Benefits of inline functions in C++?

Question

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?

Answer 1

I'd like to add that inline functions are crucial when you are building shared library. Without marking function inline, it will be exported into the library in the binary form. It will be also present in the symbols table, if exported. On the other side, inlined functions are not exported, neither to the library binaries nor to the symbols table.

It may be critical when library is intended to be loaded at runtime. It may also hit binary-compatible-aware libraries. In such cases don't use inline.

Answer 2

inline allows you to place a function definition in a header file and #include that header file in multiple source files without violating the one definition rule.

Answer 3

Inlining is a suggestion to the compiler which it is free to ignore. It's ideal for small bits of code.

If your function is inlined, it's basically inserted in the code where the function call is made to it, rather than actually calling a separate function. This can assist with speed as you don't have to do the actual call.

It also assists CPUs with pipelining as they don't have to reload the pipeline with new instructions caused by a call.

The only disadvantage is possible increased binary size but, as long as the functions are small, this won't matter too much.

I tend to leave these sorts of decisions to the compilers nowadays (well, the smart ones anyway). The people who wrote them tend to have far more detailed knowledge of the underlying architectures.

Answer 4

Our computer science professor urged us to never use inline in a c++ program. When asked why, he kindly explained to use that modern compilers should detect when to use inline automatically.

So yes, the inline can be an optimization technique to be used wherever possible, but apparently this is something that is already done for you whenever it's possible to inline a function anyways.

Answer 5

Why not make all functions inline by default? Because it's an engineering trade off. There are at least two types of "optimization": speeding up the program and reducing the size (memory footprint) of the program. Inlining generally speeds things up. It gets rid of the function call overhead, avoiding pushing then pulling parameters from the stack. However, it also makes the memory footprint of the program bigger, because every function call must now be replaced with the full code of the function. To make things even more complicated, remember that the CPU stores frequently used chunks of memory in a cache on the CPU for ultra-rapid access. If you make the program's memory image big enough, your program won't be able to use the cache efficiently, and in the worst case inlining could actually slow your program down. To some extent the compiler can calculate what the trade offs are, and may be able to make better decisions than you can, just looking at the source code.

Answer 6

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);