c++ - When Does Move Constructor get called?

Question

I'm confused about when a move constructor gets called vs a copy constructor. I've read the following sources:

Move constructor is not getting called in C++0x

Move semantics and rvalue references in C++11

msdn

All of these sources are either overcomplicated(I just want a simple example) or only show how to write a move constructor, but not how to call it. Ive written a simple problem to be more specific:

const class noConstruct{}NoConstruct;
class a
{
private:
    int *Array;
public:
    a();
    a(noConstruct);
    a(const a&);
    a& operator=(const a&);
    a(a&&);
    a& operator=(a&&);
    ~a();
};

a::a()
{
    Array=new int[5]{1,2,3,4,5};
}
a::a(noConstruct Parameter)
{
    Array=nullptr;
}
a::a(const a& Old): Array(Old.Array)
{

}
a& a::operator=(const a&Old)
{
    delete[] Array;
    Array=new int[5];
    for (int i=0;i!=5;i++)
    {
        Array[i]=Old.Array[i];
    }
    return *this;
}
a::a(a&&Old)
{
    Array=Old.Array;
    Old.Array=nullptr;
}
a& a::operator=(a&&Old)
{
    Array=Old.Array;
    Old.Array=nullptr;
    return *this;
}
a::~a()
{
    delete[] Array;
}

int main()
{
    a A(NoConstruct),B(NoConstruct),C;
    A=C;
    B=C;
}

currently A,B,and C all have different pointer values. I would like A to have a new pointer, B to have C's old pointer, and C to have a null pointer.

somewhat off topic, but If one could suggest a documentation where i could learn about these new features in detail i would be grateful and would probably not need to ask many more questions.

Answer 1

A move constructor is called:

• when an object initializer is std::move(something)
• when an object initializer is std::forward<T>(something) and T is not an lvalue reference type (useful in template programming for "perfect forwarding")
• when an object initializer is a temporary and the compiler doesn't eliminate the copy/move entirely
• when returning a function-local class object by value and the compiler doesn't eliminate the copy/move entirely
• when throwing a function-local class object and the compiler doesn't eliminate the copy/move entirely

This is not a complete list. Note that an "object initializer" can be a function argument, if the parameter has a class type (not reference).

a RetByValue() {
    a obj;
    return obj; // Might call move ctor, or no ctor.
}

void TakeByValue(a);

int main() {
    a a1;
    a a2 = a1; // copy ctor
    a a3 = std::move(a1); // move ctor

    TakeByValue(std::move(a2)); // Might call move ctor, or no ctor.

    a a4 = RetByValue(); // Might call move ctor, or no ctor.

    a1 = RetByValue(); // Calls move assignment, a::operator=(a&&)
}