the language rules say so.
The constant must have its value set in the definition, as it cannot be assigned a value later. If you don't explicitly provide a value, the type must have a default constructor.
You don't have to. It's just a note, not even a warning. The rationale is that the class can't do anything useful, which is rarely intended. GCC is just checking to see if you overlooked something.
llvm-g++-4.2) does issue an error).
clang++ are you using? My version (3.0) correctly treats this as an error. Unfortunately, g++-4.6.2 doesn’t. Which is weird, since your GCC 4.6 does.
const object and that should not be legal. That said, C++ also allows other uninitialised PODs to exist and that too should actually be illegal since otherwise you’ll be forced to call operator = on an uninitialised value. EDIT and if it’s a language extension then GCC is still wrong since some compiler versions don’t issue any diagnostic, not even when compiled with -pedantic.