I know that inline is a hint or request to the compiler and is used to avoid function call overheads.
So, on what basis one can determine whether a function is a candidate for inlining or not?
In which case one should avoid inlining?
c++inlineinline-functions
I know that inline is a hint or request to the compiler and is used to avoid function call overheads.
So, on what basis one can determine whether a function is a candidate for inlining or not?
In which case one should avoid inlining?
It worth pointing out that the inline keyword is actually just a hint to the compiler. The compiler may ignore the inline and simply generate code for the function someplace.
The main drawback to inline functions is that it can increase the size of your executable (depending on the number of instantiations). This can be a problem on some platforms (eg. embedded systems), especially if the function itself is recursive.
I'd also recommend making inline'd functions very small - The speed benefits of inline functions tend to diminish as the function grows in size. At some point the overhead of the function call becomes small compared to the execution of the function body, and the benefit is lost.
As has been suggested in previous answers, use of the inline
keyword can make code faster by inlining function calls, often at the expense of increased executables. “Inlining function calls” just means substituting the call to the target function with the actual code of the function, after filling in the arguments accordingly.
However, modern compilers are very good at inlining function calls automatically without any prompt from the user when set to high optimisation. Actually, compilers are usually better at determining what calls to inline for speed gain than humans are.
Declaring functions inline
explicitly for the sake of performance gain is (almost?) always unnecessary!
Additionally, compilers can and will ignore the inline
request if it suits them. Compilers will do this if a call to the function is impossible to inline (i.e. using nontrivial recursion or function pointers) but also if the function is simply too large for a meaningful performance gain.
However, declaring an inline function using the inline
keyword has other effects, and may actually be necessary to satisfy the One Definition Rule (ODR): This rule in the C++ standard states that a given symbol may be declared multiple times but may only be defined once. If the link editor (= linker) encounters several identical symbol definitions, it will generate an error.
One solution to this problem is to make sure that a compilation unit doesn't export a given symbol by giving it internal linkage by declaring it static
.
However, it's often better to mark a function inline
instead. This tells the linker to merge all definitions of this function across compilation units into one definition, with one address, and shared function-static variables.
As an example, consider the following program:
// header.hpp
#ifndef HEADER_HPP
#define HEADER_HPP
#include <cmath>
#include <numeric>
#include <vector>
using vec = std::vector<double>;
/*inline*/ double mean(vec const& sample) {
return std::accumulate(begin(sample), end(sample), 0.0) / sample.size();
}
#endif // !defined(HEADER_HPP)
// test.cpp
#include "header.hpp"
#include <iostream>
#include <iomanip>
void print_mean(vec const& sample) {
std::cout << "Sample with x̂ = " << mean(sample) << '\n';
}
// main.cpp
#include "header.hpp"
void print_mean(vec const&); // Forward declaration.
int main() {
vec x{4, 3, 5, 4, 5, 5, 6, 3, 8, 6, 8, 3, 1, 7};
print_mean(x);
}
Note that both .cpp
files include the header file and thus the function definition of mean
. Although the file is saved with include guards against double inclusion, this will result in two definitions of the same function, albeit in different compilation units.
Now, if you try to link those two compilation units — for example using the following command:
⟩⟩⟩ g++ -std=c++11 -pedantic main.cpp test.cpp
you'll get an error saying “duplicate symbol __Z4meanRKNSt3__16vectorIdNS_9allocatorIdEEEE” (which is the mangled name of our function mean
).
If, however, you uncomment the inline
modifier in front of the function definition, the code compiles and links correctly.
Function templates are a special case: they are always inline, regardless of whether they were declared that way. This doesn’t mean that the compiler will inline calls to them, but they won’t violate ODR. The same is true for member functions that are defined inside a class or struct.
Best Answer
Avoiding the cost of a function call is only half the story.
do:
inline
instead of#define
inline
: faster code and smaller executables (more chances to stay in the code cache)don't:
when developing a library, in order to make a class extensible in the future you should:
Remember that the
inline
keyword is a hint to the compiler: the compiler may decide not to inline a function and it can decide to inline functions that were not markedinline
in the first place. I generally avoid marking functioninline
(apart maybe when writing very very small functions).About performance, the wise approach is (as always) to profile the application, then eventually
inline
a set of functions representing a bottleneck.References:
EDIT: Bjarne Stroustrup, The C++ Programming Language:
EDIT2: ISO-IEC 14882-1998, 7.1.2 Function specifiers