type cast in c compiler

Question

Consider the following example the mixed mode expression is supported: c = a + b*10.5 where a and b are integers and c is a float. Describe how the data is typecast to its appropriate format

Answer 1

You need to look up the 'usual arithmetic conversions' in your text book, or in the C standard (§6.3.1.8 in C11 — ISO/IEC 9899:2011), which reads:

6.3.1.8 Usual arithmetic conversions

1 Many operators that expect operands of arithmetic type cause conversions and yield result types in a similar way. The purpose is to determine a common real type for the operands and result. For the specified operands, each operand is converted, without change of type domain, to a type whose corresponding real type is the common real type. Unless explicitly stated otherwise, the common real type is also the corresponding real type of the result, whose type domain is the type domain of the operands if they are the same, and complex otherwise. This pattern is called the usual arithmetic conversions:

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.

Otherwise, if the corresponding real type of either operand is float, the other operand is converted, without change of type domain, to a type whose corresponding real type is float.⁶²⁾

Otherwise, the integer promotions are performed on both operands. Then the following rules are applied to the promoted operands:

If both operands have the same type, then no further conversion is needed.

Otherwise, if both operands have signed integer types or both have unsigned integer types, the operand with the type of lesser integer conversion rank is converted to the type of the operand with greater rank.

Otherwise, if the operand that has unsigned integer type has rank greater or equal to the rank of the type of the other operand, then the operand with signed integer type is converted to the type of the operand with unsigned integer type.

Otherwise, if the type of the operand with signed integer type can represent all of the values of the type of the operand with unsigned integer type, then the operand with unsigned integer type is converted to the type of the operand with signed integer type.

Otherwise, both operands are converted to the unsigned integer type corresponding to the type of the operand with signed integer type.

2 The values of floating operands and of the results of floating expressions may be represented in greater range and precision than that required by the type; the types are not changed thereby.⁶³⁾

⁶²⁾ For example, addition of a double _Complex and a float entails just the conversion of the float operand to double (and yields a double _Complex result).

⁶³⁾ The cast and assignment operators are still required to remove extra range and precision.

And the integer promotions are defined in §6.3.1.1:

6.3.1.1 Boolean, characters, and integers

1 Every integer type has an integer conversion rank defined as follows:

• No two signed integer types shall have the same rank, even if they have the same representation.
• The rank of a signed integer type shall be greater than the rank of any signed integer type with less precision.
• The rank of long long int shall be greater than the rank of long int, which shall be greater than the rank of int, which shall be greater than the rank of short int, which shall be greater than the rank of signed char.
• The rank of any unsigned integer type shall equal the rank of the corresponding signed integer type, if any.
• The rank of any standard integer type shall be greater than the rank of any extended integer type with the same width.
• The rank of char shall equal the rank of signed char and unsigned char.
• The rank of _Bool shall be less than the rank of all other standard integer types.
• The rank of any enumerated type shall equal the rank of the compatible integer type (see 6.7.2.2).
• The rank of any extended signed integer type relative to another extended signed integer type with the same precision is implementation-defined, but still subject to the other rules for determining the integer conversion rank.
• For all integer types T1, T2, and T3, if T1 has greater rank than T2 and T2 has greater rank than T3, then T1 has greater rank than T3.

2 The following may be used in an expression wherever an int or unsigned int may be used:

• An object or expression with an integer type (other than int or unsigned int) whose integer conversion rank is less than or equal to the rank of int and unsigned int.
• A bit-field of type _Bool, int, signed int, or unsigned int.

If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.⁵⁸⁾ All other types are unchanged by the integer promotions.

3 The integer promotions preserve value including sign. As discussed earlier, whether a ‘‘plain’’ char is treated as signed is implementation-defined.

⁵⁸⁾ The integer promotions are applied only: as part of the usual arithmetic conversions, to certain argument expressions, to the operands of the unary +, -, and ~ operators, and to both operands of the shift operators, as specified by their respective subclauses.