Smallest values for int8_t and int64_t

Question

With regard to to those definitions found in stdint.h, I wish to test a function for converting vectors of int8_t or vectors of int64_t to vectors of std::string.

Here are my tests:

TEST(TestAlgorithms, toStringForInt8)
{
    std::vector<int8_t> input = boost::assign::list_of(-128)(0)(127);
    Container container(input);
    EXPECT_TRUE(boost::apply_visitor(ToString(),container) == boost::assign::list_of("-128")("0")("127"));
}

TEST(TestAlgorithms, toStringForInt64)
{
    std::vector<int64_t> input = boost::assign::list_of(-9223372036854775808)(0)(9223372036854775807);
    Container container(input);
    EXPECT_TRUE(boost::apply_visitor(ToString(),container) == boost::assign::list_of("-9223372036854775808")("0")("9223372036854775807"));
}

However, I am getting a warning in visual studio for the line:

std::vector<int64_t> input = boost::assign::list_of(-9223372036854775808)(0)(9223372036854775807);

as follows:

warning C4146: unary minus operator applied to unsigned type, result still unsigned

If I change -9223372036854775808 to -9223372036854775807, the warning disappears.

What is the issue here? With regard to my original code, the test is passing.

Answer 1

It's like the compiler says, -9223372036854775808 is not a valid number because the - and the digits are treated separately.

You could try -9223372036854775807 - 1 or use std::numeric_limits<int64_t>::min() instead.

Answer 2

The issue is that integer literals are not negative; so -42 is not a literal with a negative value, but rather the - operator applied to the literal 42.

In this case, 9223372036854775808 is out of the range of int64_t, so it will be given an unsigned type. Due to the magic of modular arithmetic, you can still negate it, assign it to int64_t, and end up with the result you expect; but the compiler will warn you about the unsigned negation (if you tell it to) since that can often be the result of an error.

You could avoid the warning (and make the code more obviously correct) by using std::numeric_limits<int64_t>::min() instead.