A value, such as 42, in a program is known as a literal constant: literal because we can speak of it only in terms of its value; constant because its value cannot be changed. Every literal has an associated type. For example, 0 is an int and 3.14159 is a double. Literals exist only for the built-in types. There are no literals of class types. Hence, there are no literals of any of the library types.

字面常量的关键知识点是：不同的写法有不同的类型。

ADVICE: USING THE BUILT-IN ARITHMETIC TYPES

The number of integral types in C++(www.cppentry.com) can be bewildering. C++(www.cppentry.com), like C, is designed to let programs get close to the hardware when necessary, and the integral types are defined to cater to the peculiarities of various kinds of hardware. Most programmers can (and should) ignore these complexities by restricting the types they actually use.

In practice, many uses of integers involve counting. For example, programs often count the number of elements in a data structure such as a vector or an array. We’ll see in Chapters 3 and 4 that the library defines a set of types to use when dealing with the size of an object. When counting such elements it is always right to use the librarydefined type intended for this purpose. When counting in other circumstances, it is usually right to use an unsigned value. Doing so avoids the possibility that a value that is too large to fit results in a (seemingly) negative result.

When performing integer arithmetic, it is rarely right to use shorts. In most programs, using shorts leads to mysterious bugs when a value is assigned to a short that is bigger than the largest number it can hold. What happens depends on the machine,
but typically the value “wraps around” so that a number too large to fit turns into a large negative number. For the same reason, even though char is an integral type, the char type should be used to hold characters and not for computation. The
fact that char is signed on some implementations and unsigned on others makes it problematic to use it as a computational type.

On most machines, integer calculations can safely use int. Technically speaking, an int can be as small as 16 bits—too small for most purposes. In practice, almost all general-purpose machines use 32-bits for ints, which is often the same size used for long. The difficulty in deciding whether to use int or long occurs on machines that have 32-bit ints and 64-bit longs. On such machines, the run-time cost of doing arithmetic with longs can be considerably greater than doing the same calculation using a 32-bit int. Deciding whether to use int or long requires detailed understanding of the program and the actual run-time performance cost of using long versus int.

Determining which floating-point type to use is easier: It is almost always right to use double. The loss of precision implicit in float is significant, whereas the cost of double precision calculations versus single precision is negligible. In fact, on some machines, double precision is faster than single. The precision offered by long double usually is unnecessary and often entails considerable extra run-time cost.