This post aims to cover all the major topics that C programmers need to know before they start writing C++ programs. I kept this post as short and concise as possible to enable people to use this as a quick reference to quickly jump into C++. This post assumes that you have prior knowledge of both C and object oriented-programming concepts. Each topic is quickly covered in a code snippet and some additional explanation is provided if necessary.
Input and output in C++ is easy, you use “cout” and “cin”. When printing with “cout”, you separate what your printing with “<<”; “endl” prints a new line.
using namespace std; //namespaces talked about below
#include <iostream> //Include statement for terminal IO.
int main()
{
cout << "Hello World" << endl; // HELLO WORLD!
int a;
cin >> a; //inputs an int into a -- notice how arrows face the direction of IO
cout << "You entered: " << a << endl; //prints what you entered
return 0; // return sucess code
}Save your C++ programs with the extension “.cpp”, you then can compile and run it with g++. Compiling a C++ program with g++ is nearly the same as compiling a C program with GCC.
ex:
g++ helloWorld.cpp -o hello
./helloName spaces are used to enable you to have multiple functions/methods called the same thing and not conflict with one another. You use “namespacename::function/variable” to access something inside of a specific namespace. To prevent you from always having to type “namespacename::”, you can use a namespace which makes that namespace default.
using namespace std; //tells compiler we want to use std namespace
#include <iostream>
namespace foo //declares a namespece named foo
{
int a, b;
void fun()
{
cout << "Inside foo namespace" << endl;
}
}
namespace bar
{
void fun() //declares a function with the same name as another function
{
cout << "Inside bar namespace" << endl;
}
}
using namespace foo; //start useing foo instead of std as selected namespace
int main()
{
fun();
bar::fun();
int a = 5;
foo::a = 12;
std::cout << "a: " << a << endl; //had to use std::cout since the default namespace is foo
std::cout << "foo::a: " << foo::a << endl;
return 0;
}Global variables are similar to C, however, you can now reference a global members with the “::” accessor.
using namespace std;
#include <iostream>
double bar = 64;
int main ()
{
double bar = 12;
cout << "Local bar: " << bar << endl; //prints 12
cout << "Global bar: " << ::bar << endl; //prints 64
return 0;
}This is NOT simply a pointer. In the following example pi, and x now are treated as the same exact variable. You cannot later change the pointer destination for x.
double pi = 3.145;
double &x = pi; //pi is x
x = 2.1;
cout << "pi: " << pi << " x: " << x << endl; // prints pi: 2.1 x: 2.1In C, everything was passed by value; C++ allows us to pass variables by reference. This is very powerful, in languages like Java, only Objects are passed by reference. C++ lets you decide exactly what gets passed by reference or by value – even primitives.
using namespace std;
#include <iostream>
void change (int &r, int s) //& infront of variable means that it will get passed by reference
{
r = 100;
s = 200;
}
int main()
{
int x = 1;
int y = 2;
cout << x << ", " << y << endl;
change(x, y);
cout << x << ", " << y << endl;
return 0;
}Same code in C. This method still works in C++.
void change(double *r, double s)
{
*r = 100;
s = 200;
}
int main()
{
int x = 1;
int y = 2;
printf("%d, %d", x, y);
change(&x, y);
printf("%d, %d", x, y);
return 0;
}A function can return a value reference which can be treated as a variable. In the following example, a function returns the reference to the variable which is the smallest.
using namespace std;
#include <iostream>
int &smallest (int &x, int &y) //smallest returns a reference to a variable
{
if (x < y)
return x;
else
return y;
}
int main ()
{
int k = 33;
int m = 2;
cout << "k: " << k << " m: " << m << endl; // prints k: 33 m: 2
smallest (k, m) = 10; // MAGIC!
cout << "k: " << k << " m: " << m << endl; // prints k: 33 m: 10
return 0;
}Inline can be used to replace a function which contains very simple logic – no for loops, etc. Like a macro, this will be inserted everywhere the code is used; a drawback to inline methods is that the compiled source will be larger. But, they typically run faster.
using namespace std;
#include <iostream>
inline int square(int x) //macro like method
{
return x * x;
}
int main()
{
int k = 4;
cout << square(k) << endl; //prints 16
return 0;
}Exceptions might help you stop segmentation faulting. The important thing to notice is that you can throw about any type in a try block.
int x;
cout << "Type a number: ";
cin >> x;
cout << endl;
try
{
if(a > 150)
throw 150;
if(a < 15)
throw 15;
throw a % 2;
}
catch(int result)
{
cout << result << " was thrown." << endl;
}This is exactly like default parameters in Python. If a function is called without the parameter, it is assumed to be that value.
double multiply(double x, double y = 5) // y has the default value of 5
{
return x * y;
}
int main()
{
cout << multiply(4) << endl; // 20
cout << multiply(4, 4) endl; // 16
return 0;
}Like Java and Python, you can overload methods in C++. Not only can you overload the methods, but, the return type of the methods which are overloaded don’t have to match.
double add(double x)
{
return x;
}
double add(double x, double y)
{
return x + y;
}
int add(int x, int y)
{
return x + y;
}
int main()
{
cout << add(4) << endl; // 4
cout << add(4.0, 4.0) << endl; // 8
return 0;
}You can redefine basic operators like (+,/,-,<<,>>, +=) for certain data types by using operator overloading.
using namespace std;
#include <iostream>
struct tuple //since tuple is defined elsewhere, we need to use :: to access it
{
int x;
int y;
};
::tuple operator + (int a, ::tuple b)
{
::tuple r; //creates the tuple from our file -- tuple is defined elsewhere
r.x = a + b.x;
r.y = a + b.y;
return r;
}
::tuple operator * (int a, ::tuple b)
{
::tuple r; //creates the tuple from our file -- tuple is defined elsewhere
r.x = a * b.x;
r.y = a * b.y;
return r;
}
int main ()
{
::tuple k, m; // No need to type "struct vector"
k.x = 2; // To be able to write
k.y = -1; // k = vector (2, -1)
// see chapter 19.
m = 3 + k; // Magic!
cout << "(" << m.x << ", " << m.y << ")" << endl;
return 0;
}In C++ you can use a template class to create a method which has generic return and parameter types.
template <class ttype> //function with 1 generic type
ttype max (ttype a, ttype b)
{
ttype r;
r = a;
if (b < a)
r = b;
return r;
}
template <class type1, class type2> //function with 2 generic types
type1 maximum (type1 a, type2 b)
{
type1 r, b_converted;
r = a;
b_converted = (type1) b;
if (b_converted > a)
r = b_converted;
return r;
}Malloc and free still exists in C++, however, people typically use “new” and “delete” instead because it is cleaner.
int *i = new int; //i = malloc(sizeof(int)); //c code
*i = 55;
delete i; //free(i); // c code
i = new int[15];
i[0] = 99;
delete i;You can now add functions to structs.
struct pair
{
int i;
int x;
int sum()
{
return i + x;
}
};The syntax of a class is similar to a struct.
class Pair
{
public: // encapsulation covered in 2 sections
int i;
int x;
int sum()
{
return i + x;
}
};Class constructors are similar to constructors in Java. Class destructor are simply the name of the class with a “~” sign in front of it. It is important to free any allocated memory in the class deconstruct since C++ does not have a garbage collector.
class Pair
{
public:
int i;
int x;
Pair(int i1, int i2) //constructor
{
i = i1;
x = i2;
}
~Pair() //class deconstructor
{
//delete any memory you have!
}
int sum()
{
return i + x;
}
};
// in main or somewhere
Pair t (12, 14); //creates a tuple on the stack
Pair* tt = new Pair(12, 15); //allocates memory for the tuple on the heap
cout << t.sum() << endl; //prints 26
cout << tt->sum() << endl; //prints 27Like Java, you can declare who can view access certain members of a class.
class Person
{
protected:
int age;
string name;
public:
Person(int age, string name)
{
}
~Person()
{
}
private:
void increaseAge()
{
age++;
}
};When you use the “this” key word, you are getting the pointer to the class that you are in.
class Person
{
protected:
int age;
string name;
public:
Person(int age, string name)
{
this->age = age;
this->name = name;
}
~Person()
{
}
private:
void increaseAge()
{
age++;
}
};Classes can inherit variables and methods from other classes. The most important thing to remember is that if you ever want to override a method in a child class, you must declare the method as “virtual”.
class Pair
{
protected:
int x;
int y;
public:
Pair(int i1, int i2)
{
x = i1;
y = i2;
}
virtual int sum()
{
return x + x;
}
};
class Triple: public Pair
{
protected:
int z;
public:
Triple(int i1, int i2, int i3): Pair(i1, i2) //calls the parent's constructor
{
z = i3;
}
int sum()
{
return x + y + z;
}
};Abstract classes are simply classes which cannot be instantiated. To do this in C++ you simply set a virtual function equal to zero.
class Animal
{
public:
virtual void speak()=0;
};
class Cat: public Animal
{
public:
void speak()
{
cout << "Meow" << endl;
}
};If you wish to use a method prototype in a class, you must use namespace syntax to define the method elsewhere. This is particularly useful for breaking classes into multiple files. It is common to declare the class in a header file and then implement the functions in a cpp file.
class Animal
{
public:
virtual void speak()=0;
};
class Cat: public Animal
{
public:
void speak()
{
cout << "Meow" << endl;
}
int fly(); //method prototype
};
int Cat::fly()
{
return 42;
}Since C++ has classes, you can work with strings in a pleasant way.
using namespace std;
#include <iostream>
int main()
{
string str1 = "Hello"; // string "Hello"
string str2("World"); // string "World"
string str1Copy(str1); // string "Hello"
//initializes string by a character and number of occurrences
string str4(5, '$'); // string "$$$$$$"
//string concatenation
string greeting = str1 + " " + str2;
cout << greeting << endl;
//length of a string
int len = str1.size();
cout << "str1.size(): " << len << endl;
//clear all characters from a string
greeting.clear();
cout <<"Greeting: "<< greeting << endl;
string numbers = "0123456789";
//returns first character in string
char first = numbers.front();
//returns last character in string
char back = numbers.back();
//gets a character at a certain position
char second = numbers.at(1);
char secondAlt = numbers[1]; //array syntax still works on strings
cout << "first: " << first << endl;
cout << "back: " << back << endl;
cout << "second: " << second << endl;
//substr(a, b) function returns a substring of b length
//starting from index a. If there is no second argument, it
//goes to the end.
cout << numbers.substr(2, 7) << endl;
//replace(a, b, str) replaces b character from a index by str
string str6 = "This is a examples";
str6.replace(2, 7, "ese are test");
cout << str6 << endl;
return 0;
}File IO is significantly different from C. I will quickly glance over a few examples which should give you most of what you need to start writing basic programs.
Reading a file example by character.
using namespace std;
#include <iostream>
#include <fstream> // header for files
int main()
{
fstream f;
char c;
f.open("p022_names.txt", ios::in);
while(!f.eof())
{
f.get(c);
cout << c;
}
f.close();
}Reading lines from a file using strings.
using namespace std;
#include <iostream>
#include <fstream>
#include <string>
int main ()
{
string line;
ifstream myfile ("example.txt");
if(myfile.is_open()) //checks to see if file open successfully
{
while(getline(myfile,line)) //gets contents of file and puts them in a string
{
cout << line << '\n';
}
myfile.close();
}
else
{
cout << "Unable to open file";
}
return 0;
}Writing to a file example.
using namespace std;
#include <iostream>
#include <fstream> // Header for files
int main()
{
fstream f;
char c;
f.open("p022_names.txt", ios::out);
f << "stuff in the file " << endl;
int i = 4;
f << i << " this is also in the text file" << endl;
f.close();
}You now know enough C++ to start programming with it. If you want to take your skills to the next level, I would recommend start working on a few projects in C++ and get a comprehensive C++ book.