为什么当我返回堆分配的对象而不是堆栈分配的对象时调用复制构造函数？

Question

I have a state class which has a move assignment/constructor. The copy assingment/constructor are set to delete .

I am confused why in the following function (which returns a state object) compiles and runs fine like this:

state propagator::PROPAGATE(date & TargetDate)
{
    jmethodID jmid_PROPAGATE = ENV->GetMethodID(this->jcls_PROPAGATOR, "propagate", "(path/to/date;)path/to/state;");
    jobject jobj_PROPAGATED_STATE = ENV->CallObjectMethod(this->jobj_PROPAGATOR, jmid_PROPAGATE, TargetDate.get_DATE_JOBJECT());

    state PROPAGATED_STATE(this->ENV);
    PROPAGATED_STATE.set_STATE_JOBJECT(jobj_PROPAGATED_STATE);
    return PROPAGATED_STATE;

    //state * PROPAGATED_STATE = new state(ENV);
    //PROPAGATED_STATE->set_STATE_JOBJECT(jobj_PROPAGATED_STATE);
    //return *PROPAGATED_STATE;
}

but complains that copy constructor has been deleted when I try this:

state propagator::PROPAGATE(date & TargetDate)
{
    jmethodID jmid_PROPAGATE = ENV->GetMethodID(this->jcls_PROPAGATOR, "propagate", "(path/to/date;)path/to/state;");
    jobject jobj_PROPAGATED_STATE = ENV->CallObjectMethod(this->jobj_PROPAGATOR, jmid_PROPAGATE, TargetDate.get_DATE_JOBJECT());

    //state PROPAGATED_STATE(this->ENV);
    //PROPAGATED_STATE.set_STATE_JOBJECT(jobj_PROPAGATED_STATE);
    //return PROPAGATED_STATE;

    state * PROPAGATED_STATE = new state(ENV);
    PROPAGATED_STATE->set_STATE_JOBJECT(jobj_PROPAGATED_STATE);
    return *PROPAGATED_STATE;
}

Compiler output:

error: use of deleted function ‘state::state(const state&)’

Answer 1

Modern compilers are clever enough to do the RVO ( What are copy elision and return value optimization? )

state propagator::PROPAGATE(date & TargetDate)
{
    jmethodID jmid_PROPAGATE = ENV->GetMethodID(this->jcls_PROPAGATOR, "propagate", "(path/to/date;)path/to/state;");
    jobject jobj_PROPAGATED_STATE = ENV->CallObjectMethod(this->jobj_PROPAGATOR, jmid_PROPAGATE, TargetDate.get_DATE_JOBJECT());

    state PROPAGATED_STATE(this->ENV);
    PROPAGATED_STATE.set_STATE_JOBJECT(jobj_PROPAGATED_STATE);
    return PROPAGATED_STATE;
}

That's why here we return actually the object which was created (compiler can just create it in place for function's return value, to avoid copying).

But in second variant you are trying to create object in the stack from object in the HEAP, and RVO or move can't be used here. That's why it is trying to perform the deleted copy.

state propagator::PROPAGATE(date & TargetDate)
{
    jmethodID jmid_PROPAGATE = ENV->GetMethodID(this->jcls_PROPAGATOR, "propagate", "(path/to/date;)path/to/state;");
    jobject jobj_PROPAGATED_STATE = ENV->CallObjectMethod(this->jobj_PROPAGATOR, jmid_PROPAGATE, TargetDate.get_DATE_JOBJECT());

    state * PROPAGATED_STATE = new state(ENV);
    PROPAGATED_STATE->set_STATE_JOBJECT(jobj_PROPAGATED_STATE);
    return *PROPAGATED_STATE;
}

Also you are leaking memory by discarding pointer to heap, where you've created an object.