The 1999 C99 standard introduces several new language features, including:

A selection of new features in C99 that might be of interest to developers using them for the first time are documented.

Some examples of special cases where the language specified by the C90 standard is not a subset of C++ include support for // comments and merging of the typedef and structure tag namespaces. For example, in C90 the following code expands to x = a / b - c; because /* hello world */ is deleted, but in C++ and C99 it expands to x = a - c; because everything from // to the end of the line is deleted:

x = a //* hello world */ b
  - c;

The following code demonstrates how typedef and the structure tag are treated differently between C (90 and 99) and C++ because of their merged namespaces:

typedef int a;
{
  struct a { int x, y; };
  printf("%d\n", sizeof(a));
}

In C 90 and C99, this code defines two types with separate names whereby a is a typedef for int and struct a is a structure type containing two integer data types. sizeof(a) evaluates to sizeof(int).

In C++, a structure type can be addressed using only its tag. This means that when the definition of struct a is in scope, the name a used on its own refers to the structure type rather than the typedef, so in C++ sizeof(a) is greater than sizeof(int).