operands for sizeof operator

Question

I understood result of

int nData = 10;
printf("%d", sizeof(nData + 2.0));

is "8"

why each result of

int nData = 10;
printf("%d", sizeof(nData = 2.0));
printf("%d", sizeof(nData += 2.0));

is not 8 but 4? Why nData cannot be 12.0 or 12 by sizeof(nData += 2.0)?

Answer 1

Because 2.0 is a constant of type double, the expression nData + 2.0 has type double as per the "usual arithmetic conversions" specified in section 6.3.1.8 of the C standard:

First, if the corresponding real type of either operand is long double , the other operand is converted, without change of type domain, to a type whose corresponding real type is long double.

Otherwise, if the corresponding real type of either operand is double , the other operand is converted, without change of type domain, to a type whose corresponding real type is double

So sizeof evaluates to the size of a double.

In the case of nData = 2.0 and nData += 2.0, each expression has type int because that is the type of the left-hand side of the assignment. So sizeof evaluated to the size of an int.

Also, the operand of the sizeof operator is evaluated for its type only at compile time. This means that any assignment or increment is not evaluated at runtime. So in your second example, nData will still have the value 10 after the two lines that use sizeof. The only time the operand of sizeof is evaluated at run time is if the operand is a variable length array.

Answer 2

Why nData cannot be 12.0 or 12 by sizeof(nData += 2.0)?

Well, sizeof is a compiler time operator, and it operates on the data type, not the value.

In other words, except when the argument is VLA, the operand of the sizeof is not evaluated.

Quoting C11, chapter §6.5.3.4

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.

So, in your case,

  printf("%d", sizeof(nData + 2.0));   // data == int, 2.0 == double

is the same as

 printf("%d", sizeof(double));  //  as the resulting "type" of the addition, 
                                //   'int + double' would be  'double'

which should better be

 printf("%zu", sizeof(double));   //not %d

as sizeof yields size_t type.

Also, regarding the 2.0 being of type double, from chapter §6.4.4.2

A floating constant has a significand part that may be followed by an exponent part and a suffix that specifies its type. The components of the significand part may include a digit sequence representing the whole-number part, followed by a period (.), followed by a digit sequence representing the fraction part. [...]

and

An unsuffixed floating constant has type double. [...]

Thus, a constant value like 2.0 has the type double.