.. title:: clang-tidy - cppcoreguidelines-narrowing-conversions
cppcoreguidelines-narrowing-conversions
=======================================
Checks for silent narrowing conversions, e.g: ``int i = 0; i += 0.1;``. While
the issue is obvious in this former example, it might not be so in the
following: ``void MyClass::f(double d) { int_member_ += d; }``.
This check implements `ES.46
<https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#es46-avoid-lossy-narrowing-truncating-arithmetic-conversions>`_
from the C++ Core Guidelines.
We enforce only part of the guideline, more specifically, we flag narrowing conversions from:
- an integer to a narrower integer (e.g. ``char`` to ``unsigned char``)
if WarnOnIntegerNarrowingConversion Option is set,
- an integer to a narrower floating-point (e.g. ``uint64_t`` to ``float``)
if WarnOnIntegerToFloatingPointNarrowingConversion Option is set,
- a floating-point to an integer (e.g. ``double`` to ``int``),
- a floating-point to a narrower floating-point (e.g. ``double`` to ``float``)
if WarnOnFloatingPointNarrowingConversion Option is set.
This check will flag:
- All narrowing conversions that are not marked by an explicit cast (c-style or
``static_cast``). For example: ``int i = 0; i += 0.1;``,
``void f(int); f(0.1);``,
- All applications of binary operators with a narrowing conversions.
For example: ``int i; i+= 0.1;``.
Options
-------
.. option:: WarnOnIntegerNarrowingConversion
When `true`, the check will warn on narrowing integer conversion
(e.g. ``int`` to ``size_t``). `true` by default.
.. option:: WarnOnIntegerToFloatingPointNarrowingConversion
When `true`, the check will warn on narrowing integer to floating-point
conversion (e.g. ``size_t`` to ``double``). `true` by default.
.. option:: WarnOnFloatingPointNarrowingConversion
When `true`, the check will warn on narrowing floating point conversion
(e.g. ``double`` to ``float``). `true` by default.
.. option:: WarnWithinTemplateInstantiation
When `true`, the check will warn on narrowing conversions within template
instantiations. `false` by default.
.. option:: WarnOnEquivalentBitWidth
When `true`, the check will warn on narrowing conversions that arise from
casting between types of equivalent bit width. (e.g.
`int n = uint(0);` or `long long n = double(0);`) `true` by default.
.. option:: IgnoreConversionFromTypes
Narrowing conversions from any type in this semicolon-separated list will be
ignored. This may be useful to weed out commonly occurring, but less commonly
problematic assignments such as `int n = std::vector<char>().size();` or
`int n = std::difference(it1, it2);`. The default list is empty, but one
suggested list for a legacy codebase would be
`size_t;ptrdiff_t;size_type;difference_type`.
.. option:: PedanticMode
When `true`, the check will warn on assigning a floating point constant
to an integer value even if the floating point value is exactly
representable in the destination type (e.g. ``int i = 1.0;``).
`false` by default.
FAQ
---
- What does "narrowing conversion from 'int' to 'float'" mean?
An IEEE754 Floating Point number can represent all integer values in the range
[-2^PrecisionBits, 2^PrecisionBits] where PrecisionBits is the number of bits in
the mantissa.
For ``float`` this would be [-2^23, 2^23], where ``int`` can represent values in
the range [-2^31, 2^31-1].
- What does "implementation-defined" mean?
You may have encountered messages like "narrowing conversion from 'unsigned int'
to signed type 'int' is implementation-defined".
The C/C++ standard does not mandate two's complement for signed integers, and so
the compiler is free to define what the semantics are for converting an unsigned
integer to signed integer. Clang's implementation uses the two's complement
format.
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