Declare one name (only) per declaration

Reason

One declaration per line increases readability and avoids mistakes related to the C/C++ grammar. It also leaves room for a more descriptive end-of-line comment.

Example, bad

char *p, c, a[7], *pp[7], **aa[10];   // yuck!

Exception

A function declaration can contain several function argument declarations.

Exception

A structured binding (C++17) is specifically designed to introduce several variables:

auto [iter, inserted] = m.insert_or_assign(k, val);
if (inserted) { /* new entry was inserted */ }

Example

template<class InputIterator, class Predicate>
bool any_of(InputIterator first, InputIterator last, Predicate pred);

or better using concepts:

bool any_of(input_iterator auto first, input_iterator auto last, predicate auto pred);

Example

double scalbn(double x, int n);   // OK: x * pow(FLT_RADIX, n); FLT_RADIX is usually 2

or:

double scalbn(    // better: x * pow(FLT_RADIX, n); FLT_RADIX is usually 2
    double x,     // base value
    int n         // exponent
);

or:

// better: base * pow(FLT_RADIX, exponent); FLT_RADIX is usually 2
double scalbn(double base, int exponent);

Example

int a = 10, b = 11, c = 12, d, e = 14, f = 15;

In a long list of declarators it is easy to overlook an uninitialized variable.

Enforcement

Flag variable and constant declarations with multiple declarators (e.g., int* p, q;)