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

In archaic C and C++, inline is like register: a suggestion (nothing more than a suggestion) to the compiler about a possible optimization.

In modern C++, inline tells the linker that, if multiple definitions (not declarations) are found in different translation units, they are all the same, and the linker can freely keep one and discard all the other ones.

inline is mandatory if a function (no matter how complex or "linear") is defined in a header file, to allow multiple sources to include it without getting a "multiple definition" error by the linker.

Member functions defined inside a class are "inline" by default, as are template functions (in contrast to global functions).

//fileA.h
inline void afunc()
{ std::cout << "this is afunc" << std::endl; }

//file1.cpp
#include "fileA.h"
void acall()
{ afunc(); }

//main.cpp
#include "fileA.h"
void acall();

int main()
{ 
   afunc(); 
   acall();
}

//output
this is afunc
this is afunc

Note the inclusion of fileA.h into two .cpp files, resulting in two instances of afunc(). The linker will discard one of them. If no inline is specified, the linker will complain.

Answer 2

Fell into the same trouble with inlining functions into so libraries. It seems that inlined functions are not compiled into the library. as a result the linker puts out a "undefined reference" error, if a executable wants to use the inlined function of the library. (happened to me compiling Qt source with gcc 4.5.