Why Can't I Directly Initialize Static Non-Const Members and Static Arrays in a C Class?

Difficulties Initializing Static Non-Const Members and Static Arrays within a Class

In C , attempts to initialize non-const static members or static arrays directly within a class definition often result in compiler errors. This article examines the reasons behind these limitations and presents workarounds to achieve desired functionality.

Why can't I initialize static data members in class?

According to the C standard, static data members can only be initialized within the class definition if they are of const integral or enumeration types. This restriction ensures the following:

• Unique Definitions: Class definitions are typically included in header files, which can be incorporated into multiple translation units. To prevent duplicate object definitions, C requires that every object have a unique definition. In-class initialization of objects that require in-memory storage would violate this rule.

Why are only static const integral types & enums allowed In-class Initialization?

• Object Storage: Static const integral members are treated as compile-time constants and can be optimized by the compiler. By allowing their in-class initialization, the compiler can eliminate the need to store them in memory, thus circumventing the aforementioned restriction.
• Enforceability: Enumerations are allowed this in-class initialization because their values can be directly used in place of integers.

How about static arrays in class?

In-class initialization of static arrays is forbidden for the same reasons that apply to static non-const members. Additionally, even initializing static const arrays in-class is not allowed because:

• Limited Constantness: Static const arrays are only constant with respect to their elements, not their size. This means the compiler cannot treat them as compile-time constants and optimize them efficiently.

Workarounds

To initialize static arrays or non-const members in a class, the following techniques can be employed:

• Enum Trick: Use an enum with defined constants for the array size and elements.
• Out-of-class Definition: Define the static variable outside the class definition in a namespace scope, allowing for both non-const static members and static array initialization.

C 11 Evolution

C 11 loosens the restriction by allowing the initialization of non-static data members within class declarations and the initialization of static const literal members with constant expressions. However, these features may not be supported in all compilers yet.

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