I have a data type, say X, and I want to know its size without declaring a variable or pointer of that type and of course without using sizeof operator.
Is this possible? I thought of using standard header files which contain size and range of data types but that doesn't work with user defined data type.
You could puzzle it out by reading the ABI for your particular processor, which explains how structures are laid out in memory. It's potentially different for each processor. But unless you're writing a compiler it's surprising you don't want to just use sizeof, which is the One Right Way to solve this problem.
Try this,
#define sizeof_type( type ) ((size_t)((type*)1000 + 1 )-(size_t)((type*)1000))
For the following user-defined datatype,
struct x
{
char c;
int i;
};
sizeof_type(x) = 8
(size_t)((x*)1000 + 1 ) = 1008
(size_t)((x*)1000) = 1000