inline function

What is inline function :
Inline Function
How to make function inline:
Why to use -
PROS -
CONS -
When to use -
Key Points -
Program code-
TableofContents
Inline function and classes:

INLINE FUNCTION
o Inline function is introduced which is an optimization technique used by the compilers especially to reduce
the execution time.We will cover “what, why, when & how” of inline functions

WHAT IS INLINE FUNCTION :
¡ The inline functions are a C++ enhancement feature to increase the execution time
of a program.
¡ Functions can be instructed to compiler to make them inline so that compiler can
replace those function definition wherever those are being called.
¡ Compiler replaces the definition of inline functions at compile time instead of
referring function definition at runtime.
¡ if function is big (in term of executable instruction etc) then, compiler can ignore the
“inline” request and treat the function as normal function.

HOW TO MAKE FUNCTION INLINE:
¡ To make any function as inline, start its definitions with the keyword “inline”.
inline int add(int a, int b)
{
return (a + b);
};

WHY TO USE -
¡ In many places we create the functions for small work/functionality which contain simple
and less number of executable instruction. Imagine their calling overhead each time they
are being called by callers.
¡ When a normal function call instruction is encountered, the program stores the memory address
of the instructions immediately following the function call statement, loads the function being called
into the memory, copies argument values, jumps to the memory location of the called function, executes
the function codes, stores the return value of the function, and then jumps back to the address of the
instruction that was saved just before executing the called function. Too much run time overhead.
Cont….

WHY TO USE -
¡ The C++ inline function provides an alternative.
¡ With inline keyword, the compiler replaces the function call
statement with the function code itself (process called expansion)
and then compiles the entire code.
¡ Thus, with inline functions, the compiler does not have to jump to
another location to execute the function, and then jump back as the
code of the called function is already available to the calling program.

PROS -
1. It speeds up your program by avoiding function calling overhead.
2. It save overhead of variables push/pop on the stack, when function calling happens.
3. It save overhead of return call from a function.
4. It increases locality of reference by utilizing instruction cache.
5. By marking it as inline, you can put a function definition in a header file (i.e. it can be
included in multiple compilation unit, without the linker complaining)

CONS -
1. It increases the executable size due to code expansion.
2. C++ inlining is resolved at compile time.Which means if you change the code of the
inline function, you would need to recompile all the code using it to make sure it will be
updated
3.When used in a header, it makes your header file larger with information which users
don’t care.
4.As mentioned above it increases the executable size, which may cause thrashing in
memory. More number of page fault bringing down your program performance.
5. Sometimes not useful for example in embedded system where large executable size
is not preferred at all due to memory constraints

INLINE FUNCTION AND CLASSES:
¡ It is also possible to define the inline function inside the class. In fact, all the functions defined
inside the class are implicitly inline.Thus, all the restrictions of inline functions are also
applied here. If you need to explicitly declare inline function in the class then just declare the
function inside the class and define it outside the class using inline keyword
For example: class S
{
public:
inline int square(int s) // redundant use of inline
{
// this function is automatically inline
// function body
}
};

WHEN TO USE -
Function can be made as inline as per programmer need. Some useful recommendation
are mentioned below-
1. Use inline function when performance is needed.
2. Use inline function over macros.
3. Prefer to use inline keyword outside the class with the function
definition to hide implementation details.

KEY POINTS -
1. It’s just a suggestion not compulsion. Compiler may or may not inline the functions you marked as inline. It may also
decide to inline functions not marked as inline at compilation or linking time.
2. Inline works like a copy/paste controlled by the compiler, which is quite different from a pre-processor macro:The macro
will be forcibly inlined, will pollute all the namespaces and code, won't be easy to debug.
3.All the member function declared and defined within class are Inline by default. So no need to define explicitly.
4.Virtual methods are not supposed to be inlinable. Still, sometimes, when the compiler can know for sure the type of the
object (i.e. the object was declared and constructed inside the same function body), even a virtual function will be inlined
because the compiler knows exactly the type of the object.
5.Template methods/functions are not always inlined (their presence in an header will not make them automatically inline).
6. Most of the compiler would do in-lining for recursive functions but some compiler provides #pragmas
microsoft c++ compiler - inline_recursion(on) and once can also control
its limit with inline_depth.
In gcc, you can also pass this in from the command-line with --maxinline-
insns-recursive

PROGRAM CODE-
#include <iostream>
using namespace std;
inline int sqr(int x) output: ans is 9
{
int y;
y = x * x;
return y;
}
int main()
{
int a =3, b;
b = sqr(a);
cout <<"ans is "<<b;
return 0;
}

inline function

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