warning: '*' in boolean context, suggest '&&' instead [-Wint-in-bool-context]? - c++

Question

i've this code (which is pretty all "float"):
#define sincf(x) (x == 0.0f) ? (1.0f) : (sinf(M_PI * x) / (M_PI * x))
// ...
for (int i = 0; i < num_taps; i++)
proto[i] = 2.0f * f * sincf(2.0f * f * (i - m / 2.0f));
// ...
why gcc says warning: '' in boolean context, suggest '&&' instead [-Wint-in-bool-context]* for the second "*"?
f is float
proto is *float
i and m are int

Answer 1

After the macro has been substituted, this part
2.0f * f * sincf(2.0f * f * (i - m / 2.0f));
becomes
2.0f * f * (2.0f * f * (i - m / 2.0f) == 0.0f) ? ...
and according to operator precedence, the multiplication 2.0f * f * condition will be done before checking if the condition is true (with ?). Like so:
(2.0f * f * (2.0f * f * (i - m / 2.0f) == 0.0f)) ? ...
The quick fix:
#define sincf(x) (((x) == 0.0f) ? (1.0f) : (sinf(M_PI * (x)) / (M_PI * (x))))
(x) == 0.0f will rarely be true but since it's only used to avoid division by zero, that's probably fine.
Now, this could easily be rewritten as a function and I suggest doing that. Example:
template<class T>
T sinc(T x) {
if(x == T{}) return {1}; // avoid division by zero
auto pix = static_cast<T>(M_PI) * x;
return std::sin(pix) / pix;
}
One could also cast x to double if T is an integral type. Here's a C++20 version of that:
#include <concepts> // std::integral
#include <numbers> // std::numbers::pi_v
template<class T>
T sinc(T x) {
if(x == T{}) return 1; // avoid division by zero
// C++20 added some constants to the standard library:
auto pix = std::numbers::pi_v<T> * x;
return std::sin(pix) / pix;
}
double sinc(std::integral auto x) {
return sinc<double>(x);
}