Does every type have a corresponding literal?

问题:

According to Bjarne Stroustrup's Programming Principles and Practice Using C++ (Sixth Printing, November 2012), "each of [the] types [int, double, char, string, and bool] has its own characteristic style of literals" (p.63).

In §A.2 of the same book, additional varieties of int (unsigned; long) and floating-point-literals are mentioned, as well as the (null) pointer literal.

Does every type have a corresponding literal?

回答1:

C++ Standard n3337 § 2.14.1/1:

There are several kinds of literals.

literal:

integer-literal

character-literal

floating-literal

string-literal

boolean-literal

pointer-literal

user-defined-literal

No, not avery type has a literal. Classes do not have literal, as well as i.e void, int*, signed char, unsigned char, short, unsigned short.

---

Literals explained:

integer-literal

An integer literal is a sequence of digits that has no period or exponent part.

example: 1

character-literal

A character literal is one or more characters enclosed in single quotes, as in ’x’, optionally preceded by one of the letters u, U, or L, as in u’y’, U’z’, or L’x’, respectively.

’ c-char-sequence ’

u’ c-char-sequence ’

U’ c-char-sequence ’

L’ c-char-sequence ’

example: 'a', 'ab', '\'', L'0', '('

floating-literal

A floating literal consists of an integer part, a decimal point, a fraction part, an e or E, an optionally signed integer exponent, and an optional type suffix. The integer and fraction parts both consist of a sequence of decimal (base ten) digits. Either the integer part or the fraction part (not both) can be omitted; either the decimal point or the letter e (or E ) and the exponent (not both) can be omitted. The integer part, the optional decimal point and the optional fraction part form the significant part of the floating literal. The exponent, if present, indicates the power of 10 by which the significant part is to be scaled.

example: 123.456e-67, .1E1f, 42., 13e3

string-literal

A string literal is a sequence of characters (as defined in 2.14.3) surrounded by double quotes, optionally prefixed by R, u8, u8R, u, uR, U, UR, L, or LR, as in "...", R"(...)", u8"...", u8R"(...)", u"...", uR"* ̃(...)* ̃", U"...", UR"zzz(...)zzz", L"...", or LR"(...)", respectively.

encoding-prefixopt " s-char-sequenceopt "

encoding-prefixopt R raw-string

example: "me", L"you"

boolean-literal

The Boolean literals are the keywords false and true. Such literals are prvalues and have type bool.

false
true

pointer-literal

The pointer literal is the keyword nullptr. It is a prvalue of type std::nullptr_t.

nullptr

user-defined-literal

Overload of operator"" that allows to interpret literal using user-defined logic.

example:

typedef double signalf;
constexpr signalf operator"" _percent( long double val) { return val / 100 ; }

std::cout  << 12.34_percent; // prints 0.1234

回答2:

There's no literal for int*. The closest you get is nullptr, but its type is nullptr_t.

回答3:

Most types do not have a literal. Classes do not have one. Pointers, references, r-value references have none, as well. void does not have a literal.

You mentioned string which also does not have a literal, it only has a constructor that takes a const char* literal! C++ calls this constructor implicitly, when assigning a const char* literal to a string.

回答4:

Adding to the list, there's no literals for signed char, unsigned char, short, unsigned short.

You should probably think in terms of the limited list of types for which there are literals, rather than the types for which there are not. Literals are for:

• bool
• char, wchar_t, char16_t, char32_t
• "array of const" char, wchar_t, char16_t, char32_t (that is, string literals)
• int, unsigned int
• long, unsigned long
• long long, unsigned long long
• float, double, long double
• std::nullptr_t

plus user-defined literals.

回答5:

First of all type std::string has no its own literals. Only arithmetic types have their own literals. The exception is string literals that has type of const char arrays and not so fer introduced in the C++ 11 nullptr of type std::nullptr_t

Take into account that bool is an arithmetic type.

回答6:

Bjarne Stroustrup's Programming Principles and Practice Using C++ (Sixth Printing, November 2012) refers to ISO/IEC 14882:2003 as the relevant C++ standard. In this can be found the pertinent section, §2.13, which reads as follows:

2.13 Literals

There are several kinds of literals.

literal:
    integer-literal
    character-literal
    floating-literal
    string-literal
    boolean-literal

Apparently, this is an exhaustive list; and because there can be many more types than there are kinds of literal in that list, the answer is: no, not every type has a corresponding literal.

N.B. for the C++11 case, please see Lizusek's answer.