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 rule is part of the “Expressions and statements” profile of the C++ Core Guidelines, corresponding to rule ES.46. See
- We enforce only part of the guideline, more specifically, we flag narrowing conversions from:
an integer to a narrower integer (e.g.
char
tounsigned char
) if WarnOnIntegerNarrowingConversion Option is set,an integer to a narrower floating-point (e.g.
uint64_t
tofloat
),a floating-point to an integer (e.g.
double
toint
),a floating-point to a narrower floating-point (e.g.
double
tofloat
) 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¶
-
WarnOnIntegerNarrowingConversion
¶
When true, the check will warn on narrowing integer conversion (e.g.
int
tosize_t
). true by default.
-
WarnOnFloatingPointNarrowingConversion
¶
When true, the check will warn on narrowing floating point conversion (e.g.
double
tofloat
). true by default.
-
WarnWithinTemplateInstantiation
¶
When true, the check will warn on narrowing conversions within template instantations. false by default.
-
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.
-
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.
-
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.