Keywords in C++ are reserved words that have special meaning in the language syntax. They cannot be used as identifiers (like variable names, function names, or class names). Each keyword serves a specific purpose in defining the structure and functionality of a C++ program. Here's an overview of the keywords in C++, along with their categories and explanations.

1. Data Types Keywords

These keywords define the types of data that can be used in a program.

• int: Represents integer data type.
• char: Represents character data type.
• float: Represents floating-point data type (single precision).
• double: Represents double precision floating-point data type.
• void: Indicates that a function does not return a value.

2. Control Flow Keywords

These keywords control the flow of the program.

• if: Executes a block of code if a specified condition is true.
• else: Executes a block of code if the corresponding if condition is false.
• else if: Used to specify a new condition to test if the previous conditions were false.
• switch: Allows a variable to be tested for equality against a list of values (cases).
• case: Defines a branch in a switch statement.
• default: Specifies the code to execute if no case matches in a switch statement.
• for: Used for looping; repeats a block of code a specified number of times.
• while: Repeats a block of code while a specified condition is true.
• do: Similar to while, but guarantees at least one execution of the loop.
• break: Exits from a loop or switch statement.
• continue: Skips the current iteration of a loop and continues with the next iteration.
• return: Exits from a function and optionally returns a value.

3. Storage Class Keywords

These keywords define the storage duration and visibility of variables.

• auto: Automatically determines the variable type (in C++11 and later).
• register: Suggests that a variable should be stored in a CPU register for faster access (note that this is often ignored by modern compilers).
• static: Preserves the value of a variable across multiple function calls.
• extern: Indicates that a variable is defined in another file or scope.
• mutable: Allows a member of a class to be modified even if the class instance is constant.

4. Type Keywords

These keywords define user-defined types.

• class: Defines a class, which is a blueprint for creating objects.
• struct: Defines a structure, similar to a class but with public members by default.
• union: Defines a union, which allows storing different data types in the same memory location.
• enum: Defines an enumeration, a user-defined type consisting of named integral constants.
• typedef: Creates an alias for an existing data type.

5. Function Keywords

These keywords relate to function definitions and declarations.

• inline: Suggests to the compiler to expand the function inline (replace the function call with the function code) for optimization.
• virtual: Allows a member function to be overridden in derived classes, enabling polymorphism.
• friend: Grants access to the private and protected members of a class to a specified function or another class.

6. Exception Handling Keywords

These keywords are used for handling exceptions.

• try: Defines a block of code to be tested for exceptions.
• catch: Defines a block of code to handle exceptions thrown by the try block.
• throw: Used to throw an exception.

7. Namespace Keywords

These keywords help in organizing code into logical groups and preventing name clashes.

• namespace: Declares a namespace, which is a declarative region that provides a scope to the identifiers inside it.
• using: Allows the use of identifiers from a namespace without qualifying them with the namespace name.

8. Other Keywords

These keywords serve various purposes.

• const: Indicates that a variable's value cannot be modified after initialization.
• volatile: Tells the compiler that a variable's value may change at any time (often used in multi-threaded programs).
• sizeof: Returns the size (in bytes) of a data type or object.
• alignas: Specifies the alignment requirement for a variable.
• alignof: Returns the alignment of a type.
• noexcept: Indicates whether a function is allowed to throw exceptions.
• decltype: Inspects the declared type of an expression.
• constexpr: Indicates that the value of a variable or function can be evaluated at compile time.