F.20: For "out" output values, prefer return values to output parameters(输出结果时更应该使用返回值而不是输出参数)

Reason(原因)

A return value is self-documenting, whereas a & could be either in-out or out-only and is liable to be misused.返回值本身可以说明用途，而引用类型可以是输入/输出参数也有可能只是输出参数，容易被误用。

This includes large objects like standard containers that use implicit move operations for performance and to avoid explicit memory management.这种观点可以覆盖像标准容器那样的大对象，它们会为了性能和避免显式内存管理而使用隐式移动操作。

If you have multiple values to return, use a tuple or similar multi-member type.如果你有多个值需要返回，使用tuple或者类似的多成员类型。

译者注：tuple可以参考以下文章。

https://mp.weixin.qq.com/s/kQEYhjpfE9XYBiRgp_fJSw

Example(示例)

// OK: return pointers to elements with the value xvector find_all(const vector&, int x);// Bad: place pointers to elements with value x in-outvoid find_all(const vector&, vector& out, int x);

Note(注意)

A struct of many (individually cheap-to-move) elements may be in aggregate expensive to move.

包含多个(单独看都可以低成本移动)元素的结构体合起来移动时可能会代价高昂。

It is not recommended to return a const value. Such older advice is now obsolete; it does not add value, and it interferes with move semantics.

不推荐返回常量值。这种过时的建议现在已经被淘汰；它不会带来好处，而且其接口含有移动语义。

const vector fct(); // bad: that "const" is more trouble than it is worthvector g(const vector& vx){ // ... fct() = vx; // prevented by the "const" // ...  return fct(); // expensive copy: move semantics suppressed by the "const"}

The argument for adding const to a return value is that it prevents (very rare) accidental access to a temporary. The argument against is prevents (very frequent) use of move semantics.

建议为返回值增加const修饰的观点认为，这样会阻止(极少发生的)对临时变量的意外访问。相反的观点认为这样做会(非常多地)阻止移动语义的运用。

Exceptions(例外)

• For non-value types, such as types in an inheritance hierarchy, return the object by unique_ptr or shared_ptr.对于非值类型函数，例如处于继承关系中的类型，通过unique_ptr或者shared_ptr返回对象。译者注：两种方式都可以避免不必要的拷贝动作。
• If a type is expensive to move (e.g., array), consider allocating it on the free store and return a handle (e.g., unique_ptr), or passing it in a reference to non-const target object to fill (to be used as an out-parameter).如果某种类型(例如array)的移动成本很高，考虑从自由存储上为其申请内存并使用句柄(例如unique_prt)返回它，或者通过用于填充的非常量对象的引用来传递。译者注：POD是Plain old data structure的简称，是C++语言的标准中定义的一类数据结构，可以简单地理解只包含单纯数据类型的结构体。
• To reuse an object that carries capacity (e.g., std::string, std::vector) across multiple calls to the function in an inner loop: treat it as an in/out parameter and pass by reference.为了让处于内循环中的函数调用可以重复使用带有容量的对象(例如std::string,std::vector)：把它看做输入/输出参数并通过引用传递。

Example(示例)

struct Package { // exceptional case: expensive-to-move object char header[16]; char load[2024 - 16];};Package fill(); // Bad: large return valuevoid fill(Package&); // OKint val(); // OKvoid val(int&); // Bad: Is val reading its argument

译者注：示例代码说明的是POD使用引用传递输出值，而小数据者应该直接使用返回值。

Enforcement(实施建议)

• Flag reference to non-const parameters that are not read before being written to and are a type that could be cheaply returned; they should be "out" return values.警告那些没有在写之前读(没有输入用途)而且可以低成本返回的参数，它们应该作为返回值输出。
• Flag returning a const value. To fix: Remove const to return a non-const value instead.警告返回常数值的状况。修改方法：去掉常量修饰，返回一个非常量。

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