At the 2016 Oulu ISO C++ Standards meeting, a proposal called Inline Variables was voted into C++17 by the standards committee.
In layman's terms, what are inline variables, how do they work and what are they useful for? How should inline variables be declared, defined and used?
The first sentence of the proposal:
The ¹guaranteed effect of
inline
as applied to a function, is to allow the function to be defined identically, with external linkage, in multiple translation units. For the in-practice that means defining the function in a header, that can be included in multiple translation units. The proposal extends this possibility to variables.So, in practical terms the (now accepted) proposal allows you to use the
inline
keyword to define an external linkageconst
namespace scope variable, or anystatic
class data member, in a header file, so that the multiple definitions that result when that header is included in multiple translation units are OK with the linker – it just chooses one of them.Up until and including C++14 the internal machinery for this has been there, in order to support
static
variables in class templates, but there was no convenient way to use that machinery. One had to resort to tricks likeFrom C++17 and onwards I believe one can write just
… in a header file.
The proposal includes the wording
… which allows the above to be further simplified to just
… as noted by T.C in a comment to this answer.
Also, the
constexpr
specifier impliesinline
for static data members as well as functions.Notes:
¹ For a function
inline
also has a hinting effect about optimization, that the compiler should prefer to replace calls of this function with direct substitution of the function's machine code. This hinting can be ignored.Inline variables are very similar to inline functions. It signals the linker that only one instance of the variable should exist, even if the variable is seen in multiple compilation units. The linker needs to ensure that no more copies are created.
Inline variables can be used to define globals in header only libraries. Before C++17, they had to use workarounds (inline functions or template hacks).
For instance, one workaround is to use the Meyer's singleton with an inline function:
There are some drawbacks with this approach, mostly in terms of performance. This overhead could be avoided by template solutions, but it is easy to get them wrong.
With inline variables, you can directly declare it (without getting a multiple definition linker error):
Apart from header only libraries, there other cases where inline variables can help. Nir Friedman covers this topic in his talk at CppCon: What C++ developers should know about globals (and the linker). The part about inline variables and the workarounds starts at 18m9s.
Long story short, if you need to declare global variables that are shared between compilation units, declaring them as inline variables in the header file is straightforward and avoids the problems with pre-C++17 workarounds.
(There are still use cases for the Meyer's singleton, for instance, if you explicitely want to have lazy initialization.)
Minimal runnable example
This awesome C++17 feature allow us to:
constexpr
: How to declare constexpr extern?main.cpp
notmain.hpp
notmain.cpp
Compile and run:
GitHub upstream.
The C++ standard guarantees that the addresses will be the same. C++17 N4659 standard draft 10.1.6 "The inline specifier":
cppreference https://en.cppreference.com/w/cpp/language/inline explains that if
static
is not given, then it has external linkage.Pre-C++ 17:
extern const
Before C++ 17, and in C, we can achieve a very similar effect with an
extern const
, which will lead to a single memory location being used.The downsides over
inline
are:constexpr
with this technique, onlyinline
allows that: How to declare constexpr extern?main.cpp
notmain.cpp
notmain.hpp
GitHub upstream.
Any way to fully inline it?
TODO: is there any way to fully inline the variable, without using any memory at all?
Much like what the preprocessor does.
This would require somehow:
Related:
Tested in Ubuntu 18.10, GCC 8.2.0.