我应该总是继续使用 `sink` 构造函数或 setter 参数吗?

问题描述

struct TestConstRef {
    std::string str;
    Test(const std::string& mStr) : str{mStr} { }
};

struct TestMove {
    std::string str;
    Test(std::string mStr) : str{std::move(mStr)} { }
};

在观看 GoingNative 2013 后，我明白 sink 参数应该始终按值传递并使用 std::move 移动.TestMove::ctor 是应用这个习语的正确方法吗?是否存在 TestConstRef::ctor 更好/更高效的情况?

After watching GoingNative 2013, I understood that sink arguments should always be passed by value and moved with std::move. Is TestMove::ctor the correct way of applying this idiom? Is there any case where TestConstRef::ctor is better/more efficient?

琐碎的 setter 呢?我应该使用以下习语还是传递 const std::string&?

What about trivial setters? Should I use the following idiom or pass a const std::string&?

struct TestSetter {
    std::string str;
    void setStr(std::string mStr) { str = std::move(str); }
};

推荐答案

简单的答案是:是的.

原因也很简单，如果您按值存储，您可能需要移动(从临时)或复制(从左值).让我们看看在这两种情况下会发生什么，两种方式.

The reason is quite simple as well, if you store by value you might either need to move (from a temporary) or make a copy (from a l-value). Let us examine what happens in both situations, with both ways.

来自临时

• 如果您通过 const-ref 获取参数，则临时对象将绑定到 const-ref 并且无法再次移动，因此您最终会制作一个(无用的)副本.
• 如果您按值获取参数，则该值将从临时(移动)中初始化，然后您自己从该参数中移动，因此不会进行复制.

一个限制:一个没有高效移动构造函数的类(例如 std::array)，因为那样你做了两个副本而不是一个.

One limitation: a class without an efficient move-constructor (such as std::array<T, N>) because then you did two copies instead of one.

从左值(或 const 临时，但谁会这样做...)

From a l-value (or const temporary, but who would do that...)

• 如果你通过 const-ref 获取参数，那里什么都不会发生，然后你复制它(不能从中移动)，这样就得到了一个副本.
• 如果您按值获取参数，则将其复制到参数中，然后从参数中移出，从而生成单个副本.

一个限制:相同的...类，移动类似于复制.

One limitation: the same... classes for which moving is akin to copying.

因此，简单的答案是，在大多数情况下，通过使用接收器，您可以避免不必要的副本(通过移动来替换它们).

So, the simple answer is that in most cases, by using a sink you avoid unnecessary copies (replacing them by moves).

唯一的限制是移动构造函数与复制构造函数一样昂贵(或接近一样昂贵)的类；在这种情况下，有两个动作而不是一个副本是最糟糕的".幸运的是，这样的类很少见(数组是一种情况).

The single limitation is classes for which the move constructor is as expensive (or near as expensive) as the copy constructor; in which case having two moves instead of one copy is "worst". Thankfully, such classes are rare (arrays are one case).

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