Inline Variables

Inline Variables

Since C++17 you can define a variable as “inline”. What does this do?

Basically, it's not really much of a speedup, but makes it easier to manage global constants, global variables, or static data members in C++ classes. You can declare these variables as “inline” in a header file, with an initializer:

    inline int g_x = 3;

Then you can with wild abandon include that header file all over the place without any problems whatsoever. The C++ linker is required to:

• Merge all of them into one variable at link-time.
• Guarantee that it's initialized as specified.
• Have the same address for that variable everywhere.

I find this addition to C++ somewhat humorous because it fixes up a huge mess that's existed since old K&R C code, and I've battled against it many times trying to get my program linked. I'm not going to irritate myself by repeating all the quirks, but it was always messy whether you had a global variable that was extern or non-extern, initialized or non-initialized, in a header file or a non-header file. So, if you ask me, the way that “extern” variable declarations “worked” was always broken, and now it's fixed in C++17. Hooray! (A bit late for me.)

Overall, allowing “inline” for variables is helpful to efficiency because you can be guaranteed about constants, static members, or global variables at compile-time. And it's always nice to get your program to link.

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