Expressions and Declarations in C++ Programming

To perform a calculation in the C=++ program you need and expression. An expression is a statement that has both a value and a type. In the C++ program, a declaration is statement that defines a variable or it’s a “holding tank” for some sort of value like a number or character.

Expressions

Expressions take one of the following forms:

objName                         // for a simple object
operator expression             // for unary operators
expr1 operator expr2            // for binary operators
expr1 ? expr2 : expr3           // for the ternary operator
funcName([argument list]);      // for function calls

Declarations

Declarations use both intrinsic and user-defined types. The intrinsic types are

[<signed | unsigned >]char
[<signed | unsigned >]wchar_t
[<signed | unsigned>] [<short | long | long long>] int
float
[long] double
bool

Declarations have one of the following forms:

[<extern|static>][const] type var[=expression]; // variable
[<extern|static>][const] type array[size][={list}]; // array
[const] type object[(argument list)];           // object
[const] type object [= {argument list}];        // alternative
[const] type * [const] ptr[=pointer expression];// pointer
type& refName = object;                         // reference
type fnName([argument list]);                   // function

A function definition has the following format:

// simple function
[<inline|constexpr>] type fnName(argument list) {...}
// member function defined outside of class
[inline] type Class::func(argument list) [const] {...}
// constructor/destructors may also be defined outside of class
Class::Class([argument list]) {...}
Class::~Class() {...}
// constructors/destructor may be deleted or defaulted
// in lieu of definition
Class::Class([argument list]) = <delete|default>;
Class::~Class() = <delete|default>;

An overloaded operator looks like a function definition. Most overloaded operators may be written either as member or simple functions. When written as a member function, *this is the assumed first argument to the operator:

MyClass& operator+(const MyClass& m1, const MyClass& m2);// simple
MyClass& MyClass::operator+(const MyClass& m2); // member;

Users may define and instantiate their own enumerated types:

enum [class] Name = {label1[=value], label2[=value]...};
Name variable = Name::label1;  // used in an assignment

Users may also define their own types using the class or struct keywords:

<struct | class> ClassName [ : [virtual] [public] BaseClass]
{
     <public|protected>:
       // constructor
       ClassName([arg list]) <[: member(val),...] {...} |;>
       ClassName() [= <delete|default>;]
       // destructor
       [virtual] ~ClassName() <{...} | [=<delete|default>;>
       // public data members
       type dataMemberName;
       // public member functions
       type memberFunctionName([arg list]) [{...}]
       // const member function
       type memberFunctionName([arg list]) const [{...}]
       // virtual member functions
       virtual type memberFunctionName([arg list]) [{...}];
       // pure virtual member functions
       virtual type memberFunctionName([arg list]) = 0;
};

In addition, a constructor with a single argument may be flagged as explicit.

Template declarations have a slightly different format:

// type T is provided by the programmer at use
template <class T, {...}> type FunctionName([arg list])
template <class T, {...}> class ClassName { {...} };

dummies

Source:http://www.dummies.com/how-to/content/expressions-and-declarations-in-c-programming.html