What is bool in C/C++? A keyword or a macro?

问题:

I referred this question, in which some of the answers suggest that bool is an integral type (IDEs also treat it as a keyword).

However, none of the answers suggest the information provided in cplusplus, which says that bool is a macro which is added through <cstdbool> (In that case, the compilers might be implicitly adding this header while compiling to allow bool). Here is the g++ version of <stdbool.h>.

So what exactly the bool is? A an integral type keyword or a macro?

回答1:

In C, bool is a macro.

There is no built-in type or keyword by the name of bool in C, so typical implementations use the standard library to #define true and false to 1 and 0 respectively. Rules such as those for the if statement are defined in terms of "zero" and "non-zero" expressions, and therefore rely on the expanded macro definitions of true and false:

[C99: 6.8.4.1/2]: In both forms, the first substatement is executed if the expression compares unequal to 0. In the else form, the second substatement is executed if the expression compares equal to 0. If the first substatement is reached via a label, the second substatement is not executed.

For convenience, C99 added the built-in intermediate type _Bool, and implementations of this language typically #define bool to _Bool. This type is defined thus:

[C99: 6.2.5/2]: An object declared as type _Bool is large enough to store the values 0 and 1.

This allows for greater compatibility with C++ programs, which may include declarations of functions using the bool type; really, though, #define _Bool int would probably have sufficed.

---

In C++, bool is both a built-in type and a keyword.

The link you provided doesn't say that bool is a macro in C++. It says:

The purpose in C of this header is to add a bool type and the true and false values as macro definitions.

In C++, which supports those directly, the header simply contains a macro that can be used to check if the type is supported.

And this is correct.

Semantically (that is, in terms of "meaning" of your code), [C++11: 3.9.1/2] defines bool as an integral type in C++.

Lexically (that is, in terms of "appearance" in your code), [C++11: 2.12/1] lists it as a keyword. In fact, all tokens that are part of the names of integral types are also keywords, including (but not limited to):

• int
• unsigned
• long
• bool
• short
• signed

It is, however, never a macro in C++. Instead, you get a macro __bool_true_false_are_defined which you could use in multi-language code to switch treatment of bool depending on whether you're working in C or C++; I'm not sure I can think of a useful example, mind you.

回答2:

In C bool is a macro from stdbool.h that expands to _Bool which is the C boolean type.

回答3:

So what exactly the bool is? A an integral type keyword or a macro?

In C++, it's called Boolean literal, it's build in type.

§2.14.7

Types bool, char, char16_t, char32_t, wchar_t, and the signed and unsigned integer types are collectively called integral types.

§2.14.6

Boolean literals

boolean-literal:
false
true

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

§ 3.9.1.6

Values of type bool are either true or false. [ Note: There are no signed, unsigned, short, or long bool types or values. — end note ] Values of type bool participate in integral promotions (4.5).

回答4:

In C++ bool is a built-in data type. In C it is not, so if you're using bool in C it has been implemented as a typedef or with #define, and true and false must have been implemented with #define or perhaps are constants.

回答5:

In C there is no concept like Boolean variables, Yes Higher level languages like Java, C# and other provides us the facility to declare a Boolean variable, that we use for flagging purposes to set it either true or false.

But you can implement this using integrals like we did in C

if(1)
{
   // Because C treats 1 and any other integer as true
}
if(0)
{
   // This time our if condition will result in false
}

回答6:

In C _Bool is a type and bool, true and false are macros defined in stdbool.h

ISO C11 standard states (in section 6.2.5 Types )

An object declared as type _Bool is large enough to store the values 0 and 1.

stdbool.h defines 4 macros.

1. bool which expands to _Bool
2. true which expands to 1
3. false which expands to 0
4. __bool_true_false_are_defined which expands to 1.