Destructor for pointer and non pointer - c++

Question

Why this can't work. Is there any way to do this?
I don't want to create a separate function for pointers
#include <iostream>
using namespace std;
template<class T>
class temp
{
public:
T val;
temp(T value) : val(value) {}
~temp()
{
if(is_pointer<T>::value)
{
delete val;
}
}
};
int main()
{
string * n = new string("cat");
temp<string*>object(n);//ok
temp<string>object2("dog"); //compliation error: type 'class std::cxx11::basic_string' argument given to 'delete', expected pointer. --- (but there is if statement!!!!)
//i dont want delete in main
return 0;
}
To compile i use g++ 6.3.0
Could someone help? Maybe, I need to separate declaration from definition?

Answer 1

The issue that you have is that the branch of an if must always be syntactically valid, even if it is never taken.
You could do it with if constexpr, which is a "compile time if"
~temp()
{
if constexpr(is_pointer<T>::value)
{
delete val;
}
}
However this isn't safe.
How do you know that the pointer passed to temp<T*> was created by new and not new[], malloc, or by taking the address of an object that wasn't dynamically allocated?
Rather than assume that pointers should be deleted, you should avoid having to know which pointers to delete
#include <string>
#include <memory>
template<class T>
class temp
{
public:
T val;
temp(T value) : val(value) {}
// n.b. no need to define destructor
};
int main()
{
std::string str("cat");
temp<std::string*> object(&str);//ok
temp<std::string> object2("dog"); // also ok
std::unique_ptr<std::string> str2 = std::make_unique<std::string>("mouse");
temp<std::string *> object3(str2.get()); // ok so long as str2 outlives object3
std::shared_ptr<std::string> str3 = std::make_shared<std::string>("rabbit");
temp<std::shared_ptr<std::string>> object4(str3); // also ok
return 0;
}