用 std::move 返回 std::vector

2022-01-19 00:00:00 return move c++

我有一个非常基本的问题:使用 std::move 返回 std::vector<A> 是个好主意吗?例如:

I have a very basic question: is it a good idea to return a std::vector<A> using std::move? For, example:

class A {};
std::vector<A> && func() {
    std::vector<A> v;
    /* fill v */
    return std::move(v);
}

我应该以这种方式返回std::map、std::list..等吗?

Should I return std::map, std::list.. etc... in this way?

推荐答案

你声明一个函数通过 r-value 引用返回 - 这几乎不应该这样做(如果你通过引用返回本地对象，你最终会得到悬垂的参考).而是将函数声明为按值返回.这样，调用者的值将由函数返回的 r 值构造.返回的值也将绑定到任何引用.

You declare a function to return by r-value reference - this should almost never be done (if you return the local object by reference, you will end up with a dangling reference). Instead declare the function to return by value. This way the caller's value will be move constructed by the r-value returned by the function. The returned value will also bind to any reference.

其次，不，您应该不使用显式 std::move 返回，因为这会阻止编译器使用 RVO.没有必要，因为编译器会自动将返回的任何左值引用转换为右值引用.

Secondly, no, you should not return using an explicit std::move as this will prevent the compiler to use RVO. There's no need as the compiler will automatically convert any l-value reference returned to an r-value reference if possible.

std::vector<A> func() {
    std::vector<A> v;
    /* fill v */
    return v; // 'v' is converted to r-value and return value is move constructed.
}

更多信息:

  • 在从函数返回值时使用 std::move() 以避免复制
  • RValue 引用 (&&) 的返回是否有用?

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