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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>:: before its name 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

Templated Methods

class Show {
  public:
    void m(double d) const { cout << "Double: " << d << '\n'; }
    template <typename T>
    void m(T v) const { cout << "Value: " << v << "\n"; }
};

int main() {
    Show s;
    s.m("hello");
    s.m(1.2);
    s.m(&s);
}

Value: hello
Double: 1.2
Value: 0x7ffc25303e5f

If all you need is a templated method, then there’s no need to templatize the entire class, just the method that needs arguments of varying types.

If the number of different types is small, then avoid templates; write several methods with different argument types.