resizing c++ vectors without calling their constructors

So I have this situation where I have:

std::vector<Coord3D> thevector;

where Coord3D is just a simple struct with 3 floats: x, y and z. The constructor also only simply has x = 0; y = 0; z = 0; etc.

Now, suppose I want to append the contents of another vector into thevector.

Is it possible to resize the vector's size into thevector.size() + theothervector.size(), without making c++ call Coord3D()'s constructor?

The reason is that I want to memcpy the contents of theothervector. initialising the contents with constructors turned out to take some time after some benchmark testing. std::loopfill also takes quite a bit of time.

So yeah, is there a way to resize vectors while bypassing constructors of the elements? Will this invol开发者_运维技巧ve dodgy hacks?

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thevector.insert(thevector.end(), theothervector.begin(), theothervector.end());

If Coord3D are bitwise-copyable the compiler should be able to optimize it to the same code as memcpy.

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std::vectors all have an allocator, usually the default allocator. The allocator's construct() member function invokes the per-element constructor on your behalf.

To avoid invoking the per-element constructor, create your own allocator and ensure the construct() member function has an empty function body (or some other logic to decide at runtime whether or not to invoke the element's constructor).

After you create the vector with the custom allocator, just resize() the vector. There will be no per-element constructor invocation.

I have used this technique successfully to point a std::vector at a database's persistent storage. It avoids the per-element constructor (which would otherwise overwrite the DB's storage), and also avoids data copies (as the custom allocator was designed to know what part of the DB store to reference).

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You should enlarge their capacity and not size. For this use reserve(). And for getting current capacity use capcity().

This code will explain how:

std::vector<Coord3D> thevector;
thevector.reserve(n); //where n is size you want
Coord3D * vector_start = &(thevector[0]);
//do memcpy to vector_start

But this code is unsafe and related to implementation of std::vector.

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Is it possible to resize the vector's size into thevector.size() + theothervector.size(), without making c++ call Coord3D()'s constructor?

No, when you try to resize the size of the vector. When you resize the size of the vector, the elements of the vector needs to be copied to the new location and in this case, if I am not wrong, copy constructor of the class needs to be called.

As others said, you can any how call reserve. But the vector is empty with no elements.