I tried a simple code and found that integers variables are not overflowing instead it seems that latest C++ compiler has introduced a new functionality related to POD datatypes - if the variable crosses its max value, its values are restart from 0:
#include <iostream>
#include <cstdint>
#include <stdexcept>
int main()
{
try
{
for (uint8_t idx = 254 ; ; idx++)
{
std::cout << unsigned(idx) << std::endl;
}
}
catch(std::overflow_error e)
{
std::cout << "Error" << std::endl;
}
}
When I run the code the exception code is never executed - but is this the desired behavior?
Almost nothing throws std::overflow_error; overflow of unsigned values is already defined in the language standard to wrap around to 0, it's not considered an "exceptional" case, so no exception will ever be thrown for plain unsigned integer math. Per cppreference docs on integer arithmetic overflow:
Unsigned integer arithmetic is always performed modulo 2n
where n is the number of bits in that particular integer. E.g. for unsigned int, adding one to UINT_MAX gives 0, and subtracting one from 0 gives UINT_MAX.
Similarly, standard library modules rarely use it:
The only standard library components that throw this exception are std::bitset::to_ulong and std::bitset::to_ullong.
The mathematical functions of the standard library components do not throw this exception (mathematical functions report overflow errors as specified in math_errhandling). Third-party libraries, however, use this. For example, boost.math throws std::overflow_error if boost::math::policies::throw_on_error is enabled (the default setting).
That is the behavior for unsigned types, according to the standard:
Standard 6.7.1/4
Unsigned integers shall obey the laws of arithmetic modulo 2n where n is the number of bits in the value representation of that particular size of integer.
Which inspires the footnote:
This implies that unsigned arithmetic does not overflow because a result that cannot be represented by the resulting unsigned integer type is reduced modulo the number that is one greater than the largest value that can be represented by the resulting unsigned integer type.
it seems that latest C++ compiler has introduced a new functionality
No. This is how C++ has always been specified.
if the variable crosses its max value, its values are restart from 0
If an unsigned integer result of a calculation is not representable by the type, then the result will instead be the representable value that is congruent with the mathematical result modulo 2n where n is the number of bits in the representation (i.e. congruent with the largest representable value + 1).
In other words, (largest representable unsigned integer + 1) is 0, just like you observed.
Note that this rule does not apply to signed integers. Overflowing a signed integer results in undefined behaviour.
P.S. No operation on a fundamental type is specified to throw an exception. Most of the standard library functions don't throw std::overflow_error either.
unsigned int does not overflow or underflow, that's the core difference between signed and unsigned types in C++. unsigned types behave according to modulo arithmetic (i.e., they "wrap around") [basic.fundamental]/4. If you want to provoke an integer overflow, use a signed integer type. But even then, an integer overflow does not throw an exception but just lead to undefined behavior…
I want to define a function that takes an unsigned int as argument and returns an int congruent modulo UINT_MAX+1 to the argument.
A first attempt might look like this:
int unsigned_to_signed(unsigned n)
{
return static_cast<int>(n);
}
But as any language lawyer knows, casting from unsigned to signed for values larger than INT_MAX is implementation-defined.
I want to implement this such that (a) it only relies on behavior mandated by the spec; and (b) it compiles into a no-op on any modern machine and optimizing compiler.
As for bizarre machines... If there is no signed int congruent modulo UINT_MAX+1 to the unsigned int, let's say I want to throw an exception. If there is more than one (I am not sure this is possible), let's say I want the largest one.
OK, second attempt:
int unsigned_to_signed(unsigned n)
{
int int_n = static_cast<int>(n);
if (n == static_cast<unsigned>(int_n))
return int_n;
// else do something long and complicated
}
I do not much care about the efficiency when I am not on a typical twos-complement system, since in my humble opinion that is unlikely. And if my code becomes a bottleneck on the omnipresent sign-magnitude systems of 2050, well, I bet someone can figure that out and optimize it then.
Now, this second attempt is pretty close to what I want. Although the cast to int is implementation-defined for some inputs, the cast back to unsigned is guaranteed by the standard to preserve the value modulo UINT_MAX+1. So the conditional does check exactly what I want, and it will compile into nothing on any system I am likely to encounter.
However... I am still casting to int without first checking whether it will invoke implementation-defined behavior. On some hypothetical system in 2050 it could do who-knows-what. So let's say I want to avoid that.
Question: What should my "third attempt" look like?
To recap, I want to:
Cast from unsigned int to signed int
Preserve the value mod UINT_MAX+1
Invoke only standard-mandated behavior
Compile into a no-op on a typical twos-complement machine with optimizing compiler
[Update]
Let me give an example to show why this is not a trivial question.
Consider a hypothetical C++ implementation with the following properties:
sizeof(int) equals 4
sizeof(unsigned) equals 4
INT_MAX equals 32767
INT_MIN equals -232 + 32768
UINT_MAX equals 232 - 1
Arithmetic on int is modulo 232 (into the range INT_MIN through INT_MAX)
std::numeric_limits<int>::is_modulo is true
Casting unsigned n to int preserves the value for 0 <= n <= 32767 and yields zero otherwise
On this hypothetical implementation, there is exactly one int value congruent (mod UINT_MAX+1) to each unsigned value. So my question would be well-defined.
I claim that this hypothetical C++ implementation fully conforms to the C++98, C++03, and C++11 specifications. I admit I have not memorized every word of all of them... But I believe I have read the relevant sections carefully. So if you want me to accept your answer, you either must (a) cite a spec that rules out this hypothetical implementation or (b) handle it correctly.
Indeed, a correct answer must handle every hypothetical implementation permitted by the standard. That is what "invoke only standard-mandated behavior" means, by definition.
Incidentally, note that std::numeric_limits<int>::is_modulo is utterly useless here for multiple reasons. For one thing, it can be true even if unsigned-to-signed casts do not work for large unsigned values. For another, it can be true even on one's-complement or sign-magnitude systems, if arithmetic is simply modulo the entire integer range. And so on. If your answer depends on is_modulo, it's wrong.
[Update 2]
hvd's answer taught me something: My hypothetical C++ implementation for integers is not permitted by modern C. The C99 and C11 standards are very specific about the representation of signed integers; indeed, they only permit twos-complement, ones-complement, and sign-magnitude (section 6.2.6.2 paragraph (2); ).
But C++ is not C. As it turns out, this fact lies at the very heart of my question.
The original C++98 standard was based on the much older C89, which says (section 3.1.2.5):
For each of the signed integer types, there is a corresponding (but
different) unsigned integer type (designated with the keyword
unsigned) that uses the same amount of storage (including sign
information) and has the same alignment requirements. The range of
nonnegative values of a signed integer type is a subrange of the
corresponding unsigned integer type, and the representation of the
same value in each type is the same.
C89 says nothing about only having one sign bit or only allowing twos-complement/ones-complement/sign-magnitude.
The C++98 standard adopted this language nearly verbatim (section 3.9.1 paragraph (3)):
For each of the signed integer types, there exists a corresponding
(but different) unsigned integer type: "unsigned char", "unsigned
short int", "unsigned int", and "unsigned long int", each of
which occupies the same amount of storage and has the same alignment
requirements (3.9) as the corresponding signed integer type ; that
is, each signed integer type has the same object representation as
its corresponding unsigned integer type. The range of nonnegative
values of a signed integer type is a subrange of the corresponding
unsigned integer type, and the value representation of each
corresponding signed/unsigned type shall be the same.
The C++03 standard uses essentially identical language, as does C++11.
No standard C++ spec constrains its signed integer representations to any C spec, as far as I can tell. And there is nothing mandating a single sign bit or anything of the kind. All it says is that non-negative signed integers must be a subrange of the corresponding unsigned.
So, again I claim that INT_MAX=32767 with INT_MIN=-232+32768 is permitted. If your answer assumes otherwise, it is incorrect unless you cite a C++ standard proving me wrong.
Expanding on user71404's answer:
int f(unsigned x)
{
if (x <= INT_MAX)
return static_cast<int>(x);
if (x >= INT_MIN)
return static_cast<int>(x - INT_MIN) + INT_MIN;
throw x; // Or whatever else you like
}
If x >= INT_MIN (keep the promotion rules in mind, INT_MIN gets converted to unsigned), then x - INT_MIN <= INT_MAX, so this won't have any overflow.
If that is not obvious, take a look at the claim "If x >= -4u, then x + 4 <= 3.", and keep in mind that INT_MAX will be equal to at least the mathematical value of -INT_MIN - 1.
On the most common systems, where !(x <= INT_MAX) implies x >= INT_MIN, the optimizer should be able (and on my system, is able) to remove the second check, determine that the two return statements can be compiled to the same code, and remove the first check too. Generated assembly listing:
__Z1fj:
LFB6:
.cfi_startproc
movl 4(%esp), %eax
ret
.cfi_endproc
The hypothetical implementation in your question:
INT_MAX equals 32767
INT_MIN equals -232 + 32768
is not possible, so does not need special consideration. INT_MIN will be equal to either -INT_MAX, or to -INT_MAX - 1. This follows from C's representation of integer types (6.2.6.2), which requires n bits to be value bits, one bit to be a sign bit, and only allows one single trap representation (not including representations that are invalid because of padding bits), namely the one that would otherwise represent negative zero / -INT_MAX - 1. C++ doesn't allow any integer representations beyond what C allows.
Update: Microsoft's compiler apparently does not notice that x > 10 and x >= 11 test the same thing. It only generates the desired code if x >= INT_MIN is replaced with x > INT_MIN - 1u, which it can detect as the negation of x <= INT_MAX (on this platform).
[Update from questioner (Nemo), elaborating on our discussion below]
I now believe this answer works in all cases, but for complicated reasons. I am likely to award the bounty to this solution, but I want to capture all the gory details in case anybody cares.
Let's start with C++11, section 18.3.3:
Table 31 describes the header <climits>.
...
The contents are the same as the Standard C library header <limits.h>.
Here, "Standard C" means C99, whose specification severely constrains the representation of signed integers. They are just like unsigned integers, but with one bit dedicated to "sign" and zero or more bits dedicated to "padding". The padding bits do not contribute to the value of the integer, and the sign bit contributes only as twos-complement, ones-complement, or sign-magnitude.
Since C++11 inherits the <climits> macros from C99, INT_MIN is either -INT_MAX or -INT_MAX-1, and hvd's code is guaranteed to work. (Note that, due to the padding, INT_MAX could be much less than UINT_MAX/2... But thanks to the way signed->unsigned casts work, this answer handles that fine.)
C++03/C++98 is trickier. It uses the same wording to inherit <climits> from "Standard C", but now "Standard C" means C89/C90.
All of these -- C++98, C++03, C89/C90 -- have the wording I give in my question, but also include this (C++03 section 3.9.1 paragraph 7):
The representations of integral types shall define values by use of a
pure binary numeration system.(44) [Example: this International
Standard permits 2’s complement, 1’s complement and signed magnitude
representations for integral types.]
Footnote (44) defines "pure binary numeration system":
A positional representation for integers that uses the binary digits 0
and 1, in which the values represented by successive bits are
additive, begin with 1, and are multiplied by successive integral
power of 2, except perhaps for the bit with the highest position.
What is interesting about this wording is that it contradicts itself, because the definition of "pure binary numeration system" does not permit a sign/magnitude representation! It does allow the high bit to have, say, the value -2n-1 (twos complement) or -(2n-1-1) (ones complement). But there is no value for the high bit that results in sign/magnitude.
Anyway, my "hypothetical implementation" does not qualify as "pure binary" under this definition, so it is ruled out.
However, the fact that the high bit is special means we can imagine it contributing any value at all: A small positive value, huge positive value, small negative value, or huge negative value. (If the sign bit can contribute -(2n-1-1), why not -(2n-1-2)? etc.)
So, let's imagine a signed integer representation that assigns a wacky value to the "sign" bit.
A small positive value for the sign bit would result in a positive range for int (possibly as large as unsigned), and hvd's code handles that just fine.
A huge positive value for the sign bit would result in int having a maximum larger than unsigned, which is is forbidden.
A huge negative value for the sign bit would result in int representing a non-contiguous range of values, and other wording in the spec rules that out.
Finally, how about a sign bit that contributes a small negative quantity? Could we have a 1 in the "sign bit" contribute, say, -37 to the value of the int? So then INT_MAX would be (say) 231-1 and INT_MIN would be -37?
This would result in some numbers having two representations... But ones-complement gives two representations to zero, and that is allowed according to the "Example". Nowhere does the spec say that zero is the only integer that might have two representations. So I think this new hypothetical is allowed by the spec.
Indeed, any negative value from -1 down to -INT_MAX-1 appears to be permissible as a value for the "sign bit", but nothing smaller (lest the range be non-contiguous). In other words, INT_MIN might be anything from -INT_MAX-1 to -1.
Now, guess what? For the second cast in hvd's code to avoid implementation-defined behavior, we just need x - (unsigned)INT_MIN less than or equal to INT_MAX. We just showed INT_MIN is at least -INT_MAX-1. Obviously, x is at most UINT_MAX. Casting a negative number to unsigned is the same as adding UINT_MAX+1. Put it all together:
x - (unsigned)INT_MIN <= INT_MAX
if and only if
UINT_MAX - (INT_MIN + UINT_MAX + 1) <= INT_MAX
-INT_MIN-1 <= INT_MAX
-INT_MIN <= INT_MAX+1
INT_MIN >= -INT_MAX-1
That last is what we just showed, so even in this perverse case, the code actually works.
That exhausts all of the possibilities, thus ending this extremely academic exercise.
Bottom line: There is some seriously under-specified behavior for signed integers in C89/C90 that got inherited by C++98/C++03. It is fixed in C99, and C++11 indirectly inherits the fix by incorporating <limits.h> from C99. But even C++11 retains the self-contradictory "pure binary representation" wording...
This code relies only on behavior, mandated by the spec, so requirement (a) is easily satisfied:
int unsigned_to_signed(unsigned n)
{
int result = INT_MAX;
if (n > INT_MAX && n < INT_MIN)
throw runtime_error("no signed int for this number");
for (unsigned i = INT_MAX; i != n; --i)
--result;
return result;
}
It's not so easy with requirement (b). This compiles into a no-op with gcc 4.6.3 (-Os, -O2, -O3) and with clang 3.0 (-Os, -O, -O2, -O3). Intel 12.1.0 refuses to optimize this. And I have no info about Visual C.
The original answer solved the problem only for unsigned => int. What if we want to solve the general problem of "some unsigned type" to its corresponding signed type? Furthermore, the original answer was excellent at citing sections of the standard and analyzing some corner cases, but it did not really help me get a feel for why it worked, so this answer will try to give a strong conceptual basis. This answer will try to help explain "why", and use modern C++ features to try to simplify the code.
C++20 answer
The problem has simplified dramatically with P0907: Signed Integers are Two’s Complement and the final wording P1236 that was voted into the C++20 standard. Now, the answer is as simple as possible:
template<std::unsigned_integral T>
constexpr auto cast_to_signed_integer(T const value) {
return static_cast<std::make_signed_t<T>>(value);
}
That's it. A static_cast (or C-style cast) is finally guaranteed to do the thing you need for this question, and the thing many programmers thought it always did.
C++17 answer
In C++17, things are much more complicated. We have to deal with three possible integer representations (two's complement, ones' complement, and sign-magnitude). Even in the case where we know it must be two's complement because we checked the range of possible values, the conversion of a value outside the range of the signed integer to that signed integer still gives us an implementation-defined result. We have to use tricks like we have seen in other answers.
First, here is the code for how to solve the problem generically:
template<typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
constexpr auto cast_to_signed_integer(T const value) {
using result = std::make_signed_t<T>;
using result_limits = std::numeric_limits<result>;
if constexpr (result_limits::min() + 1 != -result_limits::max()) {
if (value == static_cast<T>(result_limits::max()) + 1) {
throw std::runtime_error("Cannot convert the maximum possible unsigned to a signed value on this system");
}
}
if (value <= result_limits::max()) {
return static_cast<result>(value);
} else {
using promoted_unsigned = std::conditional_t<sizeof(T) <= sizeof(unsigned), unsigned, T>;
using promoted_signed = std::make_signed_t<promoted_unsigned>;
constexpr auto shift_by_window = [](auto x) {
// static_cast to avoid conversion warning
return x - static_cast<decltype(x)>(result_limits::max()) - 1;
};
return static_cast<result>(
shift_by_window( // shift values from common range to negative range
static_cast<promoted_signed>(
shift_by_window( // shift large values into common range
static_cast<promoted_unsigned>(value) // cast to avoid promotion to int
)
)
)
);
}
}
This has a few more casts than the accepted answer, and that is to ensure there are no signed / unsigned mismatch warnings from your compiler and to properly handle integer promotion rules.
We first have a special case for systems that are not two's complement (and thus we must handle the maximum possible value specially because it doesn't have anything to map to). After that, we get to the real algorithm.
The second top-level condition is straightforward: we know the value is less than or equal to the maximum value, so it fits in the result type. The third condition is a little more complicated even with the comments, so some examples would probably help understand why each statement is necessary.
Conceptual basis: the number line
First, what is this window concept? Consider the following number line:
| signed |
<.........................>
| unsigned |
It turns out that for two's complement integers, you can divide the subset of the number line that can be reached by either type into three equally sized categories:
- => signed only
= => both
+ => unsigned only
<..-------=======+++++++..>
This can be easily proven by considering the representation. An unsigned integer starts at 0 and uses all of the bits to increase the value in powers of 2. A signed integer is exactly the same for all of the bits except the sign bit, which is worth -(2^position) instead of 2^position. This means that for all n - 1 bits, they represent the same values. Then, unsigned integers have one more normal bit, which doubles the total number of values (in other words, there are just as many values with that bit set as without it set). The same logic holds for signed integers, except that all the values with that bit set are negative.
The other two legal integer representations, ones' complement and sign-magnitude, have all of the same values as two's complement integers except for one: the most negative value. C++ defines everything about integer types, except for reinterpret_cast (and the C++20 std::bit_cast), in terms of the range of representable values, not in terms of the bit representation. This means that our analysis will hold for each of these three representations as long as we do not ever try to create the trap representation. The unsigned value that would map to this missing value is a rather unfortunate one: the one right in the middle of the unsigned values. Fortunately, our first condition checks (at compile time) whether such a representation exists, and then handles it specially with a runtime check.
The first condition handles the case where we are in the = section, which means that we are in the overlapping region where the values in one can be represented in the other without change. The shift_by_window function in the code moves all values down by the size of each of these segments (we have to subtract the max value then subtract 1 to avoid arithmetic overflow issues). If we are outside of that region (we are in the + region), we need to jump down by one window size. This puts us in the overlapping range, which means we can safely convert from unsigned to signed because there is no change in value. However, we are not done yet because we have mapped two unsigned values to each signed value. Therefore, we need to shift down to the next window (the - region) so that we have a unique mapping again.
Now, does this give us a result congruent mod UINT_MAX + 1, as requested in the question? UINT_MAX + 1 is equivalent to 2^n, where n is the number of bits in the value representation. The value we use for our window size is equal to 2^(n - 1) (the final index in a sequence of values is one less than the size). We subtract that value twice, which means we subtract 2 * 2^(n - 1) which is equal to 2^n. Adding and subtracting x is a no-op in arithmetic mod x, so we have not affected the original value mod 2^n.
Properly handling integer promotions
Because this is a generic function and not just int and unsigned, we also have to concern ourselves with integral promotion rules. There are two possibly interesting cases: one in which short is smaller than int and one in which short is the same size as int.
Example: short smaller than int
If short is smaller than int (common on modern platforms) then we also know that unsigned short can fit in an int, which means that any operations on it will actually happen in int, so we explicitly cast to the promoted type to avoid this. Our final statement is pretty abstract and becomes easier to understand if we substitute in real values. For our first interesting case, with no loss of generality let us consider a 16-bit short and a 17-bit int (which is still allowed under the new rules, and would just mean that at least one of those two integer types have some padding bits):
constexpr auto shift_by_window = [](auto x) {
return x - static_cast<decltype(x)>(32767) - 1;
};
return static_cast<int16_t>(
shift_by_window(
static_cast<int17_t>(
shift_by_window(
static_cast<uint17_t>(value)
)
)
)
);
Solving for the greatest possible 16-bit unsigned value
constexpr auto shift_by_window = [](auto x) {
return x - static_cast<decltype(x)>(32767) - 1;
};
return int16_t(
shift_by_window(
int17_t(
shift_by_window(
uint17_t(65535)
)
)
)
);
Simplifies to
return int16_t(
int17_t(
uint17_t(65535) - uint17_t(32767) - 1
) -
int17_t(32767) -
1
);
Simplifies to
return int16_t(
int17_t(uint17_t(32767)) -
int17_t(32767) -
1
);
Simplifies to
return int16_t(
int17_t(32767) -
int17_t(32767) -
1
);
Simplifies to
return int16_t(-1);
We put in the largest possible unsigned and get back -1, success!
Example: short same size as int
If short is the same size as int (uncommon on modern platforms), the integral promotion rule are slightly different. In this case, short promotes to int and unsigned short promotes to unsigned. Fortunately, we explicitly cast each result to the type we want to do the calculation in, so we end up with no problematic promotions. With no loss of generality let us consider a 16-bit short and a 16-bit int:
constexpr auto shift_by_window = [](auto x) {
return x - static_cast<decltype(x)>(32767) - 1;
};
return static_cast<int16_t>(
shift_by_window(
static_cast<int16_t>(
shift_by_window(
static_cast<uint16_t>(value)
)
)
)
);
Solving for the greatest possible 16-bit unsigned value
auto x = int16_t(
uint16_t(65535) - uint16_t(32767) - 1
);
return int16_t(
x - int16_t(32767) - 1
);
Simplifies to
return int16_t(
int16_t(32767) - int16_t(32767) - 1
);
Simplifies to
return int16_t(-1);
We put in the largest possible unsigned and get back -1, success!
What if I just care about int and unsigned and don't care about warnings, like the original question?
constexpr int cast_to_signed_integer(unsigned const value) {
using result_limits = std::numeric_limits<int>;
if constexpr (result_limits::min() + 1 != -result_limits::max()) {
if (value == static_cast<unsigned>(result_limits::max()) + 1) {
throw std::runtime_error("Cannot convert the maximum possible unsigned to a signed value on this system");
}
}
if (value <= result_limits::max()) {
return static_cast<int>(value);
} else {
constexpr int window = result_limits::min();
return static_cast<int>(value + window) + window;
}
}
See it live
https://godbolt.org/z/74hY81
Here we see that clang, gcc, and icc generate no code for cast and cast_to_signed_integer_basic at -O2 and -O3, and MSVC generates no code at /O2, so the solution is optimal.
You can explicitly tell the compiler what you want to do:
int unsigned_to_signed(unsigned n) {
if (n > INT_MAX) {
if (n <= UINT_MAX + INT_MIN) {
throw "no result";
}
return static_cast<int>(n + INT_MIN) - (UINT_MAX + INT_MIN + 1);
} else {
return static_cast<int>(n);
}
}
Compiles with gcc 4.7.2 for x86_64-linux (g++ -O -S test.cpp) to
_Z18unsigned_to_signedj:
movl %edi, %eax
ret
If x is our input...
If x > INT_MAX, we want to find a constant k such that 0 < x - k*INT_MAX < INT_MAX.
This is easy -- unsigned int k = x / INT_MAX;. Then, let unsigned int x2 = x - k*INT_MAX;
We can now cast x2 to int safely. Let int x3 = static_cast<int>(x2);
We now want to subtract something like UINT_MAX - k * INT_MAX + 1 from x3, if k > 0.
Now, on a 2s complement system, so long as x > INT_MAX, this works out to:
unsigned int k = x / INT_MAX;
x -= k*INT_MAX;
int r = int(x);
r += k*INT_MAX;
r -= UINT_MAX+1;
Note that UINT_MAX+1 is zero in C++ guaranteed, the conversion to int was a noop, and we subtracted k*INT_MAX then added it back on "the same value". So an acceptable optimizer should be able to erase all that tomfoolery!
That leaves the problem of x > INT_MAX or not. Well, we create 2 branches, one with x > INT_MAX, and one without. The one without does a strait cast, which the compiler optimizes to a noop. The one with ... does a noop after the optimizer is done. The smart optimizer realizes both branches to the same thing, and drops the branch.
Issues: if UINT_MAX is really large relative to INT_MAX, the above might not work. I am assuming that k*INT_MAX <= UINT_MAX+1 implicitly.
We could probably attack this with some enums like:
enum { divisor = UINT_MAX/INT_MAX, remainder = UINT_MAX-divisor*INT_MAX };
which work out to 2 and 1 on a 2s complement system I believe (are we guaranteed for that math to work? That's tricky...), and do logic based on these that easily optimize away on non-2s complement systems...
This also opens up the exception case. It is only possible if UINT_MAX is much larger than (INT_MIN-INT_MAX), so you can put your exception code in an if block asking exactly that question somehow, and it won't slow you down on a traditional system.
I'm not exactly sure how to construct those compile-time constants to deal correctly with that.
std::numeric_limits<int>::is_modulo is a compile time constant. so you can use it for template specialization. problem solved, at least if compiler plays along with inlining.
#include <limits>
#include <stdexcept>
#include <string>
#ifdef TESTING_SF
bool const testing_sf = true;
#else
bool const testing_sf = false;
#endif
// C++ "extensions"
namespace cppx {
using std::runtime_error;
using std::string;
inline bool hopefully( bool const c ) { return c; }
inline bool throw_x( string const& s ) { throw runtime_error( s ); }
} // namespace cppx
// C++ "portability perversions"
namespace cppp {
using cppx::hopefully;
using cppx::throw_x;
using std::numeric_limits;
namespace detail {
template< bool isTwosComplement >
int signed_from( unsigned const n )
{
if( n <= unsigned( numeric_limits<int>::max() ) )
{
return static_cast<int>( n );
}
unsigned const u_max = unsigned( -1 );
unsigned const u_half = u_max/2 + 1;
if( n == u_half )
{
throw_x( "signed_from: unsupported value (negative max)" );
}
int const i_quarter = static_cast<int>( u_half/2 );
int const int_n1 = static_cast<int>( n - u_half );
int const int_n2 = int_n1 - i_quarter;
int const int_n3 = int_n2 - i_quarter;
hopefully( n == static_cast<unsigned>( int_n3 ) )
|| throw_x( "signed_from: range error" );
return int_n3;
}
template<>
inline int signed_from<true>( unsigned const n )
{
return static_cast<int>( n );
}
} // namespace detail
inline int signed_from( unsigned const n )
{
bool const is_modulo = numeric_limits< int >::is_modulo;
return detail::signed_from< is_modulo && !testing_sf >( n );
}
} // namespace cppp
#include <iostream>
using namespace std;
int main()
{
int const x = cppp::signed_from( -42u );
wcout << x << endl;
}
EDIT: Fixed up code to avoid possible trap on non-modular-int machines (only one is known to exist, namely the archaically configured versions of the Unisys Clearpath). For simplicity this is done by not supporting the value -2n-1 where n is the number of int value bits, on such machine (i.e., on the Clearpath). in practice this value will not be supported by the machine either (i.e., with sign-and-magnitude or 1’s complement representation).
I think the int type is at least two bytes, so the INT_MIN and INT_MAX may change in different platforms.
Fundamental types
≤climits≥ header
My money is on using memcpy. Any decent compiler knows to optimise it away:
#include <stdio.h>
#include <memory.h>
#include <limits.h>
static inline int unsigned_to_signed(unsigned n)
{
int result;
memcpy( &result, &n, sizeof(result));
return result;
}
int main(int argc, const char * argv[])
{
unsigned int x = UINT_MAX - 1;
int xx = unsigned_to_signed(x);
return xx;
}
For me (Xcode 8.3.2, Apple LLVM 8.1, -O3), that produces:
_main: ## #main
Lfunc_begin0:
.loc 1 21 0 ## /Users/Someone/main.c:21:0
.cfi_startproc
## BB#0:
pushq %rbp
Ltmp0:
.cfi_def_cfa_offset 16
Ltmp1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp2:
.cfi_def_cfa_register %rbp
##DEBUG_VALUE: main:argc <- %EDI
##DEBUG_VALUE: main:argv <- %RSI
Ltmp3:
##DEBUG_VALUE: main:x <- 2147483646
##DEBUG_VALUE: main:xx <- 2147483646
.loc 1 24 5 prologue_end ## /Users/Someone/main.c:24:5
movl $-2, %eax
popq %rbp
retq
Ltmp4:
Lfunc_end0:
.cfi_endproc
I encountered a strange thing when I was programming under c++. It's about a simple multiplication.
Code:
unsigned __int64 a1 = 255*256*256*256;
unsigned __int64 a2= 255 << 24; // same as the above
cerr()<<"a1 is:"<<a1;
cerr()<<"a2 is:"<<a2;
interestingly the result is:
a1 is: 18446744073692774400
a2 is: 18446744073692774400
whereas it should be:(using calculator confirms)
4278190080
Can anybody tell me how could it be possible?
255*256*256*256
all operands are int you are overflowing int. The overflow of a signed integer is undefined behavior in C and C++.
EDIT:
note that the expression 255 << 24 in your second declaration also invokes undefined behavior if your int type is 32-bit. 255 x (2^24) is 4278190080 which cannot be represented in a 32-bit int (the maximum value is usually 2147483647 on a 32-bit int in two's complement representation).
C and C++ both say for E1 << E2 that if E1 is of a signed type and positive and that E1 x (2^E2) cannot be represented in the type of E1, the program invokes undefined behavior. Here ^ is the mathematical power operator.
Your literals are int. This means that all the operations are actually performed on int, and promptly overflow. This overflowed value, when converted to an unsigned 64bit int, is the value you observe.
It is perhaps worth explaining what happened to produce the number 18446744073692774400. Technically speaking, the expressions you wrote trigger "undefined behavior" and so the compiler could have produced anything as the result; however, assuming int is a 32-bit type, which it almost always is nowadays, you'll get the same "wrong" answer if you write
uint64_t x = (int) (255u*256u*256u*256u);
and that expression does not trigger undefined behavior. (The conversion from unsigned int to int involves implementation-defined behavior, but as nobody has produced a ones-complement or sign-and-magnitude CPU in many years, all implementations you are likely to encounter define it exactly the same way.) I have written the cast in C style because everything I'm saying here applies equally to C and C++.
First off, let's look at the multiplication. I'm writing the right hand side in hex because it's easier to see what's going on that way.
255u * 256u = 0x0000FF00u
255u * 256u * 256u = 0x00FF0000u
255u * 256u * 256u * 256u = 0xFF000000u (= 4278190080)
That last result, 0xFF000000u, has the highest bit of a 32-bit number set. Casting that value to a signed 32-bit type therefore causes it to become negative as-if 232 had been subtracted from it (that's the implementation-defined operation I mentioned above).
(int) (255u*256u*256u*256u) = 0xFF000000 = -16777216
I write the hexadecimal number there, sans u suffix, to emphasize that the bit pattern of the value does not change when you convert it to a signed type; it is only reinterpreted.
Now, when you assign -16777216 to a uint64_t variable, it is back-converted to unsigned as-if by adding 264. (Unlike the unsigned-to-signed conversion, this semantic is prescribed by the standard.) This does change the bit pattern, setting all of the high 32 bits of the number to 1 instead of 0 as you had expected:
(uint64_t) (int) (255u*256u*256u*256u) = 0xFFFFFFFFFF000000u
And if you write 0xFFFFFFFFFF000000 in decimal, you get 18446744073692774400.
As a closing piece of advice, whenever you get an "impossible" integer from C or C++, try printing it out in hexadecimal; it's much easier to see oddities of twos-complement fixed-width arithmetic that way.
The answer is simple -- overflowed.
Here Overflow occurred on int and when you are assigning it to unsigned int64 its converted in to 18446744073692774400 instead of 4278190080
This question is a result of another SO question.
Example Code
#include <iostream>
int main()
{
unsigned long b = 35000000;
int i = 100;
int j = 30000000;
unsigned long n = ( i * j ) / b; // #1
unsigned long m = ( 100 * 30000000 ) / b; // #2
std::cout << n << std::endl;
std::cout << m << std::endl;
}
Output
85 85
Compiling this code with g++ -std=c++11 -Wall -pedantic -O0 -Wextra gives the following warning:
9:28: warning: integer overflow in expression [-Woverflow]
Questions
Am I correct in thinking that #1 and #2 invoke undefined behavior because the intermediate result 100 * 30000000 does not fit into an int? Or is the output I am seeing well-defined?
Why do I only get a warning with #2?
Yes, it is undefined behaviour, and the result you get is usually¹ different if unsigned long is a 64-bit type.
¹ It's UB, so there are no guarantees.
Intermediate result
Yes, this is undefined behaviour. What if you just stopped right there and return m? The compiler needs to get from point A to point B, and you've told it to do it by making that calculation (which isn't possible). A compiler may choose to optimize this statement in such a way that you don't get an overflow, but as far as I know, the standard doesn't require the optimizer to do anything.
Why no error when they're variables?
You're explicitly telling gcc not to optimize at all (-O0), so my assumption is that it doesn't know the values of i and j at that point. Normally you'd learn the values because of constant folding, but like I said, you told it not to optimize.
If you re-run this and it still doesn't mention it, there's also the possibility that this warning is generated before the optimizer runs, so it's just not smart enough to do constant folding at all for this step.
1) Yes, it's undefined behavior.
2) Because #1 involves variables (not constants), so the compiler in general doesn't know whether it will overflow (although in this case it does, and I don't know why it doesn't warn).
You get a warning with two, because the compiler knows the values in the operand. The outputs are right because both use /b which is unsigned long. The temporary value to be divisible by b must be hold greater or equal datatype range, ( i * j ) or ( 100 * 30000000 ) are stored in a CPU register that has the same datatype range of the value to be divided, if b was an int the temporary result would be a int, since b is an ulong, int can't be divided by ulong, the temporary value is stored to an ulong.
It is undefined behavior if it overflows, but it's not overflowing in those cases
A program with the same structure, only changing b to int will have only two lines on .s code.
cltd
idivl (%ecx)
to b = int
movl $0,
%edx divl (%ecx)
to b = unsigned long,
idivl performs signed division, storing the value as signed
divl performs unsigned division, storing the value as unsigned
So you're right, the operation does overflows, the output is correct because of the division operation.
What is the difference of idivl and divl?
https://stackoverflow.com/a/12488534/1513286
As for 5/4 the result is undefined behavior.
However note that if you changed the types to unsigned (for the constants just add the u suffix) not only the values do fit, but according to 3.9.1/4 the arithmetic becomes a modulo arithmetic and the result is perfectly defined even for larger intermediate values that do not fit the type.