投向布尔时得不到1

Question

I have the following code:

#include <iostream>

void f(void* buffer) {
    int x = static_cast<bool>(*(char*)buffer);
    int y = static_cast<bool>(*(bool*)buffer);
    std::cout << x << " " << y;
}

int main() {
    int a = 255;
    f(&a);
}

On GCC 9.1, I'm getting the following output:

1 255

Why the cast to bool with (char*) cast results x to be 1 and the cast to bool with (bool*) cast results y to be, in this case, 255?

Answer 1

Why the cast to bool ... with (bool*) cast results ...?

Because the behaviour of the program is undefined.

Standard says (quoting the latest draft):

[basic.lval]

If a program attempts to access the stored value of an object through a glvalue whose type is not similar ([conv.qual]) to one of the following types the behavior is undefined :

• the dynamic type of the object,
• a type that is the signed or unsigned type corresponding to the dynamic type of the object, or
• a char, unsigned char, or std::byte type.

bool is not similar to dynamic type of buffer , it is neither unsigned nor unsigned corresponding type, nor is it char unsigned char , or std::byte . Therefore behaviour is undefined.

Why the cast to bool with (char*) cast results x to be 1

Because the first byte of a contained a value that did not represent a zero. Whether this will be the case depends on the CPU architecture. The order of bytes of an integer is implementation defined.

Note that the behaviour would be well defined in this case, because char is listed as one of the exceptions in the quoted rule.

Answer 2

You need to use reinterpret_cast<int *> . For example

int y = static_cast<bool>(*reinterpret_cast<int*>( buffer ));

Here is a demonstrative program

#include <iostream>

int main()
{
    int x = 255;
    void *buffer = &x;
    int y = static_cast<bool>(*reinterpret_cast<int*>( buffer ));

    std::cout << y << '\n';
}