Why do floats and unsigned integers have different static const declaration rules? [duplicate]

Question

I have a class which is essentially just holds a bunch of constant definitions used through my application. For some reason though, longs compile but floats do not:

class MY_CONSTS
{
public :
    static const long   LONG_CONST = 1;      // Compiles 
    static const float FLOAT_CONST = 0.001f; // C2864
};

Gives the following error:

1>c:\projects\myproject\Constant_definitions.h(71) : error C2864: 'MY_CONSTS::FLOAT_CONST' : only static const integral data members can be initialized within a class

Am I missing something?

Answer 1

To answer the actual question you asked: "because the standard says so".

Only variables of static, constant, integral types (including enumerations) may be initialized inside of a class declaration. If a compiler supports in-line initialization of floats, it is an extension. As others pointed out, the way to deal with static, constant, non-integral variables is to define and initialize them in the class's corresponding source file (not the header).

C++ Standard Section 9.2 "Class Members" item 4:

A member-declarator can contain a constant-initializer only if it declares a static member (9.4) of const integral or const enumeration type, see 9.4.2.

Section 9.4.2 "Static Data Members" item 2:

If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression (5.19). In that case, the member can appear in integral constant expressions. The member shall still be defined in a namespace scope if it is used in the program and the namespace scope definition shall not contain an initializer.

Answer 2

You should initialize them in the body of one of your cpp files:

class MY_CONSTS
{
public :
    static const long   LONG_CONST = 1;      // Compiles 
    static const float FLOAT_CONST;
};

const float MY_CONSTS::FLOAT_CONST = 0.001f;