7.4.1 Character classification functions
1 The functions in this subclause return nonzero (true) if and only if the value of the
argument c conforms to that in the description of the function.
7.4.1.1 The isalnum function
Synopsis
1 #include <ctype.h>
int isalnum(int c);
Description
2 The isalnum function tests for any character for which isalpha or isdigit is true.

Returns: An iterator i such that for any iterator j in the range [first, i), pred(*j) != false,
and for any iterator k in the range [i, last), pred(*j) == false. The relative order of the
elements in both groups is preserved.

if ( var == true ) {}

if(var);
if(var != false);

CS>if ( var == true ) {}
CS>

2.13.5 Boolean literals

boolean-literal:
false
true

1 The Boolean literals are the keywords false and true. Such literals have type bool. They are not lvalues.

КР>if (std::isalnum(...))
КР>if (std::isalnum(...) == true)
КР>if (std::isalnum(...) != false)
КР>if (std::isalnum(...) == !false)
КР>

#include <iostream>
#include <cctype>

int my_isalnum(int c) // эта функция полностью соответствует стандартам C99 (п. 7.4.1.1) и C++03!
{
    return std::isalnum(c) ? 123 : 0;
}

bool bad_isalnum(char c)
{
    return my_isalnum(c) == true;
}

int main()
{
    std::cout << std::boolalpha << bad_isalnum('w') << std::endl;
}

MinGW 3.4.2: false
VC 8: false
VC 7.1: false

enum { FALSE, TRUE };

bool isConditionTrue( SomeType& arg );
...
if ( true != isConditionTrue( MyArg ) )
...

int isAnotherConditionTrue( SomeType& arg );
const int NewProgrammersConstantValue = ...;
...
if ( NewProgrammersConstantValue != isAnotherConditionTrue( MyArg ) )
...

4.5/4

An rvalue of type bool can be converted to an rvalue of type int, with false becoming zero and true
becoming one.

if ( var == true ) {}

А>>>А откуда компилятор знает что var равняется 2?

if (var)

if (var == true)

if (var != false)

if (var == !false)

if (std::isalnum(...))
if (std::isalnum(...) == true)
if (std::isalnum(...) != false)
if (std::isalnum(...) == !false)

КР>if (std::isalnum(...))
КР>if (std::isalnum(...) == true)
КР>if (std::isalnum(...) != false)
КР>if (std::isalnum(...) == !false)
КР>

if (std::isalnum(...) != !true)

if (!std::isalnum(...) == false)
if (!std::isalnum(...) == !true)

Except as noted in clauses 18 through 27, the contents of each header cname shall be the same as that of the corresponding header name.h, as specified in ISO/IEC 9899:1990 Programming Languages C (Clause 7), or ISO/IEC:1990 Programming Languages—C AMENDMENT 1: C Integrity, (Clause 7), as appropriate, as if by inclusion. In the C++ Standard Library, however, the declarations and definitions (except for names which are defined as macros in C) are within namespace scope (3.3.5) of the namespace std.

160) The ".h" headers dump all their names into the global namespace, whereas the newer forms keep their names in namespace std. Therefore, the newer forms are the preferred forms for all uses except for C++ programs which are intended to be strictly compatible with C.

O>Так на чем остановились-то?

O>enum { FALSE, TRUE };
O>

O>не использовать никогда bool? почему? я не вижу причины, почему нельзя сравнить два булевских операнда — нет приведения ни к int, ни к чему другому...кто-то потом может поменять мой вызов bool isConditionTrue(); на int isAnotherConditionTrue();?

if(authData.isLogonSuccessful() == realm.isAuthenticationRequired())

auto value = objectX.hasPropertyY(); // bool?