Is there something that I can do in C but I can't do in C++ ?

Question

Is there something that I can do in C but I can't do in C++ ? I stumbled upon the question in a sample interview questions site...

Answer 1

int new = 0;

works in C, but obviously can't work in C++ because 'new' is a reserved word.

There are some other 'tricks' with reserved words, but other than that, you can pretty much do everything in C that you can do in C++.

Answer 2

The short answer is...no, not really. See http://www.research.att.com/~bs/bs_faq.html#difference

Answer 3

You can do almost everything in any of the programming languages. Of course the way of expressing it will vary, as well as the amount of code, clarity of code, ease of further maintenance. Some tasks can be coded with few lines in Prolog and few pages of code in C++, and so on.

Some limiting factors are the available libraries, available compilers, and low-level issues. However when you consider C and C++ on a typical PC, then there is no difference in things that can be done in either of them.

Unless of course you were asking for the differences between C and C++ - for these other people have given you the idea.

Answer 4

char *c = malloc(sizeof(char));

is valid in C, not C++ i.e. automatically casting void*. This of course is a syntax issue, not so much as what you can and cannot _do_ (i.e. accomplish).

Answer 5

There are some things you can say in C wihch you can't in C++ (because C++ has stricter syntax-checking, and C has a more extensive 'legacy' syntax).

Also, there may be some run-time environments (O/S+library+compiler) which support C but not C++, so you can do C on those platforms where you can't do C++.

Answer 6

In C, you can create array literals ("compound literal"), but in C++ you cannot

/* p points to the first element of an array of 4 int */
int *p = (int[]){1, 2, 3, 4};

You can also create an array with size not yet known at compile time, but C++ has no such possibility ("variable length array"):

// create array. size is known at runtime only.
int p[rand() % 5 + 1];

Answer 7

Syntactically there are a few things you could write in C that wouldn't compile in C++ (See Incompatibilities Between ISO C and ISO C++ for excruciating details.). If you're asking at a higher level, if there is some program that it's possible to write in C, but not possible to write in C++, then the answer is "No."

Answer 8

Is this referring to the latest C standard? The original C standard (ANSI 1989 or ISO 1990, with 1995 updates) is fairly close to being a subset of C++. There's differences, but they're mostly contrived (the biggest exception probably being that void * converts freely with any data pointer in C but not in C++).

However, a new C standard came out in 1999, some time after I'd stopped doing anything in the most modern C. It had new features, some of which are going into the C++ standard due this year or next, but not all.

Answer 9

Here's a detailed list of C features that are incompatible with C++.

Answer 10

In 'C' you don't need forward declarations. This allows you to pass parameters which are interpreted incorrectly. (Not that this is a great feature, but you can't do it in C++)

in file A:

float sum(float a, float b)
{
   return a+b;
}

in file B

main()
{
  printf("%f\n", sum(1,2));
}

with C, this compiles, but prints 0.000

with C++, you need a float sum(float,float); before the printf, and it gives the expected result.

Answer 11

C++ lacks C99's restrict qualifier. Therefore, there is no way to tell the compiler to perform optimizations based around knowing that pointers aren't aliases.

Answer 12

You can sparsely initialize arrays in C. I like to use it for mapping int->sometype for relatively dense static maps where an unmapped value can be interpreted as 0:

int my_array[] = { [1] = 3, [4] = 2 };
printf("%d %d %d\n", sizeof my_array, my_array[0], my_array[1]);
/* prints 20, 0, 3 */

Answer 13

C++ does not support named struct member initialization, in C you can do:

struct { int x, y; } a = { .x = 3 };

You can also combine this with the feature shown by Matt Havener:

struct { int a[3], b; } w[] = { [0].a = {1}, [1].a[0] = 2 };

Answer 14

...that is there anything I can do in C but not in C++.

Both languages are Turing complete, so in theory you can code up equally functional applications in both.

OTOH, C++ is not a superset of C. Especially C99 has some features that C++ does not have. E.g. designated initializers, variable length arrays in structs and as automatic variables. Depending on your "anything", this could be something that C++ cannot do but C can.

Answer 15

If the criteria is to solve a particular programming problem then both will do the job although it may be a bit easier in some cases to do it in C++ due to the higher level of abstraction

Answer 16

"If it can't be done in assembly, it's not worth doing!"

ok, humor aside, I THINK the OP is asking syntactically, rather than operationally.

I think that there are a few obscure things that are legal in C (at least C89) that are not legal in C++, or at least deprecated... But (if they exist) they're pretty obscure. C++ is functionally a superset of C.

Answer 17

Quite a few things. For example, in C you can write code like this:

void * v = 0;
char * p = v;

and you can create arrays like this:

int main() {
    int n = 42;
    int a[n];
    return 0;
}

neither of which will compile under C++.

Answer 18

Actually, I can think of one example:

When you create a library (.lib file or .dll file) to be shared by other applications, you're better off using C instead of C++ because the results are more portable. You can do this within a C++ compiler by using an 'extern "C"' block though.

Specifically, C++ has a quirk where there is no standard convention for name mangling - for translating your library's function signatures into more low level names used by the compiler. So for example if you have a function like 'int addNumbers (int a, int b)', different C++ compilers may translate this function into different names, which can lead to problems when you want to import the library. If you use a C compiler or surround your library importing and exporting code with a C block though you won't see this problem, since there is only one way to mangle function names in C.

Answer 19

Declare a variable named 'class', as in:

int class = 0;

Answer 20

C++ is obviously not a superset of C for a very simple reason: New keywords have been added to C++
class, virtual, new, etc and thus can no more be used as identifiers in C++.

Some of the reasons are subtler.
You can find an exhaustive answer to this question on Bjarn Stroustrup's website: The C++ programming language | Appendix B

Answer 21

Write a program for a platform that supports C but not C++.

Does anyone know of any examples?

Answer 22

The 1998 C++ standard has a list of incompatibilities with the 1990 C standard that is 13 pages long (Annex C). Granted, it's not a lot, compared to the amount of pages that describe the languages, but still covers quit a bit of ground.

Quick summary of the kind of differences that it lists:

• New keywords are added (any C program that uses them as identifiers is not C++)
• Type of character literal changed from int to char (compare sizeof('a') in C and C++!)
• String literals made const (can't do char* q = expr ? "abc" : "de";)
• "Tentative definitions" are removed from the language.
• "Compatible types" are removed from the language.
• Converting from void* to any other pointer now requires casting.
• Converting from any const/volatile pointer to void* now requires casting.
• "Implicit declarations" are removed from the language.
• Expressions can no longer create new types (as in p = (void*)(struct x {int i;} *)0; )
• results of some expressions became lvalues (compare sizeof(0, arr) for char arr[100];)

...that was the first 3 pages of Annex C.

If you go to the 1999 C standard, the differences are going to take forever to describe. Although C++0x did include some of C99 features, many of them are just inherently incompatible, like the complex type.

Answer 23

in c, variable declared inside function are public whereas in c++ , data members (variable) declared in default as Private..