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Jim Langston
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 Posted: Tue Nov 21, 2006 10:10 am Post subject: static_cast<unsigned short)( -1 ) Well defined? |
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Is the following well defined?
size_t IntVal = 65537;
unsigned short Length;
if ( IntVal > static_cast<unsigned short>( -1 ) )
{
std::cout << "Value too long to fit in a short" << std::endl;
}
else
{
std::cout << "Value fits" << std::endl;
Length = static_cast<unsigned short>( IntVal );
}
What I'm actually going to be using it for is to send a length short through
sockets, and I want to ensure that the length isn't greater than 65535.
Rather than the magic number, I was thinking that static_cast<unsigned
short>( -1 ) would be better. Is this well defined? Will it equal 65535 in
all cases where a short is a 2 byte integer? |
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Kai-Uwe Bux
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| Posted: Tue Nov 21, 2006 10:10 am Post subject: Re: static_cast<unsigned short)( -1 ) Well defined? |
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Jim Langston wrote:
| Quote: | Is the following well defined?
size_t IntVal = 65537;
unsigned short Length;
if ( IntVal > static_cast<unsigned short>( -1 ) )
{
std::cout << "Value too long to fit in a short" << std::endl;
}
else
{
std::cout << "Value fits" << std::endl;
Length = static_cast<unsigned short>( IntVal );
}
What I'm actually going to be using it for is to send a length short
through sockets, and I want to ensure that the length isn't greater than
65535.
|
If that is want you want to check, you should say so:
if ( IntVal > 65535 ) { ...
| Quote: | Rather than the magic number, I was thinking that
static_cast<unsigned short>( -1 ) would be better.
|
Why? If you want to check > 65535, then presumably, your specs contain a
magic number. Your code should reflect that.
| Quote: | Is this well defined?
|
Your code has implementation defined behavior. Moreover, it is guaranteed,
that for IntVal <= 65535, the "Value fits" branch will be taken.
| Quote: | Will it equal 65535 in all cases where a short is a 2 byte integer?
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Yes, provided your bytes have eight bits. However, your bytes maybe longer
and your shorts may have more than 2 bytes.
Now, you need to distinguish whether you want to check
(a) whether IntVal <= 65536, or
(b) whether IntVal can be faithfully represented as a short unsigned.
Your code does the later, and as far as I can see, it does so correctly.
Best
Kai-Uwe Bux |
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Salt_Peter
Guest
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| Posted: Tue Nov 21, 2006 10:10 am Post subject: Re: static_cast<unsigned short)( -1 ) Well defined? |
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Jim Langston wrote:
| Quote: | Is the following well defined?
size_t IntVal = 65537;
unsigned short Length;
if ( IntVal > static_cast<unsigned short>( -1 ) )
{
std::cout << "Value too long to fit in a short" << std::endl;
}
else
{
std::cout << "Value fits" << std::endl;
Length = static_cast<unsigned short>( IntVal );
}
What I'm actually going to be using it for is to send a length short through
sockets, and I want to ensure that the length isn't greater than 65535.
Rather than the magic number, I was thinking that static_cast<unsigned
short>( -1 ) would be better. Is this well defined? Will it equal 65535 in
all cases where a short is a 2 byte integer?
|
I doubt that can be enforced or guarenteed other than by checking
dutifully- but i may be wrong.
take a look at:
template<>
struct numeric_limits<unsigned short>
in...
#include <limits>
namespace Project {
typedef unsigned short usocket;
};
int main()
{
using Project::usocket;
using std::numeric_limits;
std::cout << "numeric_limits< usocket >::min() ";
std::cout << numeric_limits< usocket >::min();
std::cout << std::endl;
std::cout << "numeric_limits< usocket >::max() ";
std::cout << numeric_limits< usocket >::max();
std::cout << std::endl;
int n(65530);
for(size_t t = 0; t < 10; ++t)
{
std::cout << "n++ = " << n++;
usocket usock(static_cast<usocket>(n));
std::cout << "\tusock = " << usock;
std::cout << std::endl;
}
}
/*
numeric_limits< usocket >::min() 0
numeric_limits< usocket >::max() 65535
n++ = 65530 usock = 65531
n++ = 65531 usock = 65532
n++ = 65532 usock = 65533
n++ = 65533 usock = 65534
n++ = 65534 usock = 65535
n++ = 65535 usock = 0
n++ = 65536 usock = 1
n++ = 65537 usock = 2
n++ = 65538 usock = 3
n++ = 65539 usock = 4
*/ |
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