CODEWITHSUNDEEP
c++inline
Coder
Coder

Reputation: 3715

What are the inlining rules within C++ classes?

From what I read somewhere long time ago, it seems that if you want class member function to be inlined during the compilation phase, the function has to be defined inside class declaration block.

But this has a downside of a detail leak. IMHO, other programmers should only see class interface when opening .h file.

Is the first statement still true in modern C++, was it ever? Is there a way to force inlining for functions that are declared, preferably in another file altogether?

Is it generally better to keep short member functions inside class declaration block, or not?

Upvotes: 4

Views: 2505

Answers (3)

wreckgar23
wreckgar23

Reputation: 1045

Is the first statement still true in modern C++, was it ever?

As David explained, there's the inline keyword as well. It can be ignored, as Paul stated.

Is there a way to force inlining for functions that are declared, preferably in another file altogether?

Probably by configuring your compiler. It might be doing some inling behind your back anyway. Eg. gcc has -finline-functions etc. that will be switched on for certain optimisation levels

Is it generally better to keep short member functions inside class declaration block, or no?

Up to you. Be aware though that if you have an inline method used lots of times, then you can be increasing the size of your object files, and so potentially bloat the size of what you're building and maybe slow it down.

FWIW I only tend to put implementations in header files out of laziness :)

Upvotes: 0

David Rodríguez - dribeas
David Rodríguez - dribeas

Reputation: 208436

It seems that if you want class member function to be inlined during the compilation phase, the function has to be defined inside class declaration block.

That is not really true. A function that is defined inside the class definition is implicitly marked as inline. But you don't need to defined the function inside the class for it to be inline, you can explicitly request it:

struct X {
   void f();
};
inline void f() {}

The inline keyword on the other hand, does not mean that the function will be inlined, but rather that it can be defined in multiple translation units, that is, if multiple translation units include the same header that contains that definition, the linker will not fail with a multiple definition error.

Now, on actual inlining, the compiler can decide to inline or not any function, regardless of whether the function is declared as inline provided that it sees the definition of that function (the code that it will inline), which is the reason why in general functions that are meant to be inlined should be defined in the header (either inside the class definition or marked inline outside.

Additionally, newer toolchains can perform whole program optimization or other link time optimizations, by which the linker can also decide that a function should be inlined. In this case, the function definition needs not be visible at the call site, so it could be defined inside the .cpp file. But if you really want the function to be inlined it is better not to depend on this feature and just define the function in the header.

Upvotes: 7

paulsm4
paulsm4

Reputation: 121799

Q: Is there a way to force inlining for functions?

A: No

No matter how you designate a function as inline, it is a request that the compiler is allowed to ignore: it might inline-expand some, all, or none of the calls to an inline function.

Q: What are the inlining rules within C++ classes?

Inline member functions in C++

As far as Standard C++ is concerned, a inline function must be defined in every translation unit in which it is used ...

This is different from non-inline functions which must be defined only once in an entire program (one-definition-rule)...

For member-functions, if you define your function in the class, it is implicitly inline. And because it appears in the header, the rule that it has to be defined in every translation unit in which it is used is automatically satisfied.

Here is a great FAQ (one that's more "practical" than "pedantic"):

http://www.parashift.com/c++-faq-lite/inline-functions.html

Upvotes: 2

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