Bruce Eckel's Thinking in C++, 2nd Ed Contents | Prev | Next

Explicit instantiation

At times it is useful to explicitly instantiate a template; that is, to tell the compiler to lay down the code for a specific version of that template even though you’re not creating an object at that point. To do this, you reuse the template keyword as follows: 

template class Bobbin<thread>;
template void sort<char>(char*[]);

Here’s a version of the Sorted.cpp example that explicitly instantiates a template before using it: 

//: C19:ExplicitInstantiation.cpp
#include "Urand.h"
#include "Sorted.h"
#include <iostream>
using namespace std;

// Explicit instantiation:
template class Sorted<int>;

int main() {
  Sorted<int> is;
  Urand<47> rand1;
  for(int k = 0; k < 15; k++)
    is.push_back(rand1());
  is.sort();
  for(int l = 0; l < is.size(); l++)
    cout << is[l] << endl;
} ///:~

In this example, the explicit instantiation doesn’t really accomplish anything; the program would work the same without it. Explicit instantiation is only for special cases where extra control is needed.

Explicit specification of template functions

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