C ++中的空嵌套初始化器列表

Question

Consider this piece of code:

struct S {
    float b;
    int a;    
};

int main() {
    S s{{{}}};
    return s.a;
}

Godbolt

Clang 6.0.0 compiles this code, but shows a warning:

<source> warning: too many braces around scalar initializer [-Wmany-braces-around-scalar-init]

GCC 8.2 doesn't compile this code and reports an error:

<source>: In function 'int main()': <source>:9:10: error: braces around scalar initializer for type 'float'

Which one is correct? What does the specification say about this?

Answer 1

Both compilers are correct. Unless you violate a rule that says no diagnostic required the compiler should issue you a message. Whether that message is a warning or an error is up to the implementation. Normally you'll get a warning if it is something the compiler can still proceed with and an error when there is no way the compiler can continue.

Answer 2

What does the specification say about this?

dcl.init.aggr/3 :

3 When an aggregate is initialized by an initializer list as specified in [dcl.init.list], the elements of the initializer list are taken as initializers for the elements of the aggregate. The explicitly initialized elements of the aggregate are determined as follows:

3.1 If the initializer list is a designated-initializer-list, the aggregate shall be of class type, the identifier in each designator shall name a direct non-static data member of the class, and the explicitly initialized elements of the aggregate are the elements that are, or contain, those members.

3.2 If the initializer list is an initializer-list, the explicitly initialized elements of the aggregate are the first n elements of the aggregate, where n is the number of elements in the initializer list.

3.3 Otherwise, the initializer list must be {} , and there are no explicitly initialized elements.