Overloads and templates

Overloaded functions

In C++, two different functions can have the same name if their parameters are different; either because they have a different number of parameters, or because any of their parameters are of a different type. For example: 

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// overloading functions
#include <iostream>
using namespace std;

int operate (int a, int b)
{
  return (a*b);
}

double operate (double a, double b)
{
  return (a/b);
}

int main ()
{
  int x=5,y=2;
  double n=5.0,m=2.0;
  cout << operate (x,y) << '\n';
  cout << operate (n,m) << '\n';
  return 0;
}
 
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In this example, there are two functions called operate, but one of them has two parameters of type int, while the other has them of type double. The compiler knows which one to call in each case by examining the types passed as arguments when the function is called. If it is called with two int arguments, it calls to the function that has two int parameters, and if it is called with two doubles, it calls the one with two doubles.

In this example, both functions have quite different behaviors, the int version multiplies its arguments, while the double version divides them. This is generally not a good idea. Two functions with the same name are generally expected to have -at least- a similar behavior, but this example demonstrates that is entirely possible for them not to. Two overloaded functions (i.e., two functions with the same name) have entirely different definitions; they are, for all purposes, different functions, that only happen to have the same name.

Note that a function cannot be overloaded only by its return type. At least one of its parameters must have a different type.

Function templates

Overloaded functions may have the same definition. For example:

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// overloaded functions
#include <iostream>
using namespace std;

int sum (int a, int b)
{
  return a+b;
}

double sum (double a, double b)
{
  return a+b;
}

int main ()
{
  cout << sum (10,20) << '\n';
  cout << sum (1.0,1.5) << '\n';
  return 0;
}
 
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Here, sum is overloaded with different parameter types, but with the exact same body.

The function sum could be overloaded for a lot of types, and it could make sense for all of them to have the same body. For cases such as this, C++ has the ability to define functions with generic types, known as function templates. Defining a function template follows the same syntax than a regular function, except that it is preceded by the template keyword and a series of template parameters enclosed in angle-brackets <>:

template <template-parameters> function-declaration
The template parameters are a series of parameters separated by commas. These parameters can be generic template types by specifying either the class or typename keyword followed by an identifier. This identifier can then be used in the function declaration as if it was a regular type. For example, a generic sum function could be defined as:

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template <class SomeType>
SomeType sum (SomeType a, SomeType b)
{
  return a+b;
}


It makes no difference whether the generic type is specified with keyword class or keyword typename in the template argument list (they are 100% synonyms in template declarations).

In the code above, declaring SomeType (a generic type within the template parameters enclosed in angle-brackets) allows SomeType to be used anywhere in the function definition, just as any other type; it can be used as the type for parameters, as return type, or to declare new variables of this type. In all cases, it represents a generic type that will be determined on the moment the template is instantiated.

Instantiating a template is applying the template to create a function using particular types or values for its template parameters. This is done by calling the function template, with the same syntax as calling a regular function, but specifying the template arguments enclosed in angle brackets:

name <template-arguments> (function-arguments)
For example, the sum function template defined above can be called with:

 
 
x = sum<int>(10,20);


The function sum<int> is just one of the possible instantiations of function template sum. In this case, by using int as template argument in the call, the compiler automatically instantiates a version of sum where each occurrence of SomeType is replaced by int, as if it was defined as:

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int sum (int a, int b)
{
  return a+b;
}


Let's see an actual example:

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// function template
#include <iostream>
using namespace std;

template <class T>
T sum (T a, T b)
{
  T result;
  result = a + b;
  return result;
}

int main () {
  int i=5, j=6, k;
  double f=2.0, g=0.5, h;
  k=sum<int>(i,j);
  h=sum<double>(f,g);
  cout << k << '\n';
  cout << h << '\n';
  return 0;
}
 
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In this case, we have used T as the template parameter name, instead of SomeType. It makes no difference, and T is actually a quite common template parameter name for generic types.

In the example above, we used the function template sum twice. The first time with arguments of type int, and the second one with arguments of type double. The compiler has instantiated and then called each time the appropriate version of the function.

Note also how T is also used to declare a local variable of that (generic) type within sum:

 
 
T result;


Therefore, result will be a variable of the same type as the parameters a and b, and as the type returned by the function.
In this specific case where the generic type T is used as a parameter for sum, the compiler is even able to deduce the data type automatically without having to explicitly specify it within angle brackets. Therefore, instead of explicitly specifying the template arguments with:

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k = sum<int> (i,j);
h = sum<double> (f,g);


It is possible to instead simply write:
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k = sum (i,j);
h = sum (f,g);


without the type enclosed in angle brackets. Naturally, for that, the type shall be unambiguous. If sum is called with arguments of different types, the compiler may not be able to deduce the type of T automatically.

Templates are a powerful and versatile feature. They can have multiple template parameters, and the function can still use regular non-templated types. For example:

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// function templates
#include <iostream>
using namespace std;

template <class T, class U>
bool are_equal (T a, U b)
{
  return (a==b);
}

int main ()
{
  if (are_equal(10,10.0))
    cout << "x and y are equal\n";
  else
    cout << "x and y are not equal\n";
  return 0;
}
 
x and y are equal


Note that this example uses automatic template parameter deduction in the call to are_equal:

 
 
are_equal(10,10.0)


Is equivalent to:

 
 
are_equal<int,double>(10,10.0)


Since, in C++, integer literals with no suffix (such as 10) are always of type int, and floating-point literals without suffix (such as 10.0) are always of type double, there is no ambiguity possible, and thus the template arguments can be omitted in the call.

Non-type template arguments

The template parameters can not only include types introduced by class or typename, but can also include expressions of a particular type:

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// template arguments
#include <iostream>
using namespace std;

template <class T, int N>
T fixed_multiply (T val)
{
  return val * N;
}

int main() {
  std::cout << fixed_multiply<int,2>(10) << '\n';
  std::cout << fixed_multiply<int,3>(10) << '\n';
}
 
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The second argument of the fixed_multiply function template is of type int. It just looks like a regular function parameter, and can actually be used just like one.

But there exists a major difference: the value of template parameters is determined on compile-time to generate a different instantiation of the function fixed_multiply, and thus the value of that argument is never passed during runtime: The two calls to fixed_multiply in main essentially call two versions of the function: one that always multiplies by two, and one that always multiplies by three. For that same reason, the second template argument needs to be a constant expression (it cannot be passed a variable).
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