C++ Primer Plus

There are more integral promotions: The unsigned short type is converted to int if short is smaller than int. If the two types are the same size, unsigned short is converted to unsigned int. This rule ensures that there’s no data loss in promoting unsigned short. Similarly, wchar_t is promoted to the first of the following types that is wide enough to accommodate its range: int, unsigned int, long, or unsigned long.

Then there are the conversions that take place when you arithmetically combine different types, such as adding an int to a float. When an operation involves two types, the smaller is converted to the larger. For example, the program in Listing 3.11 divides 9.0 by 5. Because 9.0 is type double, the program converts 5 to type double before it does the division. More generally, the compiler goes through a checklist to determine which conversions to make in an arithmetic expression. C++11 has modified the list slightly. Here’s the C++11 version of the list, which the compiler goes through in order:

1. If either operand is type long double, the other operand is converted to long double.

2. Otherwise, if either operand is double, the other operand is converted to double.

3. Otherwise, if either operand is float, the other operand is converted to float.

4. Otherwise, the operands are integer types and the integral promotions are made.

5. In that case, if both operands are signed or if both are unsigned, and one is of lower rank than the other, it is converted to the higher rank.

6. Otherwise, one operand is signed and one is unsigned. If the unsigned operand is of higher rank than the signed operand, the latter is converted to the type of the unsigned operand.

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

8. Otherwise, both operands are converted to the unsigned version of the signed type.

ANSI C follows the same rules as ISO 2003 C++, which are slightly different from the preceding rules, and classic K&R C has yet slightly different rules. For example, classic C always promotes float to double, even if both operands are float.

This list introduces the concept of ranking the integer types. In brief, as you might expect, the basic ranking for signed integer types from high to low is long long, long, int, short, and signed char. Unsigned types have the same rank as the corresponding signed type. The three types char, signed char, and unsigned char all have the same rank. The bool type has the lowest rank. The wchar_t, char16_t, and char32_t have the same types as their underlying types.

Conversions in Passing Arguments

Normally, C++ function prototyping controls type conversions for the passing of arguments, as you’ll learn in Chapter 7, “Functions: C++’s Programming Modules.” However, it is possible, although usually unwise, to waive prototype control for argument passing. In that case, C++ applies the integral promotions to the char and short types (signed and unsigned). Also to preserve compatibility with huge amounts of code in classic C, C++ promotes float arguments to double when passing them to a function that waives prototyping.

Type Casts

C++ empowers you to force type conversions explicitly via the type cast mechanism. (C++ recognizes the need for type rules, and it also recognizes the need to occasionally override those rules.) The type cast comes in two forms. For example, to convert an int value stored in a variable called thorn to type long, you can use either of the following expressions:

(long) thorn    // returns a type long conversion of thorn

long (thorn)    // returns a type long conversion of thorn

The type cast doesn’t alter the thorn variable itself; instead, it creates a new value of the indicated type, which you can then use in an expression, as in the following:

cout << int('Q');  // displays the integer code for 'Q'

More generally, you can do the following:

(typeName) value   // converts value to typeName type

typeName (value)   // converts value to typeName type

The first form is straight C. The second form is pure C++. The idea behind the new form is to make a type cast look like a function call. This makes type casts for the built-in types look like the type conversions you can design for user-defined classes.

C++ also introduces four type cast operators that are more restrictive in how they can be used. Chapter 15, “Friends, Exceptions, and More,” covers them. Of the four, the static_cast<> operator, can be used for converting values from one numeric type to another. For example, using it to convert thorn to a type long value looks like this:

static_cast (thorn)     // returns a type long conversion of thorn

More generally, you can do the following:

static_cast<typeName> (value)   // converts value to typeName type