具有参考的自动关键字行为 [英] auto keyword behavior with references

问题描述

假设我有一个简单的c ++类，其中包含一个私有成员和一个getter:

Let's say I have a simple c++ class that contains a private member and a getter:

class MyClass
{
    private:
        double m_testValue = 1;

    public:
        double& getTestValue(){return m_testValue;}
} 

现在让我们说我要调用getter来获取我的引用并编辑该值(并在值之前/之后打印)

Now let's say I want to call the getter to get my reference and edit this value (and printing before / after values)

auto testVal = myClassInstance.getTestValue();
std::cout << myClassInstance.getTestValue() << std::endl;
std::cout << testVal << std::endl;
testVal = 3;
std::cout << myClassInstance.getTestValue() << std::endl;
std::cout << testVal << std::endl;

输出为

1
1
1
3

这与我预期的不完全相同，因为显然m_testValue没有被编辑.确实，如果我将auto替换为double& :

This is not exactly what I expected since apparently, m_testValue wasn't edited. Indeed, if I replace auto with double& :

double& testVal = myClassInstance.getTestValue();
std::cout << myClassInstance.getTestValue() << std::endl;
std::cout << testVal << std::endl;
testVal = 3;
std::cout << myClassInstance.getTestValue() << std::endl;
std::cout << testVal << std::endl;

我知道

1
1
3
3

我想要的是什么. 所以问题是: 这是auto关键字的预期行为还是错误? 如果可以预料，这种行为的原因是什么?这是技术上的限制吗?如果是设计使然，为什么?

Which is what I want. So the question is: Is this the expected behaviour of the auto keyword or is it a bug? If this is expected, what is the reason for this behaviour? is it a technical limitation? If it by design and why?

推荐答案

推导auto时，不会推导该参考.它总是推断出价值.如果要引用返回值，则可以使用auto&，const auto&或auto&&.

When auto is deduced, it is not deduced to the reference. It always deduces to the value. If you want to have a reference to returned value, you can have auto&, const auto& or auto&&.

是的，这是设计使然.实际上，任何其他行为都是非常令人惊讶的.更糟糕的是，很容易在需要时使用参考.但是，假设auto实际上可以推论为参考.您将如何(从语法上)使它具有价值?

And yes, it is by design. Any other behavior would actually be quite surprising. What is even worse, it is easy to use a reference when you want. However, imagine that auto would be actually deduced to the reference. How would you (syntactically) make it a value?

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