您应该为复制构造函数还是移动构造函数提供默认参数？

Question

Suppose you have the following class:

template<typename T>
class A
{
    ...

    A (T const &t) 
    {
        ...
    }

    A (T &&t) noexcept
    {
        ...
    }
};

Now suppose I want to provide a default argument to one of these (ie, = T{} ). Which is more efficient/consistent to default, the copy or the move? Does it even matter? My gut says it doesn't matter performance wise, which leads me to think it would be more consistent to default the copy since you should not bind a temporary to an r-value.

Thoughts?

Answer 1

Those are not copy / move constructors, so you don't have to use a reference at all .

Assuming you have distinguished those cases to elide copying t if you can, just have one construtor

template<typename T>
class A
{
    ...

    /* explicit ? */ A (T t = {}) 
    {
        ...
    }
};