Why Must C In-Class Initializers Use `=` or `{}`?

Why In-Class Initializers Require Equals or Braces

C 11 introduces in-class initializers, which allow member variables to be initialized directly within the class definition. However, these initializers must adhere to specific syntax rules.

Question:

Why is it mandated that in-class initializers use either the equals sign (=) or curly braces ({})?

Answer:

This requirement serves to eliminate potential syntax ambiguities.

Consider the following example:

class BadTimes {
    struct Overloaded;
    int Overloaded; // Legal, but unusual.

    int confusing(Overloaded); // <-- Ambiguous line
};

The problematic line could be interpreted in two ways:

1. As a function declaration with a single Overloaded parameter that returns an integer.
2. As a declaration and initialization of an integer member named confusing, where the initial value is the Overloaded data member.

This ambiguity arises due to the use of parentheses, which can denote both method declarations and object initialization.

To resolve this confusion, C 11 mandates the use of curly braces for in-class initializers. This explicitly indicates that confusing is a member variable:

class BadTimes {
    struct Overloaded;
    int Overloaded; // Legal, but unusual.

    int confusing{Overloaded}; // <-- Clear initialization
};

Thus, in-class initializers must utilize equals or curly braces to prevent syntax misunderstandings and ensure code readability.

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