convert from long long to int and the other way back in c++

Question

How to convert from long long to int and the other way back in c++ ?? also what are the properties of long long , especially its maximum size, thank in advance ..

Answer 1

int is guaranteed to be at least 16 bits wide. On modern systems, it's most commonly 32 bits (even on 64-bit systems).

long long, which didn't originally exist in C++, is guaranteed to be at least 64 bits wide. It's almost always exactly 64 bits wide.

The usual way to convert a value from one integer type to another is simply to assign it. Any necessary conversion will be done implicitly. For example:

int x = 42; long long y = 9223372036854775807; y = x; // implicitly converts from int to long long x = y; // implicitly converts from long long to int 

For a narrowing conversion, where the target type can't represent all the values of the source type, there's a risk of overflow; int may or may not be able to hold the value 9223372036854775807. In this case, the result is implementation-defined. The most likely behavior is that the high-order bits are discarded; for example, converting 9223372036854775807 to int might yield 2147483647. (This is clearer in hexadecimal; the values are 0x7fffffffffffffff and 0x7fffffff, respectively.)

If you need to convert explicitly, you can use a cast. A C-style cast uses the type name in parentheses:

(long long)x 

Or you can use a C++-style static_cast:

static_cast<long long>(x) 

which is somewhat safer than a C-style cast because it's restricted in which types it can operate on.

Answer 2

Type long long is typically 64 bits.

Type int is likely to be 32 bits, but not on all machines.

If you cast an int to a long long, you can do

my_long_long = (long long) my_int 

and it will be just fine. If you go the other direction, like

my_int = (int) my_long_long 

and the int is smaller than 64-bits, it won't be able to hold all the information, so the result may not be correct.