


Why is Direct Initialization Syntax Forbidden for Class Data Members in C ?
Nov 17, 2024 am 02:08 AMWhy 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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