Does C++ contain the entire C language? [duplicate]

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No, C++ is not a superset of the C language. While C++ contains a large part of C, there are subtle difference that can bite you badly where you least expect them. Here are some examples:

  • C has the concept of tentative definitions which doesn’t exist in C++.
  • C does not require explicit conversion on assignment of void pointers to variables of concrete type.
  • C has different rules regarding const propagation.
  • C has something called the “implicit int rule,” which, although abolished with C99, appears some times and needs to be considered.
  • The C preprocessor has some features the C++ preprocessor does not have.
  • The C language has two styles of function definition, K&R-style and Stroustrup-style. C++ only has Stroustrup-style.
  • The lexing rules for C and C++ are different with neither being a subset of the other
  • C and C++ have different sets of reserved words. This can cause weird errors because an identifier is not allowed in the other language.
  • While C++ took almost all features from ANSI C (C89), many features were added to C in subsequent standard revisions that are not available in C++.
  • C++ has a different syntax, even for some parts that aren’t new. For example, a ? b : c = d is a syntax error in C but parsed as a ? b : (c = d) in C++.
  • C guarantees that &*E is exactly identical to E, even if E is a null pointer. C++ has no such guarantee.
  • In C, a string literal initializing an array of characters can initialize an array that is at least as long as the string without the trailing \0 byte. (i.e. char foo[3] = "bar" is legal). In C++, the array has to be at least as long as the string including the trailing \0 byte.
  • In C, a character literal like 'A' has type int. In C++, it has type char.

  • C has a special rule to make type punning through unions to be legal. C++ lacks this language, making code such as

    union intfloat {
    int i;
    float f;
} fi;

fi.f = 1.0;
printf("%d\n", fi.i);

    undefined behaviour.

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