bugprone-implicit-widening-of-multiplication-result¶
The check diagnoses instances where a result of a multiplication is implicitly widened, and suggests (with fix-it) to either silence the code by making widening explicit, or to perform the multiplication in a wider type, to avoid the widening afterwards.
This is mainly useful when operating on very large buffers. For example, consider:
void zeroinit(char* base, unsigned width, unsigned height) {
for(unsigned row = 0; row != height; ++row) {
for(unsigned col = 0; col != width; ++col) {
char* ptr = base + row * width + col;
*ptr = 0;
}
}
}
This is fine in general, but if width * height
overflows,
you end up wrapping back to the beginning of base
instead of processing the entire requested buffer.
Indeed, this only matters for pretty large buffers (4GB+), but that can happen very easily for example in image processing, where for that to happen you “only” need a ~269MPix image.
Options¶
- UseCXXStaticCastsInCppSources¶
When suggesting fix-its for C++ code, should C++-style
static_cast<>()
’s be suggested, or C-style casts. Defaults totrue
.
- UseCXXHeadersInCppSources¶
When suggesting to include the appropriate header in C++ code, should
<cstddef>
header be suggested, or<stddef.h>
. Defaults totrue
.
Examples:
long mul(int a, int b) {
return a * b; // warning: performing an implicit widening conversion to type 'long' of a multiplication performed in type 'int'
}
char* ptr_add(char *base, int a, int b) {
return base + a * b; // warning: result of multiplication in type 'int' is used as a pointer offset after an implicit widening conversion to type 'ssize_t'
}
char ptr_subscript(char *base, int a, int b) {
return base[a * b]; // warning: result of multiplication in type 'int' is used as a pointer offset after an implicit widening conversion to type 'ssize_t'
}