Comments (1)
The cause is easier to see with a repro that uses a mutable variable. A header defines a variable in the anonymous namespace, then modifies it in an inline function. The code and Makefile are available in inlinefuncodr.tar.gz
// example.h
#ifndef EXAMPLE_HDR_
#define EXAMPLE_HDR_
namespace {
int x;
}
inline void inc_x() {
++x;
}
#endif
Then two very similar files:
// ex1.cpp
#include "example.h"
#include <iostream>
void a() {
inc_x();
inc_x();
std::cout << "a(): " << &x << " = " << x <<'\n';
}
and
// ex2.cpp
#include "example.h"
#include <iostream>
void b() {
inc_x();
inc_x();
std::cout << "b(): " << &x << " = " << x <<'\n';
}
and finally a main
// main.cpp
#include "example.h"
#include <iostream>
void a();
void b();
int main() {
a(); // should inc_x twice and print 2
b(); // should inc_x twice and print 4
std::cout << "main: " << &x << " = " << x <<'\n'; // should print 4
}
Expected output would be the same address for x
and 2
, 4
, 4
.
The exact output differs by compiler flags but I get either 2
, 2
, 0
; or 2
, 0
, 0
. Always with three different addresses.
The problem is an ODR violation: inc_x()
differs across translation units because the anonymous namespace is creating a different x
with each #include
. The x
in ex1.cpp
is not the same as the x
in ex2.cpp
. We get undefined behavior. (UBSan with -fsanitize=undefined
doesn't catch it)
Removing the anonymous namespace isn't enough because then we have a different ODR violation (this time with an actual linker warning about the multiple definitions of x). We need to mark x
as inline
// FIXED example.h
#ifndef EXAMPLE_HDR_
#define EXAMPLE_HDR_
inline int x;
inline void inc_x() {
++x;
}
#endif
The same fix is correct for itertools: remove the anonymous namespace around chain
and similar, and mark it as inline
. Originally I had used the anonymous namespace to avoid multiple definitions because it was pre-c++-17 so inline variables didn't yet exist.
from cppitertools.
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