在 bool 中设置额外的位使其同时为真和假

This can happen when you call a function in a different language (say fortran) where true and false definition is different than C++. For fortran if any bits are not 0 then the value is true, if all bits are zero then the value is false.

#include <iostream>
#include <bitset>

using namespace std;

void set_bits_to_1(void* val){
  char *x = static_cast<char *>(val);

  for (int i = 0; i<2; i++ ){
    *x |= (1UL << i);
  }
}

int main(int argc,char *argv[])
{

  bool T = 3;

  cout <<"       value   bits " <<endl;
  cout <<"       -----   ---- " <<endl;
  cout <<"    T:   "<< T <<"     "<< bitset<4>(T)<<endl;

  set_bits_to_1(&T);


  bitset<4> bit_T = bitset<4>(T);
  cout <<"after bit change"<<endl;
  cout <<"    T:   "<< T <<"     "<< bit_T<<endl;

  if (T ){
    cout <<"T is true" <<endl;
  }

  if ( T == false){
    cout <<"T is false" <<endl;
  }


}

////////////////////////////////使用 ifort 编译时与 C++ 不兼容的 Fortran 函数.

       logical*1 function return_true()
         implicit none

         return_true = 1;

       end function return_true

    movzx   eax, BYTE PTR [rbp-33]
    test    al, al
    je      .L22

    movzx   eax, BYTE PTR [rbp-33]
    xor     eax, 1
    test    al, al
    je      .L23

int foo(bool b, int i) {
    return i + b;
}

foo(bool, int):
        movzx   edi, dil
        lea     eax, [rdi+rsi]
        ret

./example.f90(2): warning #6916: Fortran 95 does not allow this length specification.   [1]

    logical*1, intent(in) :: x

------------^

./example.f90:2:13:
     logical*1, intent(in) :: x
             1
Error: GNU Extension: Nonstandard type declaration LOGICAL*1 at (1)

integer function logical_compare(x, y)
    logical, intent(in) :: x
    logical, intent(in) :: y
    if (x .eqv. y) then
        logical_compare = 12
    else
        logical_compare = 24
    end if
end function logical_compare

logical_compare_:
        mov     eax, DWORD PTR [rsi]
        mov     edx, 24
        cmp     DWORD PTR [rdi], eax
        mov     eax, 12
        cmovne  eax, edx
        ret

logical*1 function integer_to_logical(x)
    integer, intent(in) :: x
    integer_to_logical = x
    return
end function integer_to_logical

integer_to_logical_:
        mov     eax, DWORD PTR [rdi]
        test    eax, eax
        setne   al
        ret