What is outline function in c plus plus language?

Answer 1

Outline is the opposite of inline. An inline expanded function is any function or class method where the declaration also provides the definition (the implementation). This is known as implicit inline expansion. Where the definition is kept separate from the declaration, you may use the inline keyword to specifiy that the function should be inline. This is known as explicit inline expansion.

Inline expanded functions (whether implied or explicit) does NOT mean the function will in fact be inline expanded. It is merely a suggestion to the compiler. If the compiler's optimisers determine that there is no advantage to be gained by inline expanding a particular function, then that function becomes an outline function.

Inline expansion simply means that the body of the function is inserted in place of the function call. Function calls are expensive in terms of memory and performance, so by eliminating the function call completely, your code performs faster and uses less memory. However, functions that are called many times throughout your code will result in a much larger code size, and large code runs slower than small code. Thus the benefit of eliminating a function call has to be weighed against the increased code size. Although some compilers do allow you to override the compiler's inline expansion optimisers, this is strictly non-standard. The best judge of what to expand and what not to expand is best left in the hands of the compiler, and indiscriminate use of the inline keyword should generally be avoided.