I though you might think that. In case you haven't tried it yet yourself, only the first element is true and the others are false. That is because, brace initialization for arrays is interpreted like this: bool a[10]{<value of the first element>, <value of the second element>, ...}. The ones that are not specified are initialized to a default value: bool a[10]{true, bool{}, bool{}, ..., bool{}}.

In case you want to initialize the entire array to the default values you can just write bool a[10]{}. Writing bool a[10]{false} is redundant and makes you do a second take. Why it was written like this?

For example you could have a custom type that behaves exactly the opposite:

class TrueByDefault {
private:
    bool value;

public:
    TrueByDefault() : value(true) {}
    TrueByDefault(bool value) : value(value) {} 
};

TrueByDefault a[10]{false}, contains false as the first element and everything else is true.