One from top of my head - C++ does not support default int.
dark_charlie
2010-09-23 09:37:40
+9
A:
The elephant in the room: the following is valid C but not valid C++.
int typename = 1;
Substitute your favorite C++ reserved word.
Konrad Rudolph
2010-09-23 09:38:53
Whyever was this downvoted?
Konrad Rudolph
2010-09-23 09:47:50
+5
A:
C++ also does not support variable-length arrays, where:
int array[n];
is valid in C, but not C++. A C++ version of the above would be:
int *array = new int[n];
...
delete [] array;
Alexander Rafferty
2010-09-23 09:40:40
You might want to clarify that `n` is a variable, not a constant, otherwise it would indeed be valid in C++.
Andreas Magnusson
2010-09-23 09:46:57
I would replace `new int[]` and `delete[]` with `std::vector<int>`.
FredOverflow
2010-09-23 09:52:30
Can you explain why it is that you can do the first in C? Would `array` be allocated on the stack somehow, and what if `n` were too large for that? I don't know C well at all, and I'm curious what exactly is going on there.
notJim
2010-09-23 09:57:10
@notJim: yes, `array` would be allocated on the stack. If `n` were too large for that then if you're lucky the implementation terminates due to a signal (or other OS-driven sanction). If you're unlucky, the implementation blindly carries on trashing someone else's memory. Depends on the the OS/compiler.
Steve Jessop
2010-09-23 10:39:12
+2
A:
Simple example, consider this declaration:
int f();
This is valid C, but invalid C++: f(3, 2, -5, "wtf");
Explanation: in C, int f() is really int f(...). Declare as int f(void) if you don't want f to take parameters.
Alexandre C.
2010-09-23 09:47:00
No, `int f()` is not really `int f(...)` , there's a big semantic difference. In the first case, this means I don't know the parameters of that function and at the first encounter of the function call, the default types used as parameters declares the signature of that function, subsequent calls must adhere to that implicit prototype and the compiler should warn if you call the function with other params. With the ellipse it's not the case, each call can have different parameters without warning.
tristopia
2010-09-23 13:02:08
Absolutely, I was just nitpicking as there is a real but subtle difference. This said I haven't downvoted your contribution as the message is mainly ok.
tristopia
2010-09-23 14:51:53
+3
A:
There is a special wiki entry that summarizes a lot of issues.
Jens Gustedt
2010-09-23 09:53:19