C++14 introduced the ability to use a digit separator (') to split a literal number into groups of digits for better readability.
To ensure that readability is really improved by using digit separators, this rule verifies:
- Homogeneity
- Except for the left-most group, which can be smaller, all groups in a number should contain the same number of digits. Mixing group sizes
is at best confusing for maintainers, and at worst a typographical error that is potentially a bug.
- Standardization
- It is also confusing to regroup digits using a size that is not standard. This rule enforce the following standards:
- Decimal numbers should be separated using groups of 3 digits.
- Hexadecimal numbers should be separated using groups of 2 or 4 digits.
- Octal and Binary should be separated using groups of 2, 3 or 4 digits.
Furthermore, using groups with more than 4 consecutive digits is not allowed because they are difficult for maintainers to read.
Noncompliant code example
long decimal_int_value = 1'554'3124; // Noncompliant; mixing groups of 3 and 4 digits
double decimal_float_value = 7'91'87'14.3456; // Noncompliant; using groups of 2 instead of 3 digits
long hexadecimal_value = 0x8'3A3'248'6E2; // Noncompliant; using groups of 3 instead of 2 or 4 digits
long octal_value = 0442'03433'13726; // Noncompliant; using groups of 5 instead of 2, 3 or 4 digits.
long binary_value = 0b01010110'11101010; // Noncompliant; using groups of 8 instead of 2, 3 or 4 digits.
Compliant solution
long decimal_int_value = 15'543'124;
double decimal_float_value = 7'918'714.3456;
long hexadecimal_value = 0x83'A324'86E2;
long octal_value = 04'4203'4331'3726;
long binary_value = 0b0101'0110'1110'1010;
Exceptions
No issue is raised on the fractional or exponent part of floating point numbers, only the integral part should comply.
