# Extra Clang Tools 18.1.8 documentation

## 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 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

WarnOnIntegerNarrowingConversion

  When true, the check will warn on narrowing integer conversion (e.g. `int` to `size_t`). true by default.

WarnOnIntegerToFloatingPointNarrowingConversion

  When true, the check will warn on narrowing integer to floating-point conversion (e.g. `size_t` to `double`). true by default.

WarnOnFloatingPointNarrowingConversion

  When true, the check will warn on narrowing floating point conversion (e.g. `double` to `float`). true by default.

WarnWithinTemplateInstantiation

  When true, the check will warn on narrowing conversions within template instantiations. 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.