Function templates
Function
templates are special functions that can operate with generic types.
This allows us to create a function template whose functionality can be adapted
to more than one variable type or class without repeating the code for each
type.
This is achieved through template
parameters. A template parameter is a special kind of parameter that can be
used to pass a type as parameter. These function templates can use these
parameters as if they were regular types.
The format for declaring
function templates with type parameters is:
template <class identifier> function_declaration;
template <typename identifier> function_declaration;
template <typename identifier> function_declaration;
The only difference between
both prototypes is the use of either the keyword class or
the keyword typename. Its use is indistinct since both expressions have exactly the
same meaning and behave exactly the same way.
For example, to create a
template function that returns the greater one of two objects we could use:
template <class myType> myType GetMax (myType a, myType b) {
return (a>b?a:b);
}
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Here we have created a
template function with myType as its template parameter.
This template parameter represents a type that has not yet been specified, but
that can be used in the template function as if it were a regular type. As you
can see, the function template GetMax returns the greater of two
parameters of this still-undefined type.
To use this function template
we use the following format for the function call:
function_name <type> (parameters);
For example, to call GetMax
to compare two integer values of type int we can write:
int x,y; GetMax <int> (x,y);
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When the compiler encounters
this call to a template function, it uses the template to automatically
generate a function replacing each appearance of myType by
the type passed as the actual template parameter (int in
this case) and then calls it. This process is automatically performed by the
compiler and is invisible to the programmer.
Here is the entire example:
// function template #include <iostream> using namespace std;
template <class T> T GetMax (T a, T b) {
T result;
result = (a>b)? a : b;
return (result);
}
int main () { int i=5, j=6, k;
long l=10, m=5, n;
k=GetMax<int>(i,j);
n=GetMax<long>(l,m);
cout << k << endl;
cout << n << endl;
return 0;
}
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6
10
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In this case we have used T as the template parameter name instead of myType because it is shorter and in fact is a very common template
parameter name. But you can use any identifier you like.
In the example above we used
the function template GetMax() twice. The first time with
arguments of type int and the second one with
arguments of type long. The compiler has
instantiated and then called each time the appropriate version of the function.
As you can see, the type T is used within the GetMax()
template function even to declare new objects of that type:
T result;
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Therefore, result will be an object of the same type as the parameters a and b when the function template is
instantiated with a specific type.
In this specific case where
the generic type T is used as a parameter for GetMax the compiler can find out automatically which data type has to
instantiate without having to explicitly specify it within angle brackets (like
we have done before specifying <int> and <long>). So we could have written instead:
int i,j; GetMax (i,j);
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Since both i and j are of type int, and
the compiler can automatically find out that the template parameter can only be
int. This implicit method produces exactly the same result:
// function template II #include <iostream> using namespace std;
template <class T> T GetMax (T a, T b) {
return (a>b?a:b);
}
int main () { int i=5, j=6, k;
long l=10, m=5, n;
k=GetMax(i,j);
n=GetMax(l,m);
cout << k << endl;
cout << n << endl;
return 0;
}
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6
10
|
Notice how in this case, we
called our function template GetMax() without explicitly
specifying the type between angle-brackets <>. The
compiler automatically determines what type is needed on each call.
Because our template function
includes only one template parameter (class T) and
the function template itself accepts two parameters, both of this T type, we cannot call our function template with two objects of
different types as arguments:
int i; long l; k = GetMax (i,l);
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This would not be correct,
since our GetMax function template expects two arguments of the same type, and
in this call to it we use objects of two different types.
We can also define function
templates that accept more than one type parameter, simply by specifying more
template parameters. For example:
template <class T, class U> T GetMin (T a, U b) {
return (a<b?a:b);
}
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In this case, our function
template GetMin() accepts two parameters of different types and returns an object
of the same type as the first parameter (T) that
is passed. For example, after that declaration we could call GetMin() with:
int i,j; long l; i = GetMin<int,long> (j,l);
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or simply:
i = GetMin (j,l);
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even though j and l have different types. But the compiler
can determine the apropriate instantiation anyway.
Class templates
We
also have the possibility to write class templates, so that a class can have
members that use template parameters as types. For example:
template <class T> class pair { T values [2];
public:
pair (T first, T second)
{
values[0]=first; values[1]=second;
}
};
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The class that we have just
defined serves to store two elements of any valid type. For example, if we
wanted to declare an object of this class to store two integer values of type int with the values 115 and 36 we would write:
pair<int> myobject (115, 36);
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this same class would also be
used to create an object to store any other type:
pair<float> myfloats (3.0, 2.18);
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The only member function in
the previous class template has been defined inline within the class
declaration itself. In case that we define a function member outside the
declaration of the class template, we must always precede that definition with
the template
<...> prefix:
// class templates #include <iostream> using namespace std; template <class T> class pair { T a, b;
public:
pair (T first, T second)
{a=first; b=second;}
T getmax ();
};
template <class T> T pair<T>::getmax ()
{
T retval;
retval = a>b? a : b;
return retval;
}
int main () { pair <int> myobject (100, 75);
cout << myobject.getmax();
return 0;
}
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100
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Notice the syntax of the
definition of member function getmax:
template <class T> T pair<T>::getmax ()
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Confused by so many T's? There are three T's in
this declaration. The first one is the template parameter. The second T refers to the type returned by the function. And the third T (the one between angle brackets) is also a requirement. It
specifies that this function's template parameter is also the class template parameter.
Template specialization
If we
want to define a different implementation for a template when a specific type
is passed as template parameter, we can declare a specialization of that
template.
For example, let's suppose
that we have a very simple class called container that
can store one element of any type and that it has just one member function
called increase, that increases its value. But we find that when it stores an
element of type char it would be more convenient to have a completely different
implementation with a function member uppercase, so
we decide to declare a class template specialization for that type:
// template specialization #include <iostream> using namespace std;
template <class T> class container { T element;
public:
container (T arg) {element=arg;}
T increase () {return ++element;}
};
template <> class container <char> { char element;
public:
container (T arg) {element=arg;}
char uppercase ();
};
template <> char container<char>::uppercase() {
if ((element>='a')&&(element<='z'))
element+='A'-'a';
return element;
}
int main () { container<int> myint (7);
container<char> mychar ('j');
cout << myint.increase() << endl;
cout << mychar.uppercase() << endl;
return 0;
}
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8
J
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This is the syntax used in
the class template specialization:
template <> class container <char> { ... }; |
First of all, notice that we
precede the class template name with template<>. This is a requirement in standard C++ for all template
specializations, although some compilers, as an anachronism, support to declare
template specialization without this prefix.
But more important than this
prefix, is the <char> specialization parameter after the class template name. This
specialization parameter itself identifies the type for which we are going to
declare a template class specialization (char).
Notice the differences between the generic class template and the
specialization:
template <class T> class container { ... }; template <> class container <char> { ... }; |
The first line is the generic
template, and the second one is the specialization.
When we declare
specializations for a template class, we must also define all its members,
because there is no inheritance of members from the generic template to the
specialization.
Non-type parameters for templates
Besides
the template arguments that are preceded by the class or typename keywords , which represent types, templates can also have
regular typed parameters, similar to those found in functions. As an example,
have a look at this class template that is used to contain sequences of
elements:
// sequence template #include <iostream> using namespace std;
template <class T, int N> class sequence { T memblock [N];
public:
void setmember (int x, T value);
T getmember (int x);
};
template <class T, int N> void sequence<T,N>::setmember (int x, T value) { memblock[x]=value;
}
template <class T, int N> T sequence<T,N>::getmember (int x) {
return memblock[x];
}
int main () { sequence <int,5> myints;
sequence <double,5> myfloats;
myints.setmember (0,100);
myfloats.setmember (3,3.1416);
cout << myints.getmember(0) << '\n';
cout << myfloats.getmember(3) << '\n';
return 0;
}
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100
3.1416
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It is also possible to set
default values or types for class template parameters. For example, if the
previous class template definition had been:
template <class T=char, int N=10> class sequence {..}; |
We could create objects using
the default template parameters by declaring:
sequence<> myseq;
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Which would be equivalent to:
sequence<char,10> myseq;
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Templates and multiple-file projects
From
the point of view of the compiler, templates are not normal functions or
classes. They are compiled on demand, meaning that the code of a template
function is not compiled until an instantiation with specific template
arguments is required. At that moment, when an instantiation is required, the
compiler generates a function specifically for those arguments from the
template.
When projects grow it is
usual to split the code of a program in different source code files. In these
cases, the interface and implementation are generally separated. Taking a
library of functions as example, the interface generally consists of
declarations of the prototypes of all the functions that can be called. These
are generally declared in a "header file" with a .h extension, and
the implementation (the definition of these functions) is in an independent
file with c++ code.
Because templates are
compiled when required, this forces a restriction for multi-file projects: the
implementation (definition) of a template class or function must be in the same
file as its declaration. That means that we cannot separate the interface in a
separate header file, and that we must include both interface and
implementation in any file that uses the templates.
Since no code is generated
until a template is instantiated when required, compilers are prepared to allow
the inclusion more than once of the same template file with both declarations
and definitions in a project without generating linkage errors.
