Does it make any sense to use inline keyword with templates?
c++, inline, templates
Solution
It is not irrelevant. And no, not every function template is `inline` by default. The standard is even explicit about it in Explicit specialization ([temp.expl.spec])
Have the following:
a.cc
#include "tpl.h"
b.cc
#include "tpl.h"
tpl.h (taken from Explicit Specialization):
#ifndef TPL_H
#define TPL_H
template<class T> void f(T) {}
template<class T> inline T g(T) {}
template<> inline void f<>(int) {} // OK: inline
template<> int g<>(int) {} // error: not inline
#endif
Compile this, et voila:
g++ a.cc b.cc
/tmp/ccfWLeDX.o: In function `int g<int>(int)':
inlinexx2.cc:(.text+0x0): multiple definition of `int g<int>(int)'
/tmp/ccUa4K20.o:inlinexx.cc:(.text+0x0): first defined here
collect2: ld returned 1 exit status
Not stating `inline` when doing explicit instantiation may also lead to issues.
So in summary: For non fully specialized function templates, i.e. ones that carry at least one unknown type, you can omit `inline`, and not receive errors, but still they are not `inline`. For full specializations, i.e. ones that use only known types, you cannot omit it.
Proposed rule of thumb: Write `inline` if you mean it and just be consistent. It makes you think less about whether to or not to just because you can. (This rule of thumb is conforming to Vandevoorde's/Josuttis's C++ Template: The Complete Guide).
Problem
Since templates are defined within headers and compiler is able to determine if inlining a function is advantageous, does it make any sense? I've heard that modern compilers know better when to inline a function and are ignoring `inline` hint. edit: I would like to accept both answers, but this is not possible. To close the issue I am accepting Sebastian Mach's answer, because it received most votes and he is formally right, but as I mentioned in comments I consider Puppy's and Component 10's answers as correct ones too, from different point of view. The problem is in C++ semantics, which is not strict in case of `inline` keyword and inlining. Sebastian Mach says "write inline if you mean it", but what is actually meant by `inline` is not clear as it evolved from its original meaning to a directive that "stops compilers bitching about ODR violations" as Puppy says.