How to pass array to function template with reference

Question

I am learning c++ template concepts. I do not understand the following.

#include <iostream>
#include <typeinfo>

using namespace std;

template <typename T>
T fun(T& x)
{
 cout <<" X is "<<x;
 cout <<"Type id is "<<typeid(x).name()<<endl;
}


int main ( int argc, char ** argv)
{
   int a[100];
   fun (a);
}

What i am trying?

1) T fun (T & x)

Here x is a reference, and hence will not decayed 'a' into pointer type, but while compiling , i am getting the following error.

 error: no matching function for call to ‘fun(int [100])’

When I try non-reference, it works fine. As I understand it the array is decayed into pointer type.

Answer 1

C-style arrays are very basic constructs which are not assignable, copyable or referenceable in the way built-ins or user defined types are. To achieve the equivalent of passing an array by reference, you need the following syntax:

// non-const version
template <typename T, size_t N>
void fun( T (&x)[N] ) { ... }

// const version
template <typename T, size_t N>
void fun( const T (&x)[N] ) { ... }

Note that here the size of the array is also a template parameter to allow the function to work will all array sizes, since T[M] and T[N] are not the same type for different M , N . Also note that the function returns void. There is no way of returning an array by value, since the array is not copyable, as already mentioned.

Answer 2

The problem is in the return type: you cannot return an array because arrays are non-copiable. And by the way, you are returning nothing!

Try instead:

template <typename T>
void fun(T& x)  // <--- note the void
{
    cout <<" X is "<<x;
    cout <<"Type id is "<<typeid(x).name()<<endl;
}

And it will work as expected.

NOTE: the original full error message (with gcc 4.8) is actually:

test.cpp: In function ‘int main(int, char**)’:
test.cpp:17:10: error: no matching function for call to ‘fun(int [100])’
    fun (a);
      ^
test.cpp:17:10: note: candidate is:
test.cpp:7:3: note: template<class T> T fun(T&)
 T fun(T& x)
   ^
test.cpp:7:3: note:   template argument deduction/substitution failed:
test.cpp: In substitution of ‘template<class T> T fun(T&) [with T = int [100]]’:
test.cpp:17:10:   required from here
test.cpp:7:3: error: function returning an array

The most relevant line is the last one.