Usefulness of the "inline" feature

Question

There's two things about inlining:

• The inline keyword will be ignored if the compiler determines that the function cannot be inlined.
• There is a compiler optimization (on Visual Studio, I don't know about GCC) that tells the compiler to inline all functions where possible.

From this I conclude that I never need to bother about inlining. I just have to turn on the compiler optimization for the release build.

Or are there any situations where manually inlining would be preferred?

Answer 1

The inline keyword has two functions:

• it serves as a hint to the compiler to perform the inlining optimization (this is basically useless on modern compilers, which inline aggressively with or without the keyword)
• it tells the compiler/linker to ignore the One Definition Rule: that the inline'd symbol may be defined in multiple translation units (typically because it is defined in a header, that is included from multiple files). Normally, this would result in a linker error, but it is allowed when you use the inline keyword.