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.. title:: clang-tidy - bugprone-implicit-widening-of-multiplication-result
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:
.. code-block:: c++
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
-------
.. option:: UseCXXStaticCastsInCppSources
When suggesting fix-its for C++ code, should C++-style ``static_cast<>()``'s
be suggested, or C-style casts. Defaults to ``true``.
.. option:: UseCXXHeadersInCppSources
When suggesting to include the appropriate header in C++ code,
should ``<cstddef>`` header be suggested, or ``<stddef.h>``.
Defaults to ``true``.
Examples:
.. code-block:: c++
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'
}
|
