Where is C not a subset of C++? [closed]

Answer 1

#include <stdio.h>

int new (int n) {
    return n/2;
}

int main(void) {
    printf("%d\n", new(10));
    return 0;
}

See also the C++ FAQ entry.

Answer 2

If you use gcc, you can use the warning -Wc++-compat to give you warnings about C code which is dubious in C++ in some way. Its currently used in gcc itself, and its gotten a lot better recently (maybe try a nightly version to get the best you can).

(This doesn't strictly answer the question, but folk might like it).

Answer 3

A number of the answers here cover syntax differences that would cause C++ compilers to fail on C89 (or C99) source code. However, there are some subtle language differences that are legal in both languages but that would produce different behavior. The sizeof (char) difference that Naveen mentioned is one example, but Write a program that will print "C" if compiled as an (ANSI) C program, and "C++" if compiled as a C++ program lists some others.

Answer 4

C++ has new keywords as well. The following is valid C code but won't compile under C++:

int class = 1;
int private = 2;
int public = 3;
int virtual = 4;

Answer 5

In C, sizeof('a') is equal to sizeof(int).

In C++, sizeof('a') is equal to sizeof(char).

Answer 6

In C++, if you declare a struct, union, or enum, its name is immediately accessible without any qualifiers:

struct foo { ... };
foo x; // declare variable

In C, this won't work, because types thus declared live in their own distinct namespaces. Thus, you have to write:

struct foo { ... };
struct foo x; // declare variable

Notice the presence of struct there on the second line. You have to do the same for union and enum (using their respective keywords), or use the typedef trick:

typedef struct { ... } foo;
foo x; // declare variable

Consequently, you can have several types of different kinds named the same in C, since you can disambiguate:

struct foo { ... };
typedef enum { ... } foo;

struct foo x;
foo y;

In C++, however, while you can prefix a struct name with keyword struct whenever you reference it, the namespaces are merged, and so the above C snippet isn't valid. On the other hand, C++ specifically makes an exception to allow a type and a typedef for that type to have the same name (obviously with no effect), to allow the use of typedef trick unchanged from C.