Why Can\'t We Use Parentheses to Initialize In-Class Data Members in C ?

In-Class Data Member Initialization Anomaly: Delving into the C Standard

Unlike local data members that can be directly initialized with the () syntax, in-class data members defy this convenient method. This peculiarity has perplexed many programmers, prompting questions about its underlying rationale.

According to the C standard, direct initialization of class data members using () is prohibited to prevent parsing ambiguity. Consider the following scenario:

class S {
public:
    int i(x); // data member with initializer
};

Without the restriction, the compiler could become perplexed when trying to determine whether the declaration refers to a data member with an initializer or a member function declaration.

For instance:

struct S {
    int i(j); // member function declaration
    int j; // data member without an initializer
};

Applying the existing parsing rule that prioritizes member functions over data members in ambiguous situations could lead to incorrect interpretations. To avoid such confusion, the C standard opted to disallow direct initialization of class data members.

Nevertheless, alternative initialization methods remain available, such as using the = initializer-clause syntax:

int s = 3;

Or the initializer-list wrapped in curly braces:

int s{3};

By adhering to these methods, programmers can effectively initialize class data members, albeit with a different syntax than their local counterparts.

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