What prevents C++ from being a strict superset of C? [duplicate]

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“C subset of C++” -> Where not ? examples ?

I'm aware that C++ is not a strict superset of C. What language features prevent C++ from being a superset of C?

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The elephant in the room: the following is valid C but not valid C++.

int typename = 1;

Substitute your favorite C++ reserved word.

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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;

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There is a special wiki entry that summarizes a lot of issues.

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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 treated like int f(...) (at least at the first call site). Declare as int f(void) if you don't want f to take parameters.

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One from top of my head - C++ does not support default int.