Why does sizeof(!0) print 1 instead of 4?

Question

#include <iostream>

int main()
{
    std::cout<<sizeof(0);
    return 0;
}

Here, sizeof(0) is 4 in C++ because 0 is an integer rvalue.

But, If I write like this:

std::cout<<sizeof(!0);

here, sizeof(!0) is 1. But, !0 means it print 1, which is also, int type.

then, Why does sizeof(!0) print 1 instead of 4? What am I miss here?

Bougie

!0 is a bool – Arnettearney

@SidS it means, it is implicitly converted int to bool type. – Bougie

@SidS But, sizeof does not evaluate the expression – Bougie

@M.SChaudhari yes, it receives already the result of the evaluation – Lichter

@M.SChaudhari - sizeof does not evaluate the expression. It works out the size of the type that would hold the result of the expression. – Loginov

Okey. Thanks to all for clear my doubt. – Bougie

Also note that sizeof(int) does not have to be 4, nor does sizeof(bool) = 1. But it is enough for identification of this question. – Hilar

Answer 1

The logical negation operator:

! rhs

If the operand is not bool, it is converted to bool using contextual conversion to bool: it is only well-formed if the declaration bool t(arg) is well-formed, for some invented temporary t.

The result is a bool prvalue.

And sizeof (bool) which is implementation defined is 1 in your implementation.

Humectant

Note you are missing a key point which is we are using sizeof w/ an expression not a type – Romo

bool operator!(int); So bool has a size of 1. – Massa

Answer 2

!0 is a bool.

sizeof(bool) depends on implementation.

Arnettearney

Why anybody say ! is the short for operator!() is it that implicit? – Massa

Answer 3

By prepending the integer value with !, you convert it to a boolean - which can be represented using a single byte.

Lichter

you most probably mean a single byte. If you want to get a single bit per boolean, you would need some kind of bitfield. – Cogswell

<joke>@Grzegorz Piwowarek So you expect it to be $\frac{1}{8}$?</joke> – Hilar

Byte :) also, int is more than 4 bits! – Lichter

Answer 4

When we write 0 it is an integer, but when we write !0, it implicitly convert the integer to boolean. ! operator turns any integer into boolean, you can try by writing !1, !2....these all give size of 1 byte. if you wanna know the size of !0 as an int, you can typecast it as

sizeof(int(!0));

Bullace