While explicitly instantiating vector, what is the someType default constructor used for?

Question

It\'s an exercise from C++ Primer 5th Edition:

Exercise 16.26: Assuming NoDefault is a class that does not have a default constructor, can we e

Answer 1

As already said, the default constructor is required to create vector of objects. However, if one were to create a classic dynamic array (contiguous area of memory), it would be possible to address the lack of a default constructor using the placement new syntax:

#include 

struct Foo {
    explicit Foo(int a): a(a) {}
    int a;
};

int main() {
    void* mem = operator new[](10*sizeof(Foo));
    Foo* ptr = static_cast(mem);
    for (int i = 0; i < 10; ++i ) {
        new(&ptr[i])Foo(i);
        std::cout << ptr[i].a;
    }
    return 0;
}