CODEWITHSUNDEEP
c++global-variablesinline-functions
undermind
undermind

Reputation: 1841

Global variables with internal linkage in inline functions

I have a val.h source code file with the global variable with internal linkage and inline function that returns its address:

// val.h
#pragma once
static int val;
inline int* get_val()
{
  return &val;
}

Then this header is included in two different translation units. If I call &val in both of these units, I get two different addresses, and it's ok since val has an internal linkage and each translation unit has its own val. But if I call get_val() in both of these units, I get two equal addresses.

Is such behavior guaranteed by the standard, would we always obtain the same value returned by get_val() call from any translation unit?

And what if compiler decides to make true inlining, i.e. just to replace get_val() call by &val statement in each translation unit. Would we get different addresses for each translation unit in such case?

Upvotes: 2

Views: 862

Answers (2)

Stephan Lechner
Stephan Lechner

Reputation: 35164

As you have it you will get undefined behaviour when you include this header file in two different translation units. This is because expression return &val will refer to two different objects; Thereby, the body of getVal is different in both translation units, but getVal has external linkage. So you violate the one definition rule.

To overcome this, you'd have to define internal linkage for getVal, too, i.e. write static int* get_val() { .... Keyword inline alone will not define external or internal linkage.

Upvotes: 1

molbdnilo
molbdnilo

Reputation: 66449

Your function violates the One Definition Rule since the expression val refers to different entities in different translation units.

Its behaviour is undefined.

Upvotes: 4

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