C++ / S109: Magic number in initialization list

Rules and Languages
1

Hello,

Let the class:

class MyClass
{
    MyClass();
    ~MyClass() = default;
    int32_t m_integer;
    float_t m_float{ 5.0f };   // Won't raise S105
};

MyClass::MyClass() :
    m_data{5}   // Will raise S105
{}

Please explain me why the value for the m_integer’s initialization is more magical than the m_float’s one ?

1 Like
5

Hi @Oodini ,

Indeed, that rule only triggers for integers and not for floating point constants. I couldn’t find any history of it being a conscious decision, so I created a ticket to change that.

Thanks for raising it.

8

The point is not between integers and floats, but between initialization with declaration, and initialization with initialization list.

9

There are two subjects indeed:

  • we don’t raise on floating point numbers no matter what, so 5.0f will never raise no matter how it is used. I don’t see a good reason for that one.
  • we don’t raise on variable declaration but raise on constructor initializers. That second one is quite philosophical and can definitely be wrong sometime. A member declaration with initialization directly gives a single name to a single value, which we expect to be clear enough to not be “magical”. When you choose to use a constructor initializer instead of initializing the member variable in its declaration, it is potentially because multiple constructors give different values to that member, so a single name for multiple values.

We provide S109, but the reason it is not part of the default quality profile is that it is not a rule we are confident in. It tries to guess with a simple heuristic how clear the code is, but that comes with false positives and false negatives.