Is accessing a static struct::var without an object undefined behaviour? [duplicate]

Question

According to Compiler Explorer, only some will build this code:

struct s { static int i; };

int main( int argc, char *argv[] )
{
  s::i = 1;
  return 0;
}

Most newer C++ compilers fail at linking.

• GCC 4.7.2 works, 4.7.3 and newer fail
• Clang 3.2 works, 3.4 and newer fail
• msvc works with all versions

So, is this undefined behaviour or should this work?

Answer 1

This code violates the One Definition Rule (ODR), which requires a single definition of every entity that is used in the program.

There is no definition of s::i in the program, but you are using it, and so the code violates the ODR. Any violation of the ODR makes the code ill-formed, no diagnostic required. This means the compiler can do anything it wants, including rejecting the code, or compiling it and producing an executable program (which could do anything it wants).