Is C code still considered C++?

Question

The comment to this answer got me wondering. I've always thought that C was a proper subset of C++, that is, any valid C code is valid C++ code by extension. Am I wrong about that? Is it possible to write a valid C program that is not valid C++ code?

EDIT: This is really similar to, but not an exact duplicate of this question.

Answer 1

In general, yes C code is considered C++ code.

But C is not a proper subset in a strict sense. There are a couple of exceptions.

Here are some valid things in C that are not valid in C++:

int *new;//<-- new is not a keyword in C
char *p = malloc(1024); //void * to char* without cast 

There are more examples too, but you get the idea.

I previously wrote a more extensive answer in a similar question here.

Answer 2

Also note that C99 adds several features which aren't permitted in C++ (or are only supported using vendor extensions), such as builtin _Complex and _Imaginary data types, variable-length arrays (arrays sized at runtime rather than compile time), flexible array members (arrays declared as the last member of a struct that may contain an unspecified number of elements), and more.

For the exhaustive list of incompatibilities between C and C++, including changes with C99, see http://david.tribble.com/text/cdiffs.htm.

Answer 3

A few more:

C allows recursive calls to main, C++ does not

char foo[3] = "abc" is legal C, not C++

sizeof('A') == sizeof(int) is true in C and false in C++

There are even more changes with C99

Edit: I found a post that lists the majority of the differences. http://c-faq.com/misc/cplusplus.nr.html

Summary of why C is not proper subset of C++:

1. auto conversion from void * to any object/incomplete type
2. new keywords (a number of these)
3. due to structs becoming scoped
4. due to struct tags becoming typedefs
5. due to prototypes being required
6. implicit int rules
7. Recursive call to main
8. Due to // comments
9. due to character literals being type char and not int etc.

Answer 4

A couple of other things that are valid C but not C++:

int func();
func(0,0); //Error in C++, but not in C

Also don't underestimate the impact of C++ having more keywords:

int new;  //Obviously an error in C++

Answer 5

One thing that sets the two apart and comes up in day to day development is linkage and function name mangling. A C function compiled with a C compiler is not accessible to C++ unless the prototype is marked up with extern "C".

Answer 6

typedef struct {
     int a, b, c;
} st;

st s = {
    .a = 1,
    .b = 2,
};

This is valid C code which does not compile in most C++ compilers. It is not part of the C++ spec as far as I know. However, some C++ compilers are "liberal" with certain parts of the language and allow things they shouldn't, just as a lot miss out on a few nuances that are in the spec but almost never used.

Answer 7

I think it may be more correct to say that ANSI C is a subset of C++. Not K&R C.

Answer 8

See also Is it true that there is no need to learn C because C++ contains everything. There is a large common subset of C and C++; there are lots of extensions in C++ that are not in C; there are some bits in C that are not in C++ (mostly the bits you don't want to use); and there are some bits that are in C and C++ but are different.