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Storage classes define the scope (visibility) and lifetime of variables/functions in C++ programs. These specifiers are placed before the type they modify. The following storage classes are available in C++ programs: * **auto**: This is the default storage class specifier and can usually be omitted. Variables specified with auto have automatic storage duration, meaning their lifetime is limited to the block in which they are defined. auto variables are typically allocated on the stack. * **register**: Used to suggest the compiler store variables in CPU registers for faster access. In C++11 and later versions, register is a deprecated feature and no longer has any practical effect. * **static**: Used to define variables or functions with static storage duration, whose lifetime spans the entire program execution. Inside functions, static variable values persist between function calls. At file scope or global scope, static variables have internal linkage and can only be accessed within the file where they are defined. * **extern**: Used to declare variables or functions with external linkage, which can be shared across multiple files. By default, global variables and functions have extern storage class. Using extern to declare global variables or functions defined in another file enables cross-file sharing. * **mutable (C++11)**: Used to modify class member variables, allowing these variables to be modified within const member functions. Typically used for data that needs to be modified in const contexts, such as caches or counters. * **thread_local (C++11)**: Used to define variables with thread-local storage duration, where each thread has its own independent copy. The lifetime of thread-local variables matches the lifetime of the thread. Since C++ 17, the auto keyword is no longer a C++ storage class specifier, and the register keyword is deprecated. The storage class specifiers provide programmers with control over the lifetime and visibility of variables and functions. Proper use of storage class specifiers can improve program maintainability and performance. Since C++11, register has lost its original purpose, while