couldn't deduce template parameter ‘looptype’

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[]> .