Question

我有一个state类，该类具有移动分配/构造函数。副本分配/构造函数设置为delete。

我很困惑为什么在下面的函数（返回一个state对象）中如此编译并正常运行：

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;
}

但是我尝试这样做时抱怨复制构造函数已被删除：

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;
}

编译器输出：

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

Answer 1

现代的编译器足够聪明，可以执行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;
}

这就是为什么我们在这里实际上返回创建的对象的原因（编译器可以只为函数的返回值创建它，以避免复制）。

但是在第二种变体中，您尝试从HEAP中的对象创建堆栈中的对象，并且此处不能使用RVO或move。这就是为什么它试图执行已删除的副本。

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;
}

此外，您还会通过丢弃指向创建对象的堆的指针来泄漏内存。

为什么返回堆分配的对象而不是堆栈分配的对象时调用拷贝构造函数？

1 个答案: