auto Declarations in C++11

C++11 introduces a facility that allows the compiler to deduce a type from the type of an initialization value. For this purpose it redefines the meaning of auto, a keyword dating back to C, but one hardly ever used. (Chapter 9 discusses the previous meaning of auto.) Just use auto instead of the type name in an initializing declaration, and the compiler assigns the variable the same type as that of the initializer:

auto n = 100;     // n is int
auto x = 1.5;     // x is double
auto y = 1.3e12L; // y is long double

However, this automatic type deduction isn’t really intended for such simple cases. Indeed, you might even go astray. For example, suppose x, y, and z are all intended to be type double. Consider the following code:

auto x = 0.0;   // ok, x is double because 0.0 is double
double y = 0;   // ok, 0 automatically converted to 0.0
auto z = 0;     // oops, z is int because 0 is int

Using 0 instead of 0.0 doesn’t cause problems with explicit typing, but it does with automatic type conversion.

Automatic type deduction becomes much more useful when dealing with complicated types, such as those in the STL (Standard Template Library). For example, C++98 code might have this:

std::vector<double> scores;
std::vector<double>::iterator pv = scores.begin();

C++11 allows you to write this instead:

std::vector<double> scores;
auto pv = scores.begin();

We’ll mention this new meaning of auto again later when it becomes more relevant to the topics at hand.

Summary

C++’s basic types fall into two groups. One group consists of values that are stored as integers. The second group consists of values that are stored in floating-point format. The integer types differ from each other in the amount of memory used to store values and in whether they are signed or unsigned. From smallest to largest, the integer types are bool, char, signed char, unsigned char, short, unsigned short, int, unsigned int, long, unsigned long, and, with C++11, long long, and unsigned long long. There is also a wchar_t type whose placement in this sequence of size depends on the implementation. C++11 adds the char16_t and char32_t types, which are wide enough to hold 16-bit and 32-bit character codes, respectively. C++ guarantees that char is large enough to hold any member of the system’s basic character set, wchar_t can hold any member of the system’s extended character set, short is at least 16 bits, int is at least as big as short, and long is at least 32 bits and at least as large as int. The exact sizes depend on the implementation.

Characters are represented by their numeric codes. The I/O system determines whether a code is interpreted as a character or as a number.

The floating-point types can represent fractional values and values much larger than integers can represent. The three floating-point types are float, double, and long double. C++ guarantees that float is no larger than double and that double is no larger than long double. Typically, float uses 32 bits of memory, double uses 64 bits, and long double uses 80 to 128 bits.

By providing a variety of types in different sizes and in both signed and unsigned varieties, C++ lets you match the type to particular data requirements.

C++ uses operators to provide the usual arithmetical support for numeric types: addition, subtraction, multiplication, division, and taking the modulus. When two operators seek to operate on the same value, C++’s precedence and associativity rules determine which operation takes place first.

C++ converts values from one type to another when you assign values to a variable, mix types in arithmetic, and use type casts to force type conversions. Many type conversions are “safe,” meaning you can make them with no loss or alteration of data. For example, you can convert an int value to a long value with no problems. Others, such as conversions of floating-point types to integer types, require more care.

At first, you might find the large number of basic C++ types a little excessive, particularly when you take into account the various conversion rules. But most likely you will eventually find occasions when one of the types is just what you need at the time, and you’ll thank C++ for having it.

Chapter Review

1. Why does C++ have more than one integer type?

2. Declare variables matching the following descriptions:

a. A short integer with the value 80

b. An unsigned int integer with the value 42,110

c. An integer with the value 3,000,000,000

3. What safeguards does C++ provide to keep you from exceeding the limits of an integer type?

4. What is the distinction between 33L and 33?

5. Consider the two C++ statements that follow:

char grade = 65;
char grade = 'A';

Are they equivalent?

6. How could you use C++ to find out which character the code 88 represents? Come up with at least two ways.

7. Assigning a long value to a float can result in a rounding error. What about assigning long to double? long long to double?

8. Evaluate the following expressions as C++ would:

a. 8 * 9 + 2

b. 6 * 3 / 4

c. 3 / 4 * 6

d. 6.0 * 3 / 4

e. 15 % 4

9. Suppose x1 and x2 are two type double variables that you want to add as integers and assign to an integer variable. Construct a C++ statement for doing so. What if you want to add them as type double and then convert to int?

10. What is the variable type for each of the following declarations?

a. auto cars = 15;

b. auto iou = 150.37f;

c. auto level = 'B';

d. auto crat = U'/U00002155';

e. auto fract = 8.25f/2.5;

Programming Exercises

1. Write a short program that asks for your height in integer inches and then converts your height to feet and inches. Have the program use the underscore character to indicate where to type the response. Also use a const symbolic constant to represent the conversion factor.

2. Write a short program that asks for your height in feet and inches and your weight in pounds. (Use three variables to store the information.) Have the program report your body mass index (BMI). To calculate the BMI, first convert your height in feet and inches to your height in inches (1 foot = 12 inches). Then convert your height in inches to your height in meters by multiplying by 0.0254. Then convert your weight in pounds into your mass in kilograms by dividing by 2.2. Finally, compute your BMI by dividing your mass in kilograms by the square of your height in meters. Use symbolic constants to represent the various conversion factors.

3. Write a program that asks the user to enter a latitude in degrees, minutes, and seconds and that then displays the latitude in decimal format. There are 60 seconds of arc to a minute and 60 minutes of arc to a degree; represent these values with symbolic constants. You should use a separate variable for each input value. A sample run should look like this:

Enter a latitude in degrees, minutes, and seconds:
First, enter the degrees: 37
Next, enter the minutes of arc: 51
Finally, enter the seconds of arc: 19
37 degrees, 51 minutes, 19 seconds = 37.8553 degrees

4. Write a program that asks the user to enter the number of seconds as an integer value (use type long, or, if available, long long) and that then displays the equivalent time in days, hours, minutes, and seconds. Use symbolic constants to represent the number of hours in the day, the number of minutes in an hour, and the number of seconds in a minute. The output should look like this:

Enter the number of seconds: 31600000
31600000 seconds = 365 days, 17 hours, 46 minutes, 40 seconds

5. Write a program that requests the user to enter the current world population and the current population of the U.S. (or of some other nation of your choice). Store the information in variables of type long long. Have the program display the percent that the U.S. (or other nation’s) population is of the world’s population. The output should look something like this:

Enter the world's population: 6898758899
Enter the population of the US: 310783781
The population of the US is 4.50492% of the world population.

You can use the Internet to get more recent figures.

6. Write a program that asks how many miles you have driven and how many gallons of gasoline you have used and then reports the miles per gallon your car has gotten. Or, if you prefer, the program can request distance in kilometers and petrol in liters and then report the result European style, in liters per 100 kilometers.

7. Write a program that asks you to enter an automobile gasoline consumption figure in the European style (liters per 100 kilometers) and converts to the U.S. style of miles per gallon. Note that in addition to using different units of measurement, the U.S. approach (distance / fuel) is the inverse of the European approach (fuel / distance). Note that 100 kilometers is 62.14 miles, and 1 gallon is 3.875 liters. Thus, 19 mpg is about 12.4 l/100 km, and 27 mpg is about 8.7 l/100 km.