Question

I noticed that when you declare an array, the default constructor must be needed. Is that right?
Is there any exception?

For example,

struct Foo{
Foo(int i  ) {}
};

int main () {
    Foo f[5];
    return 0;
}

The code above does not compile.

Answer 1

Other answers are all right but, for completeness: You could also use the array initialization syntax:

Foo f[5] = {1,2,3,4,5};

This works if Foo’s ctor is not explicit. If it was, you’d have to be…. explicit:

Foo f[5] = {Foo(1), Foo(2), Foo(3), Foo(4), Foo(5)};

Note¹: There is a difference between the two cases that may not be obvious and is thus worth noting: In the first, the array elements are directly constructed from the ints in the initialization list, by invoking the Foo(int) ctor. In the second, the initialization list is made of Foos constructed with the explicit Foo(int) ctor, and the array elements are copy constructed from the elements in the initialization list. A copy ctor for Foo is thus required in the latter case.

[1] Thanks to MSalters for the comment.