Asked  7 Months ago    Answers:  5   Viewed   37 times

What is the advantages/disadvantages of using inline functions in C++? I see that it only increases performance for the code that the compiler outputs, but with today's optimized compilers, fast CPUs, huge memory etc. (not like in the 1980< where memory was scarce and everything had to fit in 100KB of memory) what advantages do they really have today?

 Answers

82

Inline functions are faster because you don't need to push and pop things on/off the stack like parameters and the return address; however, it does make your binary slightly larger.

Does it make a significant difference? Not noticeably enough on modern hardware for most. But it can make a difference, which is enough for some people.

Marking something inline does not give you a guarantee that it will be inline. It's just a suggestion to the compiler. Sometimes it's not possible such as when you have a virtual function, or when there is recursion involved. And sometimes the compiler just chooses not to use it.

I could see a situation like this making a detectable difference:

inline int aplusb_pow2(int a, int b) {
  return (a + b)*(a + b) ;
}

for(int a = 0; a < 900000; ++a)
    for(int b = 0; b < 900000; ++b)
        aplusb_pow2(a, b);
Tuesday, June 1, 2021
 
adjco
answered 7 Months ago
70

Preprocessor macros are just substitution patterns applied to your code. They can be used almost anywhere in your code because they are replaced with their expansions before any compilation starts.

Inline functions are actual functions whose body is directly injected into their call site. They can only be used where a function call is appropriate.

Now, as far as using macros vs. inline functions in a function-like context, be advised that:

  • Macros are not type safe, and can be expanded regardless of whether they are syntatically correct - the compile phase will report errors resulting from macro expansion problems.
  • Macros can be used in context where you don't expect, resulting in problems
  • Macros are more flexible, in that they can expand other macros - whereas inline functions don't necessarily do this.
  • Macros can result in side effects because of their expansion, since the input expressions are copied wherever they appear in the pattern.
  • Inline function are not always guaranteed to be inlined - some compilers only do this in release builds, or when they are specifically configured to do so. Also, in some cases inlining may not be possible.
  • Inline functions can provide scope for variables (particularly static ones), preprocessor macros can only do this in code blocks {...}, and static variables will not behave exactly the same way.
Tuesday, June 1, 2021
 
Fernando
answered 7 Months ago
29

Finally in .NET 4.5, the CLR allows one to hint/suggest1 method inlining using MethodImplOptions.AggressiveInlining value. It is also available in the Mono's trunk (committed today).

// The full attribute usage is in mscorlib.dll,
// so should not need to include extra references
using System.Runtime.CompilerServices; 

...

[MethodImpl(MethodImplOptions.AggressiveInlining)]
void MyMethod(...)

1. Previously "force" was used here. Since there were a few downvotes, I'll try to clarify the term. As in the comments and the documentation, The method should be inlined if possible. Especially considering Mono (which is open), there are some mono-specific technical limitations considering inlining or more general one (like virtual functions). Overall, yes, this is a hint to compiler, but I guess that is what was asked for.

Friday, June 4, 2021
 
o_flyer
answered 7 Months ago
41

The linkage of a name has nothing to do with where or how it is defined, just with where the name may be used to refer to a particular object or function.

Declaring a function inline does not force it to be inlined; it just relaxes the One Definition Rule to allow a definition in each translation unit in which it's used (and require one in each translation unit in which it's called), to make it easier to inline. It doesn't prevent a non-inline version being generated, if the compiler decides not to inline a particular call to it, or if you take the address of it.

So "external linkage" and "inline" are not exclusive; "external linkage" means that the function may be referred to in any translation unit, and "inline" means that it must be defined in any translation unit that calls it.

Thursday, July 29, 2021
 
RahulG
answered 5 Months ago
97

Yes, functions that are defined inside a class body are implicitly inline.

(As with other functions declared inline it doesn't mean that the complier has to perform inline expansion in places where the function is called, it just enables the permitted relaxations of the "one definition rule", combined with the requirement that a definition must be included in all translation units where the function is used.)

Friday, July 30, 2021
 
AlterPHP
answered 5 Months ago
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