Why Doesn't C 11 Support Designated Initializers Like C99?

C 11's Exclusion of Designated Initializers: The Committee's Rationale

In C99, designated initializers offer a convenient way to initialize specific members of a struct, as illustrated by the following example:

struct Person {
    int height;
    int weight;
    int age;
};

int main() {
    Person p = { .age = 18 };
}

However, in C 11, this syntax is not valid. The C 11 standard committee has repeatedly rejected proposals to include designated initializer support, despite its practicality in C99.

Reasons for Exclusion

The committee's primary concern stems from the indeterminate order of evaluation for subexpressions in C99 designated initializers. Consider the following example:

struct X {
    int c;
    char a;
    float b;
};

struct X foo = { .a = (char)f(), .b = g(), .c = h() };

In C99, the order in which these function calls (f(), g(), and h()) would execute is undefined. This ambiguity could lead to indeterminate program behavior if these functions interact or have side effects.

C 's Stringent Initializer Requirements

In contrast to C99's flexible evaluation order, C mandates a strict sequential evaluation of initializer-clauses. As a result, implementing designated initializers in C would have required a different behavior:

f();
g();
h();

This would have broken compatibility with existing C99 implementations.

Proposed Solutions and Limitations

The c 20 standard partially addresses this issue with the introduction of P0329R4, which allows limited support for designated initializers. However, this support is restricted to avoid the complexities and potential pitfalls associated with indeterminate evaluation order.

Conclusion

The C 11 standard committee's decision to exclude designated initializers was driven by concerns about maintaining a consistent and predictable initialization process. While Boost provides an implementation for designated initializers, the standard committee has emphasized the challenges of integrating such a feature into C without introducing complexities or breaking compatibility.

The above is the detailed content of Why Doesn't C 11 Support Designated Initializers Like C99?. For more information, please follow other related articles on the PHP Chinese website!