readability-magic-numbers¶

Detects magic numbers, integer or floating point literals that are embedded in code and not introduced via constants or symbols.

Many coding guidelines advise replacing the magic values with symbolic constants to improve readability. Here are a few references:

• Rule ES.45: Avoid “magic constants”; use symbolic constants in C++ Core Guidelines
• Rule 5.1.1 Use symbolic names instead of literal values in code in High Integrity C++
• Item 17 in “C++ Coding Standards: 101 Rules, Guidelines and Best Practices” by Herb Sutter and Andrei Alexandrescu
• Chapter 17 in “Clean Code - A handbook of agile software craftsmanship.” by Robert C. Martin
• Rule 20701 in “TRAIN REAL TIME DATA PROTOCOL Coding Rules” by Armin-Hagen Weiss, Bombardier
• http://wiki.c2.com/?MagicNumber

Examples of magic values:

double circleArea = 3.1415926535 * radius * radius;

double totalCharge = 1.08 * itemPrice;

int getAnswer() {
   return -3; // FILENOTFOUND
}

for (int mm = 1; mm <= 12; ++mm) {
   std::cout << month[mm] << '\n';
}

Example with magic values refactored:

double circleArea = M_PI * radius * radius;

const double TAX_RATE = 0.08;  // or make it variable and read from a file

double totalCharge = (1.0 + TAX_RATE) * itemPrice;

int getAnswer() {
   return E_FILE_NOT_FOUND;
}

for (int mm = 1; mm <= MONTHS_IN_A_YEAR; ++mm) {
   std::cout << month[mm] << '\n';
}

For integral literals by default only 0 and 1 (and -1) integer values are accepted without a warning. This can be overridden with the IgnoredIntegerValues option. Negative values are accepted if their absolute value is present in the IgnoredIntegerValues list.

As a special case for integral values, all powers of two can be accepted without warning by enabling the IgnorePowersOf2IntegerValues option.

For floating point literals by default the 0.0 floating point value is accepted without a warning. The set of ignored floating point literals can be configured using the IgnoredFloatingPointValues option. For each value in that set, the given string value is converted to a floating-point value representation used by the target architecture. If a floating-point literal value compares equal to one of the converted values, then that literal is not diagnosed by this check. Because floating-point equality is used to determine whether to diagnose or not, the user needs to be aware of the details of floating-point representations for any values that cannot be precisely represented for their target architecture.

For each value in the IgnoredFloatingPointValues set, both the single-precision form and double-precision form are accepted (for example, if 3.14 is in the set, neither 3.14f nor 3.14 will produce a warning).

Scientific notation is supported for both source code input and option. Alternatively, the check for the floating point numbers can be disabled for all floating point values by enabling the IgnoreAllFloatingPointValues option.

Since values 0 and 0.0 are so common as the base counter of loops, or initialization values for sums, they are always accepted without warning, even if not present in the respective ignored values list.