为什么我不能在模板中返回数组的第一个元素？

Question

Consider:

#include <iostream>

template <typename T> T getArray( T &arr ) {
    return *arr;
}

int main() {

    int a[] = {5, 3, 6};

    std::cout << getArray(a);

}

It's suppose to print the first element in the array but it is not not working. Why is that?

It gives me the error:

error: no matching function for call to 'getArray(int [3])'

Answer 1

The type of a is int[3] , so the type of T is int[3] . Arrays cannot be returned from functions.

In C++11, you can do this:

template <typename T>
auto getArray(T &arr) -> decltype(*arr)
{ 
    return *arr; 
} 

Or this:

// requires <type_traits>

template <typename T>
typename std::remove_extent<T>::type& getArray(T &arr)
{ 
    return *arr; 
} 

In C++03 you can do this, but it's not quite the same:

template <typename T>
T getArray(T* arr /* not really an array */)
{ 
    return *arr; 
} 

Or:

template <typename T, std::size_t N>
T getArray(T (&arr)[N])
{ 
    return *arr; 
} 

Answer 2

Try

template <typename T, size_t N>
T getArray( T (&arr)[N] ) {
    return *arr;
}

so that T is the type of the element, not the array.

Answer 3

It does not even compile on MSVC++ 2010 Express. As I expected, this is because you are using a reference as a parameter, a scalar as a return value and a pointer is passed to the function call. I do not know the exact rules regarding templates (ie how exactly the type is determined) but that code has to be confusing the hell out of the compiler. The following code returns what you expected it to return.

#include <iostream>

template <typename T> T getArray( T* arr ) {
    return *arr;
}

int main() {

    int a[] = {5, 3, 6};

    std::cout << getArray(a);

}

Answer 4

Could you try :

#include <iostream>

template <typename T> T getArray( T arr[] ) {
    return *arr;
}

int main() {

    int a[] = {5, 3, 6};

    std::cout << getArray(a);

}