Why Can\'t Class Data Members Be Initialized Using Brace-Enclosed Syntax?

Why Class Data Member Direct Initialization Syntax is Forbidden

In C , class data members cannot be initialized using the ( ) brace-enclosed syntax. Consider the following code:

class test {
public:
    void fun() {
        int a(3);
        std::cout << a << '\n';
    }

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

Compilation fails with errors like:

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

So, why this limitation?

According to early C proposals and standard discussions, the prohibition aims to prevent parsing ambiguities. If brace-enclosed initialization were allowed, the compiler could potentially misinterpret declarations in situations like:

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

Here, it's unclear if i(x) is a data member initializer or a member function declaration, as both follow the same syntax.

To address this ambiguity, C opted for the following restrictions:

• Data members must be initialized using either the = initializer-clause or { initializer-list } syntax.
• Member functions must always use the () {} syntax for declarations.

This decision avoids parsing issues and ensures clarity in code.

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