How to get the size of a type in C++?

Question

I am looking for a C++ function that returns the size of a type. For example :

#include <stdint.h> 

int16_t my_var; 
int32_t size_of_var; 

// magical_function is what I'm looking for
// size_of_var should be equal to 16 because my_var is a int16_t
size_of_var = my_var.magical_function() ; 

I know that size() exists to get the length of a string, so I guess there is a function indicating that too.

Moreover, I'm working with Clang library, so I can get a Type (http://clang.llvm.org/doxygen/classclang_1_1Type.html) but I currently have no idea to compare 2 types and to know the bigger between those two.

Answer 1

In case you want 16 instead of 2 for int16_t

sizeof(my_var) * CHAR_BIT

sizeof give you how many bytes, and CHAR_BIT give you how many bits in a byte (normally 8)

Answer 2

Available syntaxes for the command (sizeof) are:

• sizeof( type )
• sizeof expression

Both of them return std::size_t.

Examples of types are int, float, double, whereas expressions can evaluate the size of an object. For clarification purposes I'll add the following:

sizeof(int);     // Return size of int type.
sizeof(float);   // Return size of float type.
sizeof(double);  // Return size of double type.

struct my_struct {};  // Creates an empty struct named my_struct.
my_struct alpha;      // Initializes alpha as a my_struct.
sizeof alpha;         // Returns size of alpha.

Further information can be found here.

Hope it helps!

Answer 3

To calculate the size of a type in bits, use the CHAR_BIT macro in conjunction with the sizeof operator:

#include <cstdint> 
#include <climits>

int16_t my_var; 
int32_t size_of_var; 

// size_of_var should be equal to 16 because my_var is a int16_t
size_of_var = CHAR_BIT * sizeof(int16_t);

Answer 4

You may try like this:

size_of_var = sizeof(my_var);

In your case you may try with:

sizeof(my_var) * CHAR_BIT

On a side note:

C99 standard requires that

sizeof(short) <= sizeof(int) <= sizeof(long) < sizeof(long long)

Type           C99 Minimum     Windows 32bit
char           8               8
short          16              16
int            16              32
long           32              32
long long      64              64

Also int8_t is guaranteed to be 8 bits, and int16_t is guaranteed to be 16 bits, etc.