module05 - achrafelkhnissi/CPP_Modules GitHub Wiki

Table of contents

introduction to templates

  • Templates are parameterized by one or more template parameters, of three kinds: type template parameters, non-type template parameters, and template template parameters.

  • Function template is a blueprint for defining a family of functions. The compiler uses a function template to generate a function definition when necessary. A function definition that is generated from a template is an instance or an instantiation of the template. A function template is a parametric function definition, where a particular function instance is created by one or more parameter values. The compiler generates each template instance once.

  • Need to particularly careful when using pointer types as template arguments, as you could pass the address, instead of dereferenced value as a parameter.

  • Template specialization defines a behaviour that is different from the standard template. The definition of a template specialization must come after a declaration or definition of the original template. The specialization must also appear before its first use. Otherwise, the program won't compile.

  • It is possible to overload a function / template with another function / template.

  • Class templates are templates the compiler can use to create classes. Class templates are a powerful mechanism for generating new class types automatically. A significant portion of the Standard Library is built entirely on the ability to define templates, particularly the Standard Template Library, which includes many class and function templates.

  • A class template is a parameterized type - a recipe for creating a family of class types, using one or more parameters. It is not a class, but just a recipe for creating classes, because this is the reason for many of the constraints on how you define class templates.

class_template

  • There are many applications for class templates but they are perhaps most commonly used to define container classes. These are classes that can contain sets of objects of a given type, organized in a particular way. In a container class the organization of the data is independent of the type of objects stored.

  • Instantiation of a class template doesn't instantiate any of its member functions unless they are also used. At link time, identical instantiations generated by different translation units are merged.

From C - parametric macros

  • macros have limitations with the edge effect
#include <stdio.h>

int max_int (int x, int y) {return (x>=y ? x : y);}
float max_float (float x, float y) {return (x>=y ? x : y);}
char max_char (char x, char y) {return (x>=y ? x : y);}

int foo(int x) {printf("Long computing time\n"); return x; }

#define max(x, y) (((x) > = (y))? (x) : (y)) // parametric macro
// cpp (C pre-processor) will find all the defines
// when it has an edge effect, it can be messy

int main(void)
{
	int a = 21;
	int b = 42;

	printf("Max of %d and %d is %d\n", a, b, max_int(a, b));
    printf("Max of %d and %d is %d\n", a, b, max(a, b));

    float c = -1.7f;
    float d = 4.2f;

    printf("Max of %f and %f is %f\n", c, d, max_float(c, d));
    printf("Max of %f and %f is %f\n", c, d, max(c, d));

	char e = 'a';
	char f = 'b';

	printf("Max of %c and %c is %c\n", e, f, max_int(e, f));
	printf("Max of %c and %c is %c\n", e, f, max(e, f));

	//*but .....
	printf("Max of %d and %d is %d\n", a, b, max_int(foo(a), foo(b)));
	printf("Max of %d and %d is %d\n", a, b, max(foo(a), foo(b))); // it can be problematic when foo(a) executes twice after the macro is executed

	return (0);
}
  • In C, void * is an option to take in different types of data. A lot of dereferencing is required when you run a big program, which can influence program performance
struct list_s {
	void * content;
	size_t size; // needs to know the size to move
	struct list * next;
} list_t;

list_t* list_new(void * content, size_t size);
void list_delete(list ** list);

templates

  • We need to ask the compiler to instantiate our template. There are two ways: explicit instantiation and implicit instantiation
#include <iostream>

template<typename T> // tell the compiler that we're writing a template

T const & max(T const & x, T const & y) { // use the address and not a copy will save space
	return (x >= y? x : y); // apart from a scala type, it could also used with instances of class if they've implemented the operator overload
}

int foo(int x) {
	std::cout << "Long computing time" << std::endl;
	return x;
}

int main(void)
{
	int a = 21;
	int b = 42;

	std::cout << "Max of " << a << " and " << b << " is ";
	std::cout << max<int>(a, b) << std::endl; // explicit instantiation -> this is preferred
    std::cout << "Max of " << a << " and " << b << " is ";
    std::cout << max(a, b) << std::endl; // implicit instantiation -> it might not work for complex classes

    float c = -1.7f;
    float d = 4.2f;

    std::cout << "Max of " << c << " and " << d << " is ";
    std::cout << max<float>(c, d) << std::endl; // explicit instantiation
    std::cout << "Max of " << c << " and " << d << " is ";
    std::cout << max(c, d) << std::endl; // implicit instantiation

	char e = 'a';
	char f = 'z';

	std::cout << "Max of " << e << " and " << f << " is ";
	std::cout << max<char>(e, f) << std::endl; // explicit instantiation
	std::cout << "Max of " << e << " and " << f << " is ";
    std::cout << max(e, f) << std::endl; // implicit instantiation

    // no problem here

    int ret = max<int>(foo(a), foo(b)); // explicit instantiation -> it will not be macros but actual functions, which are written by the compiler. So foo(a) and foo(b) will only be run once, and the results will be passed as parameters
	std::cout << "Max of " << a << " and " << b << " is ";
	std::cout << ret << std::endl;

	return 0;
}
  • Compilers can also write template for classes and structures.

template for structure

#include <iostream>

template<typename T>
class List {

public:
    List<T>(T const & content) {
    	// etc...
    }

    List<T>(List<T> const & list) {
    	//etc...
    }

    ~List<T>(void) {
        //etc...
    }

    //etc...

private:
	T * _content; // it works the same without *
    List<T> * _next;
};

/*******************************************************/

int main(void)
{
	List<int> a(42);
	List<float> b(3.14f);
	List<List<int>> c(a); // A list of list of integers

	//etc...

	return 0;
}

default types

  • tpp file can be used as a naming convention for templates

  • Default type means if I don't tell you what the type is, the compiler can assume that it's this type

Template<typename T = float>

class Vertex {
public:
	Vertex (T const & x, T const & y, T const & z): _x(x), _y(y), _z(z) {}
	~Vertex(void){}

	T const & getx(void) const {return this->_x};
    T const & gety(void) const {return this->_y};
    T const & getz(void) const {return this->_z};

    // etc...

private:
	T const _x;
    T const _y;
    T const _z;

    Vertex(void);
};

template<typename T>
std::ostream & operator<<(std::ostream & o, Vertex<T> const & v) {
	std::cout.precision(1);
	o << setiosflag(std::ios::fixed);
	o << "Vertex( ";
	o << v.getX() << ", ";
	o << v.getY() << ", ";
	o << v.getZ();
	o << " )";
	return o;
}

/*******************************************************/

int main(void)
{
	Vertex<int> v1(12, 23, 34);
	Vertex<> v2(12, 23, 34); // 12, 23, 34 will be implicitly converted to floats

	std::cout << v1 << std::endl;
    std::cout << v2 << std::endl;

    return 0;
}

Vertex( 12, 23, 34);
Vertex( 12.0, 23.0, 34.0);

template specialization

  • Full or partial template specialization are the same as overload. Partial specializations are only allowed for class templates.

  • A class template by itself is not a type, or an object, or any other entity. No code is generated from a source file that contains only template definitions. In order for any code to appear, a template must be instantiated: the template arguments must be provided so that the compiler can generate an actual class (or function, from a function template).

/***************FULL SPECIALISATION**********************/

template <typename T, typename U>
class Pair {

public:
	Pair<T, U>(T const & lhs, T const & rhs) : _lhs(lhs), _rhs(rhs) {
		std::cout << "Generic template" << std::endl;
		return;
	}

	~Pair<T, U>(void) {}

	T const & fst(void) const {return this->lhs;}
    U const & snd(void) const {return this->rhs;}

private:

	T const & _lhs;
    U const & _rhs;

    Pair<T, U>(void);
};

/***************PARTIAL SPECIALISATION**********************/

template <typename U>
class Pair<int, U> { // syntax is different here

    public:
    Pair<T, U>(int & lhs, T const & rhs) : _lhs(lhs), _rhs(rhs) {
        std::cout << "Int partial specialization" << std::endl;
        return;
    }

    ~Pair<T, U>(void) {}

    int     & fst(void) const {return this->lhs;}
    U const & snd(void) const {return this->rhs;}

    private:

    int     & _lhs;
    U const & _rhs;

    Pair<T, U>(void);
};

/*******************************************************/

template <>
class Pair<bool, bool> { // syntax is different here

public:
    Pair<bool, bool>(bool lhs, bool rhs) : _lhs(lhs), _rhs(rhs) {
        std::cout << "bool/ bool full specialization" << std::endl;
        this->_n = 0;
        this->_n |= static_cast<int>(lhs) << 0; // the first bit
		this->_n |= static_cast<int>(rhs) << 1; // the second bit
        return;
    }

    ~Pair<bool, bool>(void) {}

	bool fst(void) const {return (this->_n & 0x01);}
	bool snd(void) const {return (this->_n & 0x02);}

private:

    int     _n;

    Pair<bool, bool>(void);
};

/*******************************************************/

template<typename T, typename U>
std::ostream & operator<<(std::ostream & o, Pair<T, U> const & p) {
    o << "Pari(" << p.fst() << ", " << p.snd() << " )";
    return o;
}

std::ostream & operator<<(std::ostream & o, Pair<bool, bool> const & p) {
    o << std::boolalpha << "Pari(" << p.fst() << ", " << p.snd() << " )"; // std::boolalpha will print true/false instead of 1 / 0
	return o;
}

/*******************************************************/

int main(void){
	Pair<int, int> p1(4, 2); // if one parameter matches, it will use the partial specialization
	Pair<std::string, float> p2(std::string "Pi", 3.14f);
	Pair<float, bool> p3(4.2f, true);
	Pair<bool, bool> p4(true, false);

	std::cout << p1 << std::endl;
	std::cout << p2 << std::endl;
	std::cout << p3 << std::endl;
    std::cout << p4 << std::endl;

    return 0;
}
Int partial specialization
Generic template
Generic template
bool/ bool full specialization
Pair(4, 2)
Pair(Pi, 3.14f)
Pair(4.2f, 1)
Pair(true, false)

resources

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