Why is this an issue?
A magic number is a number that comes out of nowhere, and is directly used in a statement. Magic numbers are often used, for instance to limit the
number of iterations of a loops, to test the value of a property, etc.
Using magic numbers may seem obvious and straightforward when you’re writing a piece of code, but they are much less obvious and straightforward at
debugging time.
That is why magic numbers must be demystified by first being assigned a name. This is classically done by using a constant (constexpr
or const if your compiler does not support constexpr yet) or an enumeration.
-1, 0 and 1 are not considered magic numbers.
Note that since C++20, some well known mathematical constants, such as pi, are defined in the header <numbers>, and should
be preferred over defining your own version (see S6164).
Noncompliant code example
void doSomething(int var) {
for(int i = 0; i < 42; i++) { // Noncompliant - 42 is a magic number
// ...
}
if (var == 42) { // Noncompliant - magic number
// ...
}
}
Compliant solution
enum Status {
STATUS_KO = 0,
STATUS_OK = 42,
};
void doSomething(Status var) {
constexpr int maxIterations = 42; // Compliant - in a declaration
for(int i = 0; i < maxIterations ; i++){ // Compliant: 0 is excluded, and maxIterations is a named constant
// ...
}
if (STATUS_OK == var) { // Compliant - number comes from an enum
// ...
}
}
