Am I missing something? (Tutorial #13: Inline Functions)

[Bonnet]

I just read Tutorial #13: Inline Functions on this website, and I think I might be missing something.

It seemed like inline functions are completely useless. Isn't that the point of a function in the first place? So that you only have to write the code once? Let me use the example used in the tutorial:

#include <iostream>
using namespace std;

inline void helloWorld(){
     cout<<"Hello World!";
}
int main(){
     helloWorld();
     cin.get();
}

Outputs

Hello World!

#include <iostream>
using namespace std;

void helloWorld(){
     cout<<"Hello World!";
}
int main(){
     helloWorld();
     cin.get();
}

Outputs

Hello World!

Again, am I missing something here? What's the point of inline functions?

[tabstop]

Speeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeed.

You still only write the code once, just like any other function. But the compiler can (if it wants) simply replace the line of the function call with the actual body of the function, to save the overhead of doing a "real" function call.

[Bonnet]

So without the "inline" keyword, the compiler tells the program to look at whatever code that function has, and run it, whenever the function is called, but with the "inline" keyword, the compiler literally replaces helloWorld(); with the code that is in the "helloWorld" function? If so, that makes sense, I suppose. :P

[AndrewHunter]

So without the "inline" keyword, the compiler tells the program to look at whatever code that function has, and run it, whenever the function is called, but with the "inline" keyword, the compiler literally replaces helloWorld(); with the code that is in the "helloWorld" function? If so, that makes sense, I suppose. :P

Yes and no. Technically, the compiler will do whatever it believes to be the fasted way to execute the code, depending on the level of optimization selected.

There is overhead in making a function call, e.g. setting up the stack, placing parameters on the stack, calling the function, executing code, returning from the function, resetting the stack, ect. This things take real time and hence can produce possible bottlenecks in code execution.

The inline specificier is a suggestion to the compiler to make the function call as fast as possible. How it actually makes that call fast is up to the implementation. In some cases it will replace the function call with the actual code, in others it won't. The point of functions is to modularize your code, whether or not that results in a speed increase varies on a case by case basis.

[Bonnet]

Okay, thanks for the reply. I knew I was missing something, because it sounded like inline functions were completely useless.

[whiteflags]

To tell the truth, they are. When a lot of people write inline functions they're usually following a rule of thumb like "one liners are candidates for inlining" and don't worry about the details, since the compiler basically does what it wants.

[cyberfish]

I believe (don't quote me on this) modern versions of GCC actually ignores the inline keyword altogether.

[cyberfish]

Sorry I remembered wrong. Inline is used, though at -O3, all functions are considered for inlining (not just those marked "inline"). It's the "register" keyword that's ignored.

[tabl3six]

Are inline functions important for working with classes? Since I'm new to C++ I find it hard to follow the flow of the code.

[Alexandre]

inline is a suggestion to the compiler which may or may not be taken into account and which may result only in a slight increase in processing speed. My guess is that you could never use inline and still be a great programmer.

[whiteflags]

Are inline functions important for working with classes? Since I'm new to C++ I find it hard to follow the flow of the code.

Yes. Template classes are usually completely defined and implemented in header files.

[iMalc]

I would say that so far this thread has totally overlooked what is nodways arguably the more important use of inline.
It is for preventing multiple definitions. When it comes to code in header files, which is the norm for templated code, then without the explicit inline keyword being there or the function being implicitly inlined by being inside the class definition, then link errors would be a massive problem.
Regardless of whether the compiler does or does not actually inline a function call is one thing, but it always prevents multiple definitions.