What language standards allow ignoring null terminators on fixed size arrays?

Question

We are transitioning C code into C++.
I noticed that the following code is well defined in C,

int main(){

  //length is valid. '\0' is ignored
  char  str[3]="abc";
}

as it is stated in Array initialization that:

"If the size of the array is known, it may be one less than the size of the string literal, in which case the terminating null character is ignored."

However, if I were to build the same code in C++, I get the following C++ error:

error: initializer-string for array of chars is too long
[-fpermissive]    char  str[3]="abc";

I'm hoping someone can expound on this.

Questions:
Is the code example valid in all C language standards?
Is it invalid in all C++ language standards?
Is there a reason that is valid in one language but not another?

Answer 1

What you see here is a difference in the initialization rules for cstring in C and C++. In C11 §6.7.9/14 we have

An array of character type may be initialized by a character string literal or UTF−8 string literal, optionally enclosed in braces. Successive bytes of the string literal (including the terminating null character if there is room or if the array is of unknown size) initialize the elements of the array.

^{emphasis mine}

So as long as the array is large enough for the string excluding the null terminator it is valid. So

char  str[3]="abc";

Is valid C. In C++14 however the rule that governs this found in [dcl.init.string]/2 states

There shall not be more initializers than there are array elements.

And goes on to show that the following code is an error

char cv[4] = "asdf"; // error

So in C++ you have to have enough storage for the entire string literal including the null terminator.

Answer 2

Is the code example valid in all C language standards?

Note that only one ISO standard is in effect at a time; C2011 supercedes C99, which superceded C89.

I believe it should be valid under any one of those standards, though.

Is it invalid in all C++ language standards?

Same as above, just change "valid" to "invalid".

Is there a reason that is valid in one language but not another?

Most likely, it was left valid in C so as not to break any legacy code that relied on the behavior. C++ came along about a decade or so after C and tried to address some of C's shortcomings, and this was one of the holes that got plugged.

Many modern programming languages are iterations and improvements on earlier languages; C is B with a type system, C++ is C with OO support and better type safety, Java and C# are C++ with less undefined behavior, etc.