Title: The Wall

Dear Francis,

This is an interesting one, I have just found this out whilst using Visual Studio 6 SP3.

int main() {
  int iVal;
  iVal = 1; 
  {
    printf("iVal == %d\n", iVal);
    int iVal = 2;
    printf("iVal == %d\n", iVal);
  }
  printf("iVal == %d\n", iVal);
  return 0;
}

Output is:

1
2
1

I was not actually aware that you could do this in C++, and just spotted it as a typo during a code review, declaring a duplicate variable appears to hide the outer scope declaration, could be nasty. I would expect a warning to be raised but Visual Studio does not display any warning at all even with the warning level set to 4, I trust this is not right! :-)

NB. I have gone through the list of warnings VS uses and there does not appear to be any applicable one listed, this behaviour also happens in C and C++ mode.

Darren Jefford

The code you provide is faultless C++ though a C compiler should have thrown it out because a declaration after an executable statement at block scope. Note the past tense, the new C standard allows late declarations.

This hiding rule has always been the case. Reusing an identifier in an inner scope hides the one in the outer scope. C++ provides mechanisms for referring to the hidden name if that was declared at global, namespace or class scope.

In strictly conforming mode a C++ compiler must not issue a diagnostic (though it is up to the implementers to determine whether a warning constitutes a diagnostic). However, I would expect a quality compiler to issue a warning for declaring a name that had already been used in the current scope to refer to something in an outer scope. FG.