Why can templates only be implemented in the header file?

 

Templates in C++ can be implemented in header files because they need to be available at compile time to generate code for different data types. This is because template code is not compiled until an instantiation with a specific data type occurs. If the template implementation were in a separate source (.cpp) file, the compiler wouldn't have access to the template code during the compilation of other source files that use the template.

Here's an example to illustrate this:

Let's say you want to create a simple template function that swaps two values of any data type:

cpp

// Swap.h (Header File)
#ifndef SWAP_H
#define SWAP_H

template <typename T>
void swap(T &a, T &b) {
    T temp = a;
    a = b;
    b = temp;
}

#endif

Now, you have a separate source file that uses this template:

cpp

// main.cpp (Source File)
#include <iostream>
#include "Swap.h"

int main() {
    int x = 5, y = 10;
    std::cout << "Before swap: x = " << x << ", y = " << y << std::endl;
    
    swap(x, y);  // Compiler needs to know how to instantiate swap<int>(int&, int&)
    
    std::cout << "After swap: x = " << x << ", y = " << y << std::endl;
    return 0;
}

When you compile main.cpp, the compiler needs to generate code for the swap<int>(int&, int&) function instantiation. It doesn't know the details of how to do this unless it can see the template implementation (from the Swap.h header) at the time of compilation.

If you were to put the template implementation in a separate source file (e.g., Swap.cpp), the compiler wouldn't have access to it when compiling main.cpp. As a result, it wouldn't be able to generate the necessary code for the swap<int>(int&, int&) function instantiation, leading to linker errors.

So, in summary, templates are typically implemented in header files to ensure that the template code is available during the compilation of source files that use the template, allowing the compiler to generate specialized code for different data types as needed.