An inline function in C++ is a function that is expanded in line when it is called, meaning that the code of the function is substituted at the point of the function call. This substitution is performed by the C++ compiler at compile time. The main advantage of inline functions is that they can reduce the function call overhead and improve the execution time and speed of the program. However, inlining is only a request to the compiler, not a command, and the compiler can ignore the request for inlining. The compiler may not perform inlining in certain circumstances, such as if the function contains a loop, static variables, a switch or goto statement, or if it is recursive. Additionally, inline functions may not be useful for many embedded systems because inlining might increase the size of the binary executable file, causing thrashing in memory and reducing the performance of the computer. To declare an inline function, the keyword "inline" is placed before the function name in the function definition. A given inline member function must be declared the same way in every compilation unit, and there must be exactly one definition of an inline function. A class member function defaults to external linkage unless a definition for that function contains the inline specifier.