convert int to short in C

Question

I have:

int a = 2147483647;
short b = (short)a;

and I get b = -1 whereas I expect int32 to be converted to int16(short). I expect to see some value and not -1.

Please someone help me with this.

Answer 1

This is implementation defined behavior, for example gcc Integers Implementation document says:

For conversion to a type of width N, the value is reduced modulo 2^N to be within range of the type; no signal is raised.

This can differ from compiler to compiler, I am not able to dig up similar documents for clang nor visual studio.

From the draft C++ standard, section 4.7 Integral conversions paragraph 3:

If the destination type is signed, the value is unchanged if it can be represented in the destination type (and bit-field width); otherwise, the value is implementation-defined.

If this was unsigned then you would have perfectly well defined behavior, as per paragraph 2:

If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source integer (modulo 2n where n is the number of bits used to represent the unsigned type). [ Note: In a two’s complement representation, this conversion is conceptual and there is no change in the bit pattern (if there is no truncation). —end note ]

The language is similar in the C99 draft standard section 6.3.1.3 Signed and unsigned integers.

Answer 2

Your int A is larger than the size of short. When you convert A to short, you get a 1 in the left most bit, which is going to indicate that it is a negative number. Since you're getting -1, I suppose you're getting 1s in all 16 bits, which is going to give you -2^15 + 2^14 + 2^13... + 2^0, which will give you -1. In short (no pun intended), you can't convert the integer to a short if it is too large.