declaration with width

Can any one explain me this declaration,
unsigned int is_key : 1;
And where can this be used?
What operations can't be performed and what opeations can be done using this variable?operations like +,-,&,&&..
Thanks in advance,
masg

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unsigned int is_key : 1;

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That is a bitfied.

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And where can this be used?

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You can use them when a whole byte might be a waste. You might find them very usefull when dealing with various communication protocols.

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What operations can't be performed and what opeations can be done using this variable?operations like +,-,&,&&

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You cannot take the address of the bitfield. You can use any of the +,-,&,&&.

This declares a "C" Bitfield e.g. in a struct. AFAIK only integral types (signed, unsigned, int) are allowed. You basically say that the is_key variable should use 1 bit. Example:

Code:

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struct SFoo {
unsigned short hand : 1;
unsigned short finger : 3;
unsigned short age : 7;
};

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This structure uses 2 bytes. hand ranges from [0;1], finger from [0;7] and age from [0;127].

You can use at least +/- operators as usual and the values will be kept in range. [edit]I'm not sure about other operators.[/edit]

It is generally not recommended to use bitfields except if it really helps and you can make assumptions about the data types, but it can truely be a porting nightmare. Moreover, although less memory is used, you can expect that access to these data members is somewhat slower.

C++ bit fields