Why is Direct Initialization Syntax Forbidden for Class Data Members in C ?

Why the Direct Initialization Syntax is Forbidden for Class Data Members

C class data members cannot be initialized using the direct initialization syntax (expression-list) due to potential parsing ambiguities. This is evident in the example provided:

class test {
    private:
        int s(3); // Compiler error
};

The compiler raises errors:

• "expected identifier before numeric constant"
• "expected ',' or '...' before numeric constant"

According to C Standards, data members can only be initialized using the following syntax:

• Assignment initialization: = initializer-clause
• Brace initialization: { initializer-list }

Reasons for the Restriction

The direct initialization syntax was intentionally omitted for class data members to avoid ambiguity. Parsing the following declarations could be challenging:

struct S {
    int i(x); // data member with initializer?
    static int x;
};

struct T {
    int i(x); // member function declaration?
    typedef int x;
};

If the direct initialization syntax were allowed, it would be unclear whether int i(x) represents a data member with an initializer or a member function declaration. This ambiguity would also affect templates.

To resolve this parsing problem, the C Standards Committee decided to prohibit the direct initialization syntax for class data members and reserve it for function declarations and local variables.

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