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CS253 Template Implementation

Pick One

• There are two fundamentally different ways to implement a template class:
  1. The *.h file contains inline method bodies (code), all in the class declaration itself. Method bodies are inside class Foo { ... };.
  2. The *.h file contains the class declaration, with no method bodies. After that, comes each method body.
• The first technique is shorter, but mixes interface & implementation, and so is hard to read.
• Neither technique involves a *.cc file. Though the C++ standard allows for this, current compilers can’t handle it.
• No *.h file should contain a using namespace declaration.

Inline

#include <iostream>

template<typename T>
class Foo {
    T data;
  public:
    Foo(const T &value) : data(value) { }
    T get() const { return data; }
};

template<typename T>
std::ostream & operator<<(std::ostream &os, const Foo<T> &f) {
    return os << f.get();
}

• The ctor and get() method bodies are inside the class.
• operator<< is outside of the class, because it’s not a method.

Not Inline

#include <iostream>

template<typename T>
class Bar {
    T data;
  public:
    Bar(const T &);
    T get() const;
};

template<typename T>
Bar<T>::Bar(const T &value) : data(value) {
}

template<typename T>
T Bar<T>::get() const {
    return data;
}

template<typename T>
std::ostream & operator<<(std::ostream &os,
                          const Bar<T> &b) {
    return os << b.get();
}

• The class declaration itself contains no method bodies.
• Each body is preceded by template<typename T>, because each is defining a family of functions.
• Each method needs Bar<T>:: to indicate that it’s a method and not a free function.

Example

from ~cs253/Example/Templates:

$ cp ~cs253/Example/Templates/* .
$ ls
Bar.h  Foo.h  index.php  Makefile  test.cc
$ cat test.cc
#include "Foo.h"
#include "Bar.h"
#include <iostream>
int main() {
    Foo<short> f(12);
    Bar<long> b(34);
    std::cout << f << ' ' << f.get() << ' ' << sizeof(f) << '\n'
	      << b << ' ' << b.get() << ' ' << sizeof(b) << '\n';
}
$ make && ./test
g++ -Wall -Wextra -Wpedantic -Werror -Wfatal-errors  -o test test.cc
12 12 2
34 34 8