類別資料成員可以直接在C中初始化嗎？

Can Class Data Members Be Directly Initialized?

In C++, class data members cannot be initialized using the direct initialization syntax, (), as seen in the following example:

#include <iostream>

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

int main() {
    test t;
    t.fun();
    return 0;
}

The compilation fails with errors:

11    9 [Error] expected identifier before numeric constant
11    9 [Error] expected ',' or '...' before numeric constant

Why is this the case?

The C++ standard explicitly prohibits this syntax for class data member initialization. Early proposals for the feature's introduction cited parsing problems as the reason.

Consider this ambiguous example:

struct S {
    int i(x); // data member with initializer or...
    // ...
    static int x;
    int i(y); // member function declaration
    // ...
    typedef int y;
};

The standard proposes a solution:

To eliminate ambiguity, the C++ standard allows only the following syntax for class data member initialization:

• = initializer-clause
• { initializer-list }

This resolution ensures clarity and avoids the potential for misunderstanding in cases where a declaration could resemble both an object and a function declaration.

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