couldn't deduce template parameter ‘looptype’ - c++

Question

#include<cstdlib>
#include<iostream>
#include<math.h>
#include<string>
using namespace std;
class getAverage{
public:
template<class add>
add computeAverage(add input[], int nosOfElem){
add sum = add();//calling default constructor to initialise it.
for(int index=0;index<=nosOfElem;++index){
sum += input[index];
}
return double(sum)/nosOfElem;
}
template<class looptype>
looptype* returnArray(int sizeOfArray){
looptype* inputArray= (int*)malloc(sizeof(int)*sizeOfArray);
for(int index=0;index<=sizeOfArray;++index){
inputArray[index]=index;//=rand() % sizeOfArray;
}
return inputArray;
}
};
int main(){
int sizeOfArray=2;
int inputArray;
getAverage gA;
int* x= gA.returnArray(sizeOfArray);
for(int index=0;index<=2;++index){
cout<<x[index];
}
cout<<endl;
cout<<gA.computeAverage(x,sizeOfArray);
free(x);
return 0;
}
I want to create a template function through which I can create dynamic arrays of different type(Int,long,string ..etc.). I tried doing it,and realised that the "looptype" never gets the type value. Can someone suggest a way to do this.
Thanks

Answer 1

template<class looptype>
looptype* returnArray(int sizeOfArray){
looptype* inputArray= (int*)malloc(sizeof(int)*sizeOfArray);
for(int index=0;index<=sizeOfArray;++index){
inputArray[index]=index;//=rand() % sizeOfArray;
}
return inputArray;
}
template parameters can only be deduced from the function-template arguments. Here the only argument your function template takes is sizeOfArray which is an int. How does the compiler know what typename looptype is? Since it cannot be deduced, you have to explicitly specify it.
int* x= gA.returnArray<int>(sizeOfArray);
rather than:
int* x= gA.returnArray(sizeOfArray);
BTW, what's the point of have a template parameter looptype when I know it can only be an int as sold by this line of your code:
...
looptype* inputArray= (int*)malloc(sizeof(int)*sizeOfArray);
...
Your use of malloc is scary. For virtually same performance, you are making simple tasks complicated. Prefer std::vector<int> or worse case std::unique_ptr<int[]>.