Question

I am sorry for my newbie question, but I do not know much about C++. Can anybody answer why I get the error “error: a call to a constructor cannot appear in a constant-expression” when compiling following code;

class EliminationWeight
{
 public:
    typedef double Type;
    static const Type MAX_VALUE = __DBL_MAX__;
    static const Type MIN_VALUE = -__DBL_MAX__;
};

I use Ubuntu 12.04 and gcc that comes with it. It is not my code and I know that this code probably it works OK 100% (perhaps in older version of gcc or other compiler). Is there a quick way to fix it?

Thanks in advance for any answers, this is actually my first time asking something at SO.

Answer 1

Call to a constructor cannot appear in a constant-expression is a GCC error message which doesn’t really make sense to me here. Clang, for instance, accepts your code with some warnings:

test.cpp:31:23: warning: in-class initializer for static data member of type
      'const Type' (aka 'const double') is a GNU extension [-Wgnu]
    static const Type MAX_VALUE = __DBL_MAX__;
                      ^           ~~~~~~~~~~~

Anyway, initializing double in a class body is non-standard. You should do initialization separately:

class EliminationWeight
{
 public:
    typedef double Type;
    static const Type MAX_VALUE;
    static const Type MIN_VALUE;
};

and then in exactly one source file (not a header file):

const EliminationWeight::Type EliminationWeight::MAX_VALUE = __DBL_MAX__;
const EliminationWeight::Type EliminationWeight::MIN_VALUE = -__DBL_MAX__;

In general, you can only initialize static member variables having integral types in class body, although this has been extended in C++ 0x11. See also Initializing const member within class declaration in C++