What is the value of `c` in the following C/C code: `int a = 2; bool b = a; int c = 3 b;`?

Conversion from bool to int: Guaranteed Values of 0 or 1

A common misconception arises when converting bool values to int. While many compilers appear to store booleans as 0 or 1, the question remains: is this behavior guaranteed?

Question:

Consider the following C code snippet:

int a = 2;
bool b = a;
int c = 3 + b; // 4 or 5?

What will be the value of c: 4 or 5?

Answer:

Yes, c will always be 4 in C and C.

Explanation:

• C : According to the C standard (§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." Therefore, b is converted to 0.
• C: In C (§6.3.1.2/1), "When any scalar value is converted to _Bool, the result is 0 if the value compares equal to 0; otherwise, the result is 1." This implies that a is converted to 1 and stored in b.
• Conversion to int: Converting from _Bool to int is straightforward because int can hold both 0 and 1 (§6.3.1.3). In both cases, the value of b remains unchanged, resulting in c being 3 0 = 4.

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