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;.

Arithmetic with smaller integer types than int trigger implicit conversions, as explained under “Integral Promotion” on cppreference.com. This check diagnoses more instances of narrowing than the compiler warning -Wconversion does. The example below demonstrates this behavior.

// The following function definition demonstrates usage of arithmetic with
// integer types smaller than `int` and how the narrowing conversion happens
// implicitly.
void computation(short argument1, short argument2) {
  // Arithmetic written by humans:
  short result = argument1 + argument2;
  // Arithmetic actually performed by C++:
  short result = static_cast<short>(static_cast<int>(argument1) + static_cast<int>(argument2));
}

void recommended_resolution(short argument1, short argument2) {
  short result = argument1 + argument2;
  //           ^ warning: narrowing conversion from 'int' to signed type 'short' is implementation-defined

  // The cppcoreguidelines recommend to resolve this issue by using the GSL
  // in one of two ways. Either by a cast that throws if a loss of precision
  // would occur.
  short result = gsl::narrow<short>(argument1 + argument2);
  // Or it can be resolved without checking the result risking invalid results.
  short result = gsl::narrow_cast<short>(argument1 + argument2);

  // A classical `static_cast` will silence the warning as well if the GSL
  // is not available.
  short result = static_cast<short>(argument1 + argument2);
}

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.