Basic data types & declaration in C++

Fundamental data types

When programming, we store the variables in our computer’s memory, but the computer has to know what kind of data we want to store in them, since it is not going to occupy the same amount of memory to store a simple number than to store a single letter or a large number, and they are not going to be interpreted the same way.

The memory in our computers is organized in bytes. A byte is the minimum amount of memory that we can manage in C++. A byte can store a relatively small amount of data: one single character or a small integer (generally an integer between 0 and 255). In addition, the computer can manipulate more complex data types that come from grouping several bytes, such as long numbers or non-integer numbers.

Next you have a summary of the basic fundamental data types in C++, as well as the range of values that can be represented with each one:

charCharacter or small integer.1bytesigned: -128 to 127
unsigned: 0 to 255
short int(short)Short Integer.2bytessigned: -32768 to 32767
unsigned: 0 to 65535
intInteger.4bytessigned: -2147483648 to 2147483647
unsigned: 0 to 4294967295
long int (long)Long integer.4bytessigned: -2147483648 to 2147483647
unsigned: 0 to 4294967295
boolBoolean value. It can take one of two values: true or false.1bytetrue or false
floatFloating point number.4bytes+/- 3.4e +/- 38 (~7 digits)
doubleDouble precision floating point number.8bytes+/- 1.7e +/- 308 (~15 digits)
long doubleLong double precision floating point number.8bytes+/- 1.7e +/- 308 (~15 digits)
wchar_tWide character.or 4 bytes1 wide character

* The values of the columns Size and Range depend on the system the program is compiled for. The values shown above are those found on most 32-bit systems. But for other systems, the general specification is that int has the natural size suggested by the system architecture (one “word”) and the four integer types charshortint andlong must each one be at least as large as the one preceding it, with char being always one byte in size. The same applies to the floating point types floatdouble and long double, where each one must provide at least as much precision as the preceding one.

Declaration of variables

In order to use a variable in C++, we must first declare it specifying which data type we want it to be. The syntax to declare a new variable is to write the specifier of the desired data type (like int, bool, float…) followed by a valid variable identifier. For example:

int a;
float mynumber;

These are two valid declarations of variables. The first one declares a variable of type int with the identifier a. The second one declares a variable of type float with the identifier mynumber. Once declared, the variables a andmynumber can be used within the rest of their scope in the program.

If you are going to declare more than one variable of the same type, you can declare all of them in a single statement by separating their identifiers with commas. For example:

int a, b, c;

This declares three variables (ab and c), all of them of type int, and has exactly the same meaning as:

int a;
int b;
int c;

The integer data types charshortlong and int can be either signed or unsigned depending on the range of numbers needed to be represented. Signed types can represent both positive and negative values, whereas unsigned types can only represent positive values (and zero). This can be specified by using either the specifiersigned or the specifier unsigned before the type name. For example:

unsigned short int NumberOfSisters;
signed int MyAccountBalance;

By default, if we do not specify either signed or unsigned most compiler settings will assume the type to be signed, therefore instead of the second declaration above we could have written:

int MyAccountBalance;

with exactly the same meaning (with or without the keyword signed)

An exception to this general rule is the char type, which exists by itself and is considered a different fundamental data type from signed char and unsigned char, thought to store characters. You should use either signed orunsigned if you intend to store numerical values in a char-sized variable.

short and long can be used alone as type specifiers. In this case, they refer to their respective integer fundamental types: short is equivalent to short int and long is equivalent to long int. The following two variable declarations are equivalent:

short Year;
short int Year;

Finally, signed and unsigned may also be used as standalone type specifiers, meaning the same as signed int andunsigned int respectively. The following two declarations are equivalent:

unsigned NextYear;
unsigned int NextYear;

To see what variable declarations look like in action within a program, we are going to see the C++ code of the example about your mental memory proposed at the beginning of this section:

// operating with variables

#include <iostream>
using namespace std;

int main ()
  // declaring variables:
  int a, b;
  int result;

  // process:
  a = 5;
  b = 2;
  a = a + 1;
  result = a - b;

  // print out the result:
  cout << result;

  // terminate the program:
  return 0;

Do not worry if something else than the variable declarations themselves looks a bit strange to you. You will see the rest in detail in coming sections.