Does the 'if' statement in C/C++ cast the operand to an integer?

Question

When you use the if statement in C/C++, or any other logical operator, is the operand you pass to the statement cast to an integer for evaluation?

My current understanding was that the operand passed to the statement is cast to an integer to test whether or not it is non-zero (true), and that if you pass a pointer, this can be cast to an integer to evaluate with a 0/null value being defined as false.

I was under the impression that C++'s standard Bool values were simply typedef of an unsigned char with a value of 0 and 1.

Can anyone explain what's actually happening behind the scenes with this behavior?

Answer 1

In C++ bool is a standalone type that has nothing to do with unsigned char. And in C++ language the expression under if is indeed implicitly cast to bool type. Note: to bool specifically, not to int. For scalar types other than bool, the implicit conversion to bool is essentially defined through non-equality comparison to literal 0. I.e. values that compare non-equal are converted to true, while values that compare equal - to false.

In other words, in C++ statement

if (a)

is interpreted as

bool tmp = (bool) a;
// `tmp` is either `true` or `false`
if (tmp)

which for scalar types is interpreted as

bool tmp = a != 0;
if (tmp)

Note that this works "as expected" for pointer types as well. But instead of converting the pointer to int type, it actually works the other way around: it converts literal 0 to the proper pointer type.

For other types it could be something different, like a call to a user-defined conversion operator operator bool().

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In C language the expression under if has to have scalar type. And it is implicitly compared to constant 0. Note that this does not involve converting the controlling expression it to int. Comparison to constant 0 is defined separately for all scalar types. E.g. for int it has its natural meaning, while for pointer types it is interpreted as comparison to null pointer. Now, the result of such comparison in C has type int and evaluates to either 0 or 1. That 0 or 1 is what controls what if branch is taken.

In other words, in C statement

if (a)

is immediately interpreted as

int tmp = a != 0;
// `tmp` is either `0` or `1`
if (tmp)

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Additionally, your assumption that null pointer produces a zero int value when converted to int type is incorrect. Neither language makes such guarantee. Null pointer is not guaranteed to be represented by zero address value and is not guaranteed to produce zero value when converted to int.