我是真的返回地址还是提及临时地址

Question

I am implementing a generic function that finds minimum in an array. It works fine and looks like this:

template<typename T>
auto FindMinimum(T &input, int size) -> decltype(input[0])
{
    auto TempMin=input[0];
    int leastIndex=0;
    for (int i = 1; i < size ; i++)
    {
        if(input[i]<TempMin)
        {
            leastIndex=i;
            TempMin=input[i];
        }
    }

    return TempMin;
}

But in the IDE, I get a warning: returning address or reference to a temporary . If I change TempMin to input[leastIndex] , then the warning disappears.

I am wondering that I tend to return by value and also I haven't used & anywhere, but why does it still return by reference or address?

Thoughts?

Thanks.

EDIT

In decltype(input[0]) , I have passed a subscript to input array. So shouldn't it really correspond to a value and not a reference or address to a temporary?

Answer 1

As "Effective Modern C++" says:

In C++11, perhaps the primary use for decltype is declaring function templates where the function's return type depends on its parameter types. For example, suppose we'd like to write a function that takes a container that supports indexing via square brackets (ie, the use of “[] ”) plus an index, then authenticates the user before returning the result of the indexing operation. The return type of the function should be the same as the type returned by the indexing operation.

operator[] on a container of objects of type T typically returns a T&. This is the case for std::deque, for example, and it's almost always the case for std::vector. For std::vector, however, operator[] does not return a bool&. Instead, it returns a brand new object ... but what's important here is that the type returned by a container's opera tor[] depends on the container.

So this code:

template<typename T>
auto FindMinimum(T &input, int size) -> decltype(input[0])

auto maybe T&.

And this code:

auto temp = input[0]

auto maybe T.

I think the compiler hints you, maybe that's the reason.