Invoke default constructor for each element in constructed std::vector

Question

Is there a way to construct std::vector<C> of N elements by invoking the default constructor for each?

The constructor from size_type just calls C's constructor once and then uses its copy constructor for the rest of the elements.

Answer 1

The constructor from size_type just calls C's constructor once and then uses its copy constructor for the rest of the elements.

Not true since C++11. Look at std::vector::vector documentation:

...

vector( size_type count, const T& value, const Allocator& alloc = Allocator()); (2)

explicit vector( size_type count, const Allocator& alloc = Allocator() ); (3)

...

And then:

...

2) Constructs the container with count copies of elements with value value.

3) Constructs the container with count default-inserted instances of T. No copies are made.

...

So you need the 3rd constructor std::vector<C>(size)

---

It seems like this behavior exists only since c++11.

I can't find a way of doing this before c++11. Since no constructor can do this, the option would have been to create an empty vector, reserve and then emplace_back elements. But emplace_back is since c++11 so... we're back to square one.

Answer 2

Just do this:

std::vector<C> v(size)

Example:

#include <iostream>
#include <string>
#include <vector>
class C{
    public:
    C(){
        std::cout << "constructor\n";
    }
    C(C const&){
        std::cout << "copy/n";
    }
};
int main()
{
    std::vector<C> v(10);

}

Result: (C++11/14)

constructor 
constructor 
constructor 
constructor
constructor
constructor 
constructor
constructor
constructor
constructor

Results: (C++98)

constructor
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy

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