[cfe-dev] Move constructor forces copy assignment to be implicitly defaulted?

[Howard Hinnant]

On May 28, 2012, at 2:52 AM, Suman Kar wrote:

>> Right.  I mean that they don't exist, just as in C++98/03.  Deleted move members are generally problematic as they inhibit "copying" from rvalues.  If you have valid copy members and deleted move members, you can't return such an object from a function.
>> 
> 
> Okay, I still can't wrap my head around the last sentence. This looks ominous.

struct A
{
    A() = default;
    A(const A&) = default;
    A& operator=(const A&) = default;
    A(A&&) = delete;
    A& operator=(A&&) = delete;
};

A
make()
{
    A a;
    return a;
}

int main()
{
    A a = make();
}

test.cpp:67:7: error: call to deleted constructor of 'A'
    A a = make();
      ^   ~~~~~~
test.cpp:54:5: note: function has been explicitly marked deleted here
    A(A&&) = delete;
    ^
1 error generated.

But:

struct A
{
    A() = default;
    A(const A&) = default;
    A& operator=(const A&) = default;
};

A
make()
{
    A a;
    return a;
}

int main()
{
    A a = make();
}

Compiles and runs fine.

Howard