large negative integer literals

Question

On Visual Studio 2010 the following program

#include <iostream>
using std::cout;

int main()
{
    cout << -2147483646 << '\n';
    cout << -2147483647 << '\n';
    cout << -2147483648 << '\n';    // numeric_limits<int>::min()
    cout << -2147483649 << '\n';
    cout << -2147483650 << '\n';
    cout << "..." << '\n';
    cout << -4294967293 << '\n';
    cout << -4294967294 << '\n';
    cout << -4294967295 << '\n';    // -numeric_limits<unsigned int>::max()
    cout << -4294967296 << '\n';
    cout << -4294967297 << '\n';
}

generates the following output

-2147483646
-2147483647
2147483648
2147483647
2147483646
...
3
2
1
-4294967296
-4294967297

What is going on?

Is this standard behavior or a Visual Studio bug?

Edit: As several people have pointed out, there is no such thing as a negative integer literal. See Keith Thompson's excellent answer below for more details.

Answer 1

-2147483648, for example, is not an integer literal; it's an expression consisting of a unary - operator applied to the literal 2147483648.

Prior to the new C++ 2011 standard, C++ doesn't require the existence of any type bigger than 32 bits (C++2011 adds long long), so the literal 2147483648 is non-portable.

A decimal integer literal is of the first of the following types in which its value fits:

int
long int
long long int (new in C++ 2011)

Note that it's never of an unsigned type in standard C++. In the 1998 and 2003 versions of the C standard (which don't have long long int), a decimal integer literal that's too big to fit in long int results in undefined behavior. In C++2011, if a decimal integer literal doesn't fit in long long int, then the program is "ill-formed".

But gcc (at least as of release 4.6.1, the latest one I have) doesn't implement the C++2011 semantics. The literal 2147483648, which doesn't fit in a 32-bit long, is treated as unsigned long, at least on my 32-bit system. (That's fine for C++98 or C++2003; the behavior is undefined, so the compiler can do anything it likes.)

So given a typical 32-bit 2's-complement int type, this:

cout << -2147483647 << '\n';

takes the int value 2147483647, negates it, and prints the result, which matches the mathematical result you'd expect. But this:

cout << -2147483648 << '\n';

(when compiled with gcc 4.6.1) takes the long or unsigned long value 2147483648, negates it as an unsigned int, yielding 2147483648, and prints that.

As others have mentioned, you can use suffixes to force a particular type.

Here's a small program that you can use to show how your compiler treats literals:

#include <iostream>
#include <climits>

const char *type_of(int)                { return "int"; }
const char *type_of(unsigned int)       { return "unsigned int"; }
const char *type_of(long)               { return "long"; }
const char *type_of(unsigned long)      { return "unsigned long"; }
const char *type_of(long long)          { return "long long"; }
const char *type_of(unsigned long long) { return "unsigned long long"; }

int main()
{
    std::cout << "int: " << INT_MIN << " .. " << INT_MAX << "\n";
    std::cout << "long: " << LONG_MIN << " .. " << LONG_MAX << "\n";
    std::cout << "long long: " << LLONG_MIN << " .. " << LLONG_MAX << "\n";

    std::cout << "2147483647 is of type " << type_of(2147483647) << "\n";
    std::cout << "2147483648 is of type " << type_of(2147483648) << "\n";
    std::cout << "-2147483647 is of type " << type_of(-2147483647) << "\n";
    std::cout << "-2147483648 is of type " << type_of(-2147483648) << "\n";
}

When I compile it, I get some warnings:

lits.cpp:18:5: warning: this decimal constant is unsigned only in ISO C90
lits.cpp:20:5: warning: this decimal constant is unsigned only in ISO C90

and the following output, even with gcc -std=c++0x:

int: -2147483648 .. 2147483647
long: -2147483648 .. 2147483647
long long: -9223372036854775808 .. 9223372036854775807
2147483647 is of type int
2147483648 is of type unsigned long
-2147483647 is of type int
-2147483648 is of type unsigned long

I get the same output with VS2010, at least with default settings.

Answer 2

When I compile this in GCC, I get the following message:

warning: this decimal constant is unsigned only in ISO C90 [enabled by default]

It occurs for every line after (and including)

cout << -2147483648 << '\n';    // numeric_limits<int>::min()

So what's happening is both Visual Studio's compiler and GCC allow us to write these literals, and they just treat them as though there were marked as unsigned. This explains the behaviour of what gets printed, and it makes me pretty confident that the output is correct (supposing we had placed a u suffix on the numbers).

I still find it interesting that -2147483648 is not a valid signed integer literal, even though it is a valid signed integer value. Any thought on that anyone?