Identifiers,data type and variables deceleration

By: Unknown on 00:21

A quality identifier is a grouping of one or more letters, digits, or underscore characters (_). Spaces, punctuation imprints, and images can't be a piece of an identifier. Also, identifiers might dependably start with a letter. They can likewise start with an underline character (_), however such identifiers are -on most cases- acknowledged held for compiler-particular magic words or outside identifiers, and in addition identifiers holding two progressive underscore characters anyplace. In no case would they be able to start with a digit.

C++ uses a number of keywords to identify operations and data descriptions; therefore, identifiers created by a programmer cannot match these keywords. The standard reserved keywords that cannot be used for programmer created identifiers are:
alignas, alignof, and, and_eq, asm, auto, bitand, bitor, bool, break, case, catch, char, char16_t, char32_t, class, compl, const, constexpr, const_cast, continue, decltype, default, delete, do, double, dynamic_cast, else, enum, explicit, export, extern, false, float, for, friend, goto, if, inline, int, long, mutable, namespace, new, noexcept, not, not_eq, nullptr, operator, or, or_eq, private, protected, public, register, reinterpret_cast, return, short, signed, sizeof, static, static_assert, static_cast, struct, switch, template, this, thread_local, throw, true, try, typedef, typeid, typename, union, unsigned, using, virtual, void, volatile, wchar_t, while, xor, xor_eq
Here is the complete list of fundamental types in C++:

 


Group	Type names*	Notes on size / precision
Character types	char	Exactly one byte in size. At least 8 bits.
	char16_t	Not smaller than char. At least 16 bits.
	char32_t	Not smaller than char16_t. At least 32 bits.
	wchar_t	Can represent the largest supported character set.
Integer types (signed)	signed char	Same size as char. At least 8 bits.
	signed short int	Not smaller than char. At least 16 bits.
	signed int	Not smaller than short. At least 16 bits.
	signed long int	Not smaller than int. At least 32 bits.
	signed long long int	Not smaller than long. At least 64 bits.
Integer types (unsigned)	unsigned char	(same size as their signed counterparts)
	unsigned short int
	unsigned int
	unsigned long int
	unsigned long long int
Floating-point types	float
	double	Precision not less than float
	long double	Precision not less than double
Boolean type	bool
Void type	void	no storage
Null pointer	decltype(nullptr)

 

Size	Unique representable values	Notes
8-bit	256	= 28
16-bit	65 536	= 216
32-bit	4 294 967 296	= 232 (~4 billion)
64-bit	18 446 744 073 309 551 616	= 264 (~18 billion billion)

Declaration of variables

C++ is a strongly-typed language, and requires every variable to be declared with its type before its first use. This informs the compiler the size to reserve in memory for the variable and how to interpret its value. The syntax to declare a new variable in C++ is straightforward: we simply write the type followed by the variable name (i.e., its identifier). For example:

1 2	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 and mynumber can be used within the rest of their scope in the program.
If declaring more than one variable of the same type, they can all be declared in a single statement by separating their identifiers with commas. For example:

int a, b, c;

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

1 2 3

int a; int b; int c;

To see what variable declarations look like in action within a program, let's have a look at the entire C++ code of the example about your mental memory proposed at the beginning of this chapter:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

// 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; }

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Don't be worried if something else than the variable declarations themselves look a bit strange to you. 
                                                              
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