在 C++ 中将具有 unique_ptr 的对象推入向量 [英] Pushing an object with unique_ptr into vector in C++

问题描述

我有一个简单的类结构，对离散模拟进行建模，带有一个状态向量，每个状态包含许多转换，作为智能指针的向量.我使用智能指针来保存转换，因为在我的完整应用程序中我需要多态性.

I have a simple class structure modelling a discrete simulation, with a vector of States, which each contain a number of Transitions, held as a vector of smart pointers. I've used smart pointers to hold the transitions as in my full application I need polymorphism.

#include <vector>
#include <memory>

class Transition {
    public:
        Transition() {}
};


class State {
    public:
        State(int num) : num(num), transitions() {}
        void add_transition(std::unique_ptr<Transition> trans) {
            transitions.push_back(std::move(trans));
        }

    private:
        int num;
        std::vector<std::unique_ptr<Transition>> transitions;
};


int main() {
    std::vector<State> states;
    for (int i = 0; i < 10; i++) {
        State nstate = State(i);
        for (int j = 0; j < 2; j++) {
            nstate.add_transition(std::move(std::unique_ptr<Transition>(new Transition())));
        }
        // This line causes compiler errors
        states.push_back(nstate);
    }
}

将新状态对象添加到向量时出现编译器错误:

I get compiler errors when adding the new state object to the vector:

Error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = Transition; _Dp = std::default_delete<Transition>]’
 { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }

我想这是由于向量制作了 State 对象的副本，该对象也试图制作 unique_ptrs 的向量的副本，这是不允许的.我已经看到 emplace_back 不像 push_back 那样制作副本，但我仍然遇到同样的错误.

I imagine this is due to vector making a copy of the State object which is also trying to make a copy of the vector of unique_ptrs which isn't allowed. I've seen that emplace_back doesn't make copies like push_back does but I still get the same error.

将 State 对象直接添加到向量中是可行的，但我更愿意避免这种变通方法，因为在我的实际代码中，我对 State 对象做了更多的工作，而不仅仅是添加转换并且不想继续访问向量.

Adding the State object directly into the vector works, but I'd prefer to avoid this workaround as in my actual code I do more work with the State object rather than just adding transitions and don't want to keep accessing the vector.

int main() {
    std::vector<State> states;
    for (int i = 0; i < 10; i++) {
        states.push_back(State(i));
        for (int j = 0; j < 2; j++) {
            states[i].add_transition(std::move(std::unique_ptr<Transition>(new Transition())));
        }
    }
}

推荐答案

State 不可复制，只能移动；但是对于 states.push_back(nstate);，nstate 是一个左值(作为命名变量)，不能从中移动.然后尝试执行副本，但不允许.

State is not copyable, but only moveable; But for states.push_back(nstate);, nstate is an lvalue (as a named variable), which can't be moved from. Then a copy is tried to perform but it's not allowed.

要解决它，您可以使用 std::move(将其转换为右值):

To solve it you could use std::move (to turn it to an rvalue):

states.push_back(std::move(nstate));

现场直播

注意，在移动操作之后，nstate的数据成员(包括向量及其内容)也会被移动.

Note that after the move operation, the data member of nstate (including the vector and its content) will be moved too.

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