C++11 for loop in a Template Function - c++

Question

I am trying to write a function which would Print Data on the console. The function is to be templated as it should accept different types of Data.
The code is as shown below:
template<typename DataType>
void PrintData(DataType *X)
{
for (DataType Data : X)
{
cout << Data << "\t";
}
cout << endl;
}
int main()
{
int nArray[7] = { 7, 5, 4, 3, 9, 8, 6 };
double dArray[5] = { 4.3, 2.5, -0.9, 100.2, 3.0 };
PrintData(nArray);
PrintData(dArray);
system("pause");
return EXIT_SUCCESS;
}
I get an error that variable Data is undeclared in the templated function PrintData.
error C2065: 'Data' : undeclared identifier
error C3312: no callable 'begin' function found for type 'double *'
error C3312: no callable 'begin' function found for type 'int *'
error C3312: no callable 'end' function found for type 'double *'
error C3312: no callable 'end' function found for type 'int *'
Any help would be appreciated.
Thanks

Answer 1

Assuming you have included the iostream header file and the using namespace std; directive. Then your problems are:
You should not use DataType *. Your code makes X a pointer, which is different from array. Use DataType const& or DataType&& instead.
You have to include the iterator header file which provides the begin and end function for C-style array.
The following code works for me.
#include <iostream>
#include <iterator>
using namespace std;
template<typename DataType>
void PrintData(DataType const& X)
{
for (auto Data : X)
{
cout << Data << "\t";
}
cout << endl;
}
int main()
{
int nArray[7] = { 7, 5, 4, 3, 9, 8, 6 };
double dArray[5] = { 4.3, 2.5, -0.9, 100.2, 3.0 };
PrintData(nArray);
PrintData(dArray);
return EXIT_SUCCESS;
}
As commented by Igor Tandetnik, you may use template<struct DataType, size_t N> if you want to refer the size of the array.
Update:
With template<struct DataType> and DataType *X, DataType is deduced as int and double, and X is a pointer which is not a container.
With template<struct DataType, size_t N> and DataType (&X)[N], DataType is deduced as int and double, and X is an array which can be used with range-based for loop.
With template<struct DataType> and DataType&& X or DataType const& X, DataType is deduced as int[7] or double[5], and X is an array as well.

Answer 2

The problem is that you're passing the name of the array (e.g., nArray), which is just a pointer to the first element, basically. However, new-style for loops expect something on which you can call begin and end - a range.
The following changes make it work by using a vector instead of an array. Note that there are very few differences otherwise, and, in general, very few reason to use C-style arrays in contemporary C++.
#include <iostream>
#include <vector>
using namespace std;
template<typename DataType>
void PrintData(const DataType &X)
{
for (const auto &Data : X)
{
cout << Data << "\t";
}
cout << endl;
}
int main()
{
vector<int> nArray = { 7, 5, 4, 3, 9, 8, 6 };
vector<double> dArray = { 4.3, 2.5, -0.9, 100.2, 3.0 };
PrintData(nArray);
PrintData(dArray);
system("pause");
return EXIT_SUCCESS;

Answer 3

You have some problems:
1) DataType must be a container. So, try to use:
std::vector<int> nArray = { 7, 5, 4, 3, 9, 8, 6 };
std::vector<double> dArray = { 4.3, 2.5, -0.9, 100.2, 3.0 };
2) You are not going to change the container. It's better to pass container by const reference:
template<typename DataType>
void PrintData(const DataType & X);
3) Change the loop like this:
for (const auto & value : X) {
std::cout << value << "\t";
}
Code example:
#include <iostream>
#include <vector>
template<typename DataType>
void PrintData(const DataType & X) {
for (const auto & value : X) {
std::cout << value << "\t";
}
std::cout << std::endl;
}
int main() {
std::vector<int> nArray = { 7, 5, 4, 3, 9, 8, 6 };
std::vector<double> dArray = { 4.3, 2.5, -0.9, 100.2, 3.0 };
PrintData(nArray);
PrintData(dArray);
return 0;
}

Answer 4

As Igor suggested, If at all you wish to use DataType * then you need to do something like this
#include <iostream>
#include <iterator>
using namespace std;
template <typename DataType, size_t N>
void PrintData(DataType (&X)[N])
{
for (auto i : X)
cout << i << "\t";
cout << endl;
}
int main()
{
int nArray[7] = { 7, 5, 4, 3, 9, 8, 6 };
double dArray[5] = { 4.3, 2.5, -0.9, 100.2, 3.0 };
PrintData(nArray);
PrintData(dArray);
return EXIT_SUCCESS;
}
Output
7 5 4 3 9 8 6
4.3 2.5 -0.9 100.2 3
Explanation:
If you see void PrintData(int* nArray); & void PrintData(int (&nArray)[7] );
are similar declarations, except that seconds one tells where array ends.
Template function
template <typename DataType, size_t N>
void PrintData(DataType (&X)[N])
is deduced as'
void PrintData(int (&nArray)[7] )
You could also write
void PrintData(int (&nArray)[7] )
{
for (auto i : nArray)
cout << i << "\t";
cout << endl;
}